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Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032 (2023)

Chapter: 18 Planetary Defense: Defending Earth Through Applied Planetary Science

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Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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18

Planetary Defense: Defending Earth Through Applied Planetary Science

Planetary defense is an international cooperative enterprise aimed at providing protection to the nations of the world from devastating asteroid and comet impacts. Through the use of the knowledge and tools gained through planetary science and exploration, it is now possible to develop realistic and cost-effective detection and mitigation strategies against these natural disasters. As awareness of the hazard posed to life and property by Earth-approaching asteroids and comets has grown, the U.S. Congress and presidential administrations have directed NASA, the National Science Foundation (NSF), and other government agencies (e.g., Department of Energy [DOE], Department of Defense [DoD], Department of Homeland Security [DHS], etc.) to pursue activities in support of planetary defense. This chapter discusses the status of planetary defense activities and recommends directions for the next decade.

Our planet orbits the Sun within a swarm of cosmic debris in the form of asteroids and comets, collectively called near-Earth objects (NEOs).1 The scars of previous NEO collisions in the form of impact craters are evident on Earth (e.g., Meteor Crater in Arizona) and major collisions in the past have substantially altered the course of life on the planet. The most famous of these collisions is the one that created the 150 km diameter Chicxulub crater 66 Ma in the Yucatan Peninsula of Mexico. A consequence of this event was the end to the reign of the dinosaurs.

Fortunately, large NEOs are very much less numerous than small NEOs (Figure 18-1). Smaller, more frequent events can also produce serious local to regional (i.e., an area equal to several U.S. states or a small nation) damage. For example, the disintegration in 1908 of a ~40–80 m NEO in the atmosphere above Tunguska, Siberia delivered between 3 and 20 megatons of TNT-equivalent energy to that remote region, scorching and blasting down trees across 2,000 square kilometers (Boslough and Crawford 2008; Morrison 2018). Although this event led to minimal loss of life, it was only because it occurred over a sparsely populated area. A similar but smaller event took place in 2013 over the Siberian city of Chelyabinsk. In this case a 20-meter-diameter asteroid detonated in an airburst, releasing nearly 440 kt of energy, and injuring more than 1,600 people.

Such impacts occur across Earth with an average interval of several decades for Chelyabinsk-sized events to a few millennia for Tunguska-sized events. However, there are considerable uncertainties regarding how much damage is produced by a bolide’s2 energy, considering that a projectile passing through the atmosphere distributes energy along its path as it disintegrates and that the blast energy itself has downward momentum. This means bolide airbursts are more akin to a descending linear detonation in the sky than the explosion of a nuclear weapon.

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1 A glossary of acronyms and technical terms can be found in Appendix F.

2 A bolide is a term describing an object that burns up in the atmosphere, creating an exceptionally bright “fireball.”

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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FIGURE 18-1 The known and extrapolated population of near-Earth asteroids (N) in terms of estimated diameter (D) and absolute magnitude (H). The blue shaded area represents the estimated population that remains to be discovered. Collisions with the millions of ~20 m objects, the size of the Chelyabinsk asteroid, will likely occur every 50–100 years, compared to ~10 km Chicxulub-size impactors, which strike Earth every 100 million years on average. The various symbols used are explained in the box at upper right. SOURCE: Reprinted and adapted from A.W. Harris and P.W. Chodas, 2021, “The Population of Near-Earth Asteroids Revisited and Updated,” Icarus 365:114452, https://doi.org/10.1016/j.icarus.2021.114452. Copyright 2021, with permission from Elsevier.

The amount of damage produced by such events is not easily predicted owing to uncertainties related to the object’s physical properties and structure, the breakup process, and atmospheric energy deposition. Some smaller objects may penetrate deeper into the atmosphere and deliver more energy than anticipated.

Orbiting spacecraft routinely detect the high-altitude break-up of smaller NEOs entering Earth’s atmosphere, the vast majority of which are harmless (Figure 18-2). Although the annual probability of Earth being struck by a larger asteroid or comet is small, the consequences of such a collision are so serious that prudence dictates that society assess the nature of the threat and be prepared to respond.

To date, numerous NEOs have been found, mainly by U.S.-funded surveys, and cataloged by NASA (see Figures 18-1 and 18-3). Characteristics of the NEO population include:

  • There are 1,000 or so NEOs greater than 1 km in diameter that are potentially capable of causing global impact effects. Approximately 95 percent of these bodies have been found, and fortunately none of them are a current threat.
Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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FIGURE 18-2 The chart above shows reported fireball events for which geographic location data are known. Each event’s calculated total impact energy is indicated by its relative size and color. The 2013 Chelyabinsk event is the largest recorded over this timeframe (marked by the large red circle on the map), with an energy of approximately 440 kt. For reference, the atomic bomb dropped on Hiroshima released some 15 kt of energy and the above data show that these events deposit roughly 1.5 to 2 times that amount into Earth’s atmosphere every year. Data are current through February 2, 2023. SOURCE: Courtesy of A.B. Chamberlin, NASA/JPL-Caltech, https://cneos.jpl.nasa.gov/fireballs.
  • There are close to 25,000 objects larger than 140 m in diameter that could cause regional devastation. Only about one-third have been detected and tracked to date.
  • An estimated 100,000 or more objects exist that are equal to or larger than 50 m in diameter and could destroy a concentrated urban area. It is estimated that fewer than 2 percent of these have been detected.
  • There are millions to tens of millions of NEOs that if they struck Earth would likely break up in Earth’s atmosphere, and could potentially cause surface damage, although atmospheric screening does provide partial shielding from blast effects. Relatively few of these small bodies have been discovered.

The public has consistently ranked planetary defense (henceforward, “PD”) as one of NASA’s top priorities, supported in 2019 by 62 percent of poll respondents (Johnson 2019). Over the past three decades, congressional action has spurred NASA to improve its PD programs.

The Spaceguard Survey, aimed at discovering and tracking NEOs 1 km and larger, was initiated in 1998 and largely completed by 2009. The 2005 George E. Brown, Jr. NEO Survey Act directed NASA to catalog, by 2020, 90 percent of NEOs larger than 140 m; impacts of this size may cause regional devastation. As shown in Figure 18-1, NASA has not yet completed the George E. Brown survey goal. NASEM (2019) stated that NASA was provided inadequate funding to accomplish this task by the 2020 date. Guided by continuing PD technology and policy studies, NASA in 2016 established a Planetary Defense Coordination Office (PDCO), and the National Science and Technology Council in 2018 published the National Near-Earth Object Preparedness Strategy and Action Plan (NSTC 2018), followed by a 2021 Report on NEO Impact Threat Emergency Protocols (NSTC 2021).

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
×
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FIGURE 18-3 Near-Earth object discoveries over time, showing the accelerating rate of NEO detection from ground-based search programs. Data current as of February 16, 2023. SOURCE: Courtesy of A.B. Chamberlin, NASA/JPL-Caltech, https://cneos.jpl.nasa.gov/stats/totals.html.

NASA’s annual PD budget within the Science Mission Directorate has risen to more than $160 million.3 These resources have enabled NASA and its partners to catalog, as of February 2023, more than 30,000 NEOs using ground-based telescopes and the NEOWISE space-based, infrared telescope (NASA 2021b) (Figure 18-3). The discovery rate is increasing, yet millions of NEOs remain undiscovered. NASA is developing the Near-Earth Object Surveyor (NEO Surveyor) infrared space telescope to complete the congressionally mandated 140 m NEO survey; a 2026 launch is planned. Additionally, the agency launched the Double Asteroid Redirection Test (DART) kinetic impact deflection demonstration mission in 2021; the spacecraft will impact the small asteroid Dimorphos, a satellite of the asteroid Didymos, in 2022. Last, outside of NASA’s PD budget line, the NSF-funded 8.4-meter Vera C. Rubin Observatory (VCRO), previously referred to as the Large Synoptic Survey Telescope, plans to begin operations in roughly two years and will discover numerous NEOs.

The International Astronomical Union Minor Planet Center (MPC), hosted at the Harvard and Smithsonian Center for Astrophysics, is the key repository for all NEO observations collected worldwide. The MPC conducts an initial analysis of observations to confirm and identify NEOs, computes orbits based on these observations, and highlights objects of interest. MPC transmits this information to the Center for Near-Earth Object Studies (CNEOS) at NASA’s Jet Propulsion Laboratory, which calculates high-precision orbits and scans all confirmed

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3 All dollar amounts are in real-year dollars unless stated otherwise.

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
×

NEOs for any potential impacts via its Sentry impact monitoring system, and monitors potential newly discovered NEOs via its Scout hazard assessment system.

NASA and NSF have broad expertise in both the scientific exploration and characterization of NEOs. NASA’s deep space operations experience also enables the development of deflection technologies for future diversion of a threatening NEO. The decadal survey is tasked with making recommendations on future steps and the level of resources needed to meet PD preparedness goals.

The PD findings and recommendations in this report are presented in the framework of the National Near-Earth Object Preparedness Strategy and Action Plan, which identifies NASA as the key U.S. government agency to lead such activities and directs NSF to provide support. The plan’s five strategic goals underpin the nation’s effort to enhance preparedness for dealing with the threat of future NEO impacts (NSTC 2018):

  1. Enhance NEO detection, tracking, and characterization capabilities.
  2. Improve NEO modeling, prediction, and information integration.
  3. Develop technologies for NEO deflection and disruption missions.
  4. Increase international cooperation on NEO preparation.
  5. Strengthen and routinely exercise NEO impact emergency procedures and action protocols.

Given the statement of task and its purview (see Appendix A), the committee’s findings and recommendations will concentrate on the first three items in the list above.

Finding: The threat to Earth from potentially hazardous objects is a low-probability yet high-consequence hazard recognized and documented in several academic and government reports. Efforts to address this threat are most effective when undertaken on a continuous, long-term basis through cooperation among NASA, NSF, U.S. Space Command, and other U.S. government entities.

Finding: The establishment in 2016 of NASA’s PDCO has brought leadership and strategic direction to U.S. planetary defense efforts.

Recommendation: NASA’s PDCO should be robustly supported and sustained as the critical organization to advance U.S. planetary defense capabilities and initiatives in the next decade and beyond.

NEO DETECTION, TRACKING, AND CHARACTERIZATION

NEO Surveys

NASA’s current efforts to detect and track NEOs are guided by the Brown Act, which requires NASA to “detect, track, catalogue, and characterize . . . near-Earth objects ≥140 meters in diameter to assess the threat of such objects to Earth.” It further set a goal of detecting 90 percent of this population of objects within 15 years—that is, by 2020. However, by 2021 only an estimated one-third of all NEOs ≥140 m have been discovered (NSTC 2018, 2021; see Figure 18-3).

These NEO survey goals are designed to quantify the risk of regional to global catastrophic impact damage by establishing a degree of completion that is both timely and cost effective. Impacts from 140 m objects would be damaging on a regional level. Such an impact would be equivalent to an explosion of more than 60 megatons of TNT, larger than the most powerful nuclear device ever tested (NASEM 2019). While such events are rare (the average interval between impacts of 140 m objects is ~20,000 years), the consequence of such an event can be large, causing mass casualties and catastrophic loss of infrastructure, particularly if the impact occurs near a populated area (NASEM 2019). A recent study estimated that the number of casualties from such an event—when averaged over the entire surface of Earth—would exceed 10,000 (NASA 2017), although the actual number of casualties will vary given the warning time and civil defense preparations. The primary benefit from a comprehensive NEO survey is to reduce the risk uncertainty associated with such high-consequence events by early identification of potentially hazardous objects—finding them before they find us.

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
×

The Brown Act survey goals are based on the findings of a 2003 NASA science definition team study (NASA 2003). A follow-on report (NASA 2017) reconfirmed these goals as a reasonable baseline to understand the current impact risk at an acceptable level within a decade after survey initiation. Although the Brown Act survey’s completion deadline was not met, its accomplishment in roughly a decade is still crucially important. Meeting such a deadline dramatically reduces the uncertainties in the risk of impact from medium to large (≥140 m in diameter) NEOs, optimizes the use of facilities and trained personnel, and obtains maximum benefit from the lessons provided by other current space-based detection tools, for example, NEOWISE.

Even though objects ≥140 m in size are the priority for detection and cataloging owing to their grave impact consequences, objects smaller than 140 m can also cause significant damage (e.g., the Tunguska and Chelyabinsk events). They represent a tangible threat to public safety, and yet owing to their small size, are some of the most challenging objects to detect prior to impact. It should be noted that small airburst events are by far the likeliest NEO impacts that Earth will encounter in the coming century (see Figures 18-1 and 18-2).

Finding: The Brown Act survey goals still provide a reasonable size threshold (≥140 m), completion level (>90 percent), and timetable (survey completion in <10 years from now). However, the most likely impactors are NEOs less than 140 m in diameter and these can pose a significant threat of local damage. It is therefore important to detect, track, and characterize as much of this smaller population as possible in addition to meeting the act’s goals.

Current assets used by NASA for NEO surveys include both ground-based, visible-wavelength telescopes (e.g., Pan-STARRS, Catalina Sky Survey, ATLAS) and the repurposed NEOWISE space-based, infrared telescope (NASA 2021b). The rate of NEO discoveries has increased steadily over the past decade as more capable telescopes have been added to this network. However, current observational capabilities will only achieve ~50 percent completion over the next decade in detection of objects ≥140 m (NASEM 2019).

The addition of NSF’s 8.4 m, ground-based Vera C. Rubin Observatory, planned for first light in 2023, will augment existing detection capabilities. However, even if this telescope was fully dedicated to NEO search and detection operations, it would still not achieve 90 percent detection completion for decades. Visible-wavelength telescopes are unable to distinguish between small, bright objects and large, dark ones, leading to an uncertainty in the size of detected NEOs that translates into an uncertainty concerning potential impact energy. Follow-up observations would be needed to better quantify the hazard from detected objects (NASEM 2019).

NEO Surveyor

Space-based telescopes have distinct advantages over their ground-based counterparts—e.g., uninterrupted operations, a more optimal viewing geometry yielding higher detection rates, and the ability to operate at wavelengths that are blocked by Earth’s atmosphere. Space-based telescopes operating in the mid-infrared detect energy radiating directly from the object, not reflected sunlight. This resolves the uncertainty in size versus reflectivity (albedo) and enables a much more accurate determination of an object’s diameter. A recent National Academies study (NASEM 2019) concluded that a space-based mid-infrared survey is the most effective, timely option for meeting the congressional NEO survey completeness and size determination requirements.

Finding: A dedicated space-based mid-infrared survey is the most effective architecture to accomplish congressionally directed NEO survey goals. A space-based mid-infrared survey also provides real-time information on object diameter, critical for rapid impact hazard assessment.

NASA has for several years been developing a space-based, mid-infrared telescope mission, NEO Surveyor. Its precursor concept, NEOCam, was funded to mature the proposed infrared detector technology and refine the mission design. NEO Surveyor is now in Phase B aimed at launch by 2026. However, while NASA has requested

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
×

funding for full-scale development in the 2022 President’s Budget Request, funding is not yet assured, and the mission is not yet confirmed.

Finding: The first priority in planetary defense is early detection, tracking, and characterization of NEOs, whose impact may cause widespread regional damage. NEO Surveyor, in development by NASA for this purpose, is the most timely and effective means to complete the survey goal of detecting 90 percent of NEOs greater than 140 m in diameter.

Recommendation: NASA should fully support the development, timely launch, and subsequent operation of NEO Surveyor to achieve the highest priority planetary defense NEO survey goals.

Vera C. Rubin Observatory

The Vera C. Rubin Observatory (VCRO) is a multi-disciplinary facility funded by NSF and DOE and currently under construction on Cerro Pachón in north-central Chile. Although VCRO alone, or in combination with existing ground-based observatories, cannot meet the completion goals for the NEO survey within the next decade, it does complement NEO Surveyor. The observation cadence of VCRO is also optimal for sampling the rotational light curves of most asteroids, important for constraining asteroid shapes and spin states (Jones et al. 2020).

VCRO’s search area is centered away from the Sun, optimal for detection of objects in opposition, while NEOS’s search area is oriented toward the near-Sun regions of the sky (Mainzer et al. 2020). NEOS’s search strategy is optimized to increase its overall NEO detection rates. In addition, the different search areas provide opportunities for follow-up observations over longer orbital arcs, reducing errors in orbit estimation, which is important for tracking potentially hazardous objects.

Although funding for VCRO is in place, no focused programs exist specifically for analysis of solar system data, such as NEO characterization. Dedicated funding is needed to ensure maximum use of this facility for PD purposes. Because most VCRO discoveries are expected to occur during the first few years of the survey, readiness for handling NEO data prior to expected full operations as early as late 2023 is critical.

Finding: Although the VCRO cannot alone meet the Brown Act’s goals, it is complementary to NEO Surveyor. A robust, focused program for VCRO dedicated to NEO detection, follow-up tracking, and characterization, and a well-coordinated data pipeline between VCRO, the Minor Planet Center, and the Center for Near-Earth Object Studies, would provide optimal benefits for planetary defense.

NEO discovery rates have been increasing for more than two decades, taxing the capabilities of existing followup facilities to keep pace with NEO detection confirmations, arc extensions, and characterization. NEO detection rates are expected to increase ten-fold when VCRO and NEOS come on-line (Jones et al. 2020) and will put additional stress on follow-up observations. Although VCRO and NEOS can conduct self-follow-up observations, the demand for time on other telescopes to follow-up new NEO discoveries is likely to sharply increase. Follow-up observations of interesting, and often faint, objects discovered by VCRO and NEOS may require access to the new generation of extremely large telescopes now under development, or possibly even large space telescopes such as James Webb Space Telescope or Nancy Grace Roman Space Telescope. New, dedicated, large-aperture follow-up facilities and/or significant access to existing large-aperture telescopes will be required to complement the surveys’ self-follow-up plans (Seaman et al. 2020). International collaboration and cooperation will also play a key role in tracking and observing these objects (see below).

Finding: Existing observatories dedicated to NEO discovery and follow-up will still be needed after VCRO and NEO Surveyor come on-line. In addition, time on larger telescopes (both existing and in development) coordinated by NASA and NSF will be needed for tracking and characterization of faint objects that pose a nonnegligible impact probability.

Finding: The data pipeline for detection, tracking, impact assessment, and reporting to the public will be tested by the ten-fold increase in NEO detection rates when VCRO and NEO Surveyor become operational. Both facilities will benefit from coordinated rehearsals and operational readiness reviews well prior to achieving their full operational capabilities.

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
×

Mega-Satellite Constellations and NEO Surveys

An increasing threat to the ability of ground-based telescopes to detect and characterize NEOs comes from existing and planned mega-constellations of low-Earth-orbiting (LEO) satellites (e.g., Starlink). These constellations, particularly those below 600 km, are highly visible during twilight when NEO surveys begin and end nightly operations. Accordingly, they are contaminating a fraction of each night’s survey observing window with reflected light from their surfaces, making it more difficult to detect potentially hazardous objects.

The consequences of existing LEO satellites are starting to be felt by the leading NEO search facilities such as Pan-STARRS in Hawaii and the Catalina Sky Survey in Arizona. Concern is also mounting among the astronomical community over the proposed networks of 100,000 LEO satellites. If such networks come to fruition, no combination of even “best-case” mitigation measures (e.g., material selection and coatings to reduce reflective light, and orientation of solar arrays) will be capable of fully countering the constellations’ negative impact on the search for NEOs (Walker et al. 2021).

Finding: Mega-constellations of commercial satellites will likely impact the efficiency of ground-based searches for NEOs. Efforts by NASA, NSF, and the astronomical community to monitor these constellations and encourage development of mitigation mechanisms to reduce light contamination from these satellites, as well as support software algorithm improvement of NEO search pipelines, would minimize impacts to ground-based NEO surveys.

Inter-Agency Synergies

One of the key areas of overlap for NEO discoveries exists between space situational awareness (i.e., tracking objects in space, identifying them, and predicting where they will be at any given time), monitoring and characterizing orbital debris, and planetary defense. U.S. Space Command’s role in space situational awareness is to monitor near-Earth and cislunar (i.e., the space in the immediate vicinity of the Earth-Moon system) space to maintain U.S. national security interests. Some of the assets it uses are optimized for detecting small human-made objects in near-Earth space but can also detect natural objects as a byproduct of their mission objectives. Similarly, NASA’s Orbital Debris Program Office uses its own assets to characterize the debris environment in Earth orbit and has capabilities to distinguish natural objects from human-made space debris. Data from these respective organizations have the potential to provide insight into the NEO population in and around near-Earth/cislunar space and contribute to important PD initiatives.

Finding: U.S. Space Command and NASA’s Orbital Debris Program Office have capabilities to discover NEOs in cislunar and very near-Earth space environments. Such discoveries would provide useful information concerning the general NEO population. Increased cooperation between NASA and U.S. Space Command to exchange information on NEOs from their respective organizations would aid planetary defense objectives.

NEO Characterization

Policy makers, scientists, and engineers require knowledge of the physical characteristics of a NEO to determine the safest and most efficient measures to deflect a threatening object away from Earth.

The most important factor in NEO characterization is determining the object’s orbit. When detected, the trajectories of most objects often have substantial uncertainties, such that Earth impact probability is poorly constrained. More accurate knowledge of the orbit is then required from follow-up observations to determine whether that object poses a true impact threat to Earth. In such circumstances, radar observations are extremely useful to determine precise positions and velocities of these objects.

Beyond orbit determination, characterization encompasses the measurement of all properties of a NEO relevant to planetary defense (NRC 2010). The severity of a NEO impact is determined by an object’s mass and velocity at Earth encounter, as these two characteristics define the impact kinetic energy. Other factors influencing impact effects include the NEO’s diameter, density, composition, material strength, and its approach angle to the surface. These dynamical and physical characteristics determine how the energy is partitioned upon Earth impact and influence whether the object breaks up harmlessly in the upper atmosphere or whether aerodynamic stress will

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
×

cause it to detonate near/at Earth’s surface. The NEO’s physical characteristics also have bearing on the effectiveness of various space deflection/mitigation techniques. For example, a high-velocity projectile launched into a weak rubble pile object may produce unexpected results in deflection efficiency compared to striking a more consolidated asteroid.

Ground-based remote sensing techniques measure and constrain important properties such as diameter, shape, composition, and density. However, in situ spacecraft observations offer precise knowledge of these properties as well as additional ones such as mass, porosity, strength, and the presence of small companions. Such characterization is desired to develop robust mitigation strategies for different NEO object classes and may be needed for specific NEOs if it becomes necessary to deflect or disrupt them (see Next Steps for Planetary Defense Demonstration Missions section below).

Ground-based characterization efforts can deliver insights into the physical nature of individual NEOs and the population in general. However, as the catalog of discovered NEOs grows and becomes more diverse, characterization efforts can lag far behind. This is in part owing to the limited number of ground-based observatories available, the challenges of observing small, distant NEOs, and the personnel required to obtain, analyze, and interpret NEO data. This means that broad-scale characterization efforts of certain properties (e.g., color surveys, such as those performed by the Sloan Digital Sky Survey in New Mexico; diameter and albedo surveys, such as those by NEOWISE) combined with detailed observations of individual NEOs are needed to evaluate which mitigation strategies will be the most effective and to assess the specific risk from the most hazardous NEOs.

When a NEO is discovered by a ground-based survey like Pan-STARRS, the only property immediately known apart from the orbit is its optical brightness. By assuming an albedo (e.g., typical values are 0.04 for carbonaceous chondrites and 0.20 for noncarbonaceous chondrites), it is possible to get a rough estimate of the object’s diameter. Uncertainties in albedo, and limited observations of small (dim) NEOs that lead to uncertainties in brightness, can easily result in diameter estimates being off by a factor of two or more. Radar measurements obtain more precise diameter information without any need for brightness/albedo estimates, but such opportunities are limited (see next section).

Spectral similarities between a NEO and specific meteorite compositions can be used to estimate an object’s mineralogical makeup. NASA’s Infrared Telescope Facility in Hawaii has made key contributions to the spectral characterization of NEOs, and continuing such efforts is important for refining mineralogic classifications. These classifications are then used to infer bulk densities, physical strengths, and assumptions about internal structure, albeit with significant uncertainty, because porosity is not known.

Observed light curves can deliver estimates of a NEO’s physical properties such as shape, rotation period, pole orientation, and the inferred presence of satellites. Radar observations can yield those same characteristics as well as NEO size and surface roughness, but for a limited number of targets.

Finding: Because NEOs are best observed closest to Earth, when optically bright (usually soon after discovery), ground-based characterization efforts depend on obtaining scarce telescope observing time, along with availability of favorable observing geometry and sky conditions. Such challenges result frequently in missed opportunities to physically characterize many NEOs, and the lagging pace of characterization will reduce abilities to respond effectively to a future impact threat.

Research on meteors, meteorites, fireballs, and bolides also provides valuable opportunities to characterize NEOs. Data from these studies include estimates of NEOs’ compositions, strengths, internal structures, and mechanical properties. These factors are important to assess the potential damage from a NEO impact and to inform any mitigation techniques that could be employed to prevent an impact with Earth. Meteorites provide unique insights into the range of materials that make up NEOs and main belt asteroids, as well as providing scientific knowledge about the early solar system, planetesimal formation, and small body evolution. Currently, more than 65,000 meteorites are cataloged. The most common type of meteorites, ordinary chondrites, are composed of both rocky and metallic components and can have a compressive strength similar to concrete or granite when coherent (Popova et al. 2011). In contrast, other meteorite types are known to have quite different properties, from iron meteorites composed dominantly of an Fe-Ni alloy to highly friable carbonaceous chondrites (Brown et al. 2000). Small asteroids entering the atmosphere typically don’t cause direct damage, but they are the most abundant

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
×

type and are visible by many people. Modeling and accurately predicting the visibility range and brightness of these small impacting NEOs will enhance the credibility of the NEO activities and improve the predictions of the magnitude of expected explosions.

However, it is not possible to directly infer the strength of an NEO from the compressive strength of spectrally similar meteorites. Meteorites are pervasively fractured down to centimeter scales, and bolide observations have repeatedly shown that these objects are considerably weaker than coherent rocks, often breaking up high in the atmosphere. Estimates of the bulk strength of ordinary and carbonaceous chondrite bolides suggest a wide range of values, with some akin to a crumbly clod of dirt and others strong enough to reach deep in the atmosphere (Popova et al. 2011; Brown et al. 2016). The behavior of bolides is attributed to their macroscopic structure; some are highly fractured or simply have a weak structure, while others have some internal integrity. For planetary defense purposes, it is crucial to characterize an object as a whole, including its large-scale structure, especially as that structure may be a controlling factor in the strength of the object.

Finding: Meteor, fireball, and bolide events offer naturally occurring opportunities to characterize atmospheric energy deposition processes, elucidate the mechanical properties of NEO materials, and investigate the break-up process. Such knowledge informs and assists planetary defense activities related to NEO characterization, mitigation, and modeling.

Unique perspectives can also be gained from fireball and bolide events if they can be subsequently linked to a recovered meteorite. One of the most insightful examples of this paired knowledge stemmed from the airburst of 2008 TC3, which delivered the Almahata Sitta meteorites to Sudan (Jenniskens et al. 2009). The roughly 4-meter 2008 TC3 asteroid was discovered about 19 hours prior to impact and observed by numerous telescopes worldwide. After its explosion as a bright fireball, a dedicated field search collected numerous meteorites. These meteorites revealed a surprising degree of mineralogical and compositional diversity, illuminating the potential compositional heterogeneity exhibited by a single NEO. This event demonstrated the scientific value of linking and interpreting telescopic observations of small NEOs prior to Earth impact.

Conversely, although the roughly 20-meter Chelyabinsk NEO was not detected prior to atmospheric entry in 2013, the numerous samples that were collected, all ordinary chondrites (Popova et al. 2013), provide key knowledge that can be used to calibrate impact hazard models against a known event and to inform future mitigation strategies.

Finding: Meteorites provide samples of NEOs for investigation in state-of-the-art laboratories, adding to our understanding of compositional and mechanical properties and variations among the NEO population. The rapid recovery of meteorite falls linked to atmospheric entry events is uniquely valuable to combined meteorite and bolide studies.

A challenge with fireball, bolide, and atmospheric entry observations/research is that these events are largely unpredictable. Being in position to observe and follow-up when opportunities arise is essential to such studies. The flexibility to respond rapidly to a given event through international collaboration is important to obtain PD knowledge from these naturally occurring but infrequent events.

Recommendation: NASA should support planning, monitoring, and coordination among the global planetary defense, NEO observing, meteor/bolide, and meteoritics communities to take advantage of the opportunistic events provided by atmospheric entry of NEO materials, and to collect any associated meteorites in order to advance planetary defense objectives.

Importance of Radar Observations for Planetary Defense

Ground-based radar observations of NEOs provide invaluable information for long-term tracking through ultra-precise (1 part in 100 million) measurements of line-of-sight distance and velocities. Radar astrometry of NEOs routinely reduces the orbital uncertainties by several orders of magnitude after only a few minutes of observation, preventing the loss of newly discovered objects and the need for their subsequent rediscovery. This precision also enables radar to accurately predict NEO/Earth encounters on average 400 years into the future. For example,

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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initial orbit determinations of potentially hazardous NEOs (99942) Apophis and 2020 NK1 indicated a significant chance of Earth impact, but radar observations were able to quickly rule out a collision hazard.

Depending on the observing circumstances (e.g., NEO distance, size, and viewing geometry), radar imaging can provide highly detailed characterization of a NEO’s physical properties, including direct size, shape, and rotation state measurements, as well as constraints on near-surface density and roughness, surface geology, and identification of satellites (radar has discovered more than 70 percent of all known NEO satellites). Radar-enabled three-dimensional shape modeling of NEOs has proven reliable and precise; for example, the shape of Bennu determined by OSIRIS-REx was within 2 percent of the radar shape model (Nolan et al. 2019).

The level of characterization afforded by radar provides crucial information for impact mitigation strategies. Because NEO impact energy scales with density, diameter, and velocity, and radar can constrain all of these, planetary radar observations are an important post-discovery characterization technique. As such, radar plays a “unique role” in achieving the tracking and characterization goals of the Brown Act (NRC 2010).

Finding: Ultra-precise radar measurements are the most accurate ground-based means for refining NEO orbits and retiring the risk of future impacts. Furthermore, radar-enabled post-discovery characterization permits direct measurements of size and shape, as well as constrains other physical properties important for impact mitigation strategies.

A vital asset for radar observations was the NASA-funded Planetary Radar Project at the NSF’s Arecibo Observatory in Puerto Rico. The December 2020 loss of Arecibo Observatory’s 305 m radio telescope created an urgent need to recover this critical capability. Currently, the remaining key facility for planetary radar observations is NASA’s Goldstone Solar System Radar (GSSR), specifically the Deep Space Station facilities DSS-14 at X-band and DSS-13 at C-band. GSSR is a fully steerable radar telescope that can deliver more precise line-of-sight distances than could Arecibo. Although GSSR can produce longer observing tracks, Arecibo was 15× more sensitive than GSSR; in a typical year, Arecibo could observe twice as many NEOs as GSSR (Naidu et al. 2016). Thus, Arecibo’s loss has drastically reduced the capability for follow-up radar characterization of NEOs (see also Infrastructure chapter).

Finding: The loss of the Arecibo Observatory planetary radar greatly inhibits the ability to perform follow-up NEO characterization. Existing radar infrastructure can observe only half the asteroids once observable with Arecibo.

Recommendation: NASA and NSF should support studies to develop a plan for ground-based planetary radar capabilities comparable to or exceeding those of the Arecibo Observatory necessary for achieving planetary defense objectives.

The Green Bank Telescope (GBT) can operate as part of a bistatic radar facility where GSSR transmits and the GBT acts as a receiving station. The GBT is considering the addition of a radar transmitter (Bonsall et al. 2020), adding a complementary capability and some redundancy to GSSR. However, even with two active radar observatories, the national planetary radar infrastructure lacks two-fault-tolerant redundancy: a system able to survive two faults, as experienced in 2020 when both Arecibo and GSSR were inoperative owing to klystron failures (Adamo et al. 2020). Furthermore, these facilities are all in the northern hemisphere, leaving a significant gap in follow-up capability of NEOs at southern declinations.

Finding: In order to conduct the required NEO follow-up characterization observations to meet key planetary defense objectives, it would be valuable to expand and extend planetary radar capabilities to obtain coverage over the northern and southern celestial hemispheres.

NASA-NSF Cooperation

Because ground-based planetary radar observations have been conducted at shared-use facilities, in particular at NSF facilities such as Arecibo and the Green Bank Telescope, improved collaboration between NASA and NSF is needed to ensure the nation meets its PD strategic goals. In particular, the National Near-Earth Object Preparedness Strategy and Action Plan requires that NASA and NSF work together to “identify opportunities in existing

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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and planned telescope programs to improve detection and tracking” of NEOs, as well as to identify “opportunities in existing and new telescope programs to enhance characterization of NEO composition and dynamical and physical properties” (NSTC 2018).

Finding: The National Near-Earth Object Preparedness Strategy and Action Plan recommends NASA and NSF work together to support and fund opportunities in existing and planned telescope programs to improve the detection, tracking, and characterization of NEOs. Such efforts would improve characterization of objects with a nonnegligible impact probability and the NEO population as a whole.

Recommendation: As the steward of ground-based observatories with NEO observing capabilities, NSF should support and prioritize critical planetary defense observations of NEOs at its ground-based facilities.

Although NASA and NSF have signed a memorandum of understanding to advance space, Earth, biological, and the physical sciences, closer cooperation is needed to ensure that shared-use facilities can be leveraged effectively to meet PD goals. For example, prior to the collapse of Arecibo’s platform in December 2020, NSF asked NASA to investigate the “root cause of the Auxiliary M4N cable failure” that occurred in August 2020, which ultimately left the facility vulnerable to collapse (Harrigan et al. 2021). However, the agency’s June 2021 report observes that “Additional data and hardware were available to the NSF but were not available to (NASA) for examination” (Harrigan et al. 2021). Thus, a more detailed collaborative agreement that enables direct communication and input between NASA (particularly the PDCO) and NSF for decision-making on shared-use facilities is required.

Finding: NASA and NSF have largely informally cooperated at shared-use facilities, such as Arecibo and GBT, by leveraging their grantees and contractors as intermediaries. A more formalized agreement is required to ensure appropriate collaborations for planetary defense.

While the NSF is tasked to support ground-based observing infrastructure, NASA has been tasked with leading key planetary defense objectives. This leaves ground-based observatories that support PD goals in a nebulous situation in terms of funding. A collaborative agreement between NASA and NSF would enable the effective communication and cooperation required to facilitate critical planetary defense work at existing and future shared-use ground-based facilities. Such a collaboration would also facilitate the development of a ground-based planetary radar facility with planetary defense capabilities comparable to or exceeding those of the Arecibo Observatory planetary radar. Such an endeavor would also require adequate support for the development and maturation of associated radar technologies. If both agencies formally work together, it would also further the nation’s ability to expand the planetary radar infrastructure in the northern hemisphere, as well as its expansion, through partnerships or new facilities, into the southern hemisphere in order to achieve key planetary defense objectives.

Apophis, a Unique Characterization Opportunity in the Coming Decade

The close approach of asteroid (99942) Apophis to Earth in 2029, which will pass within Earth’s belt of geostationary satellites and about 6 Earth radii from the center of Earth, presents an unprecedented opportunity for both planetary science and global planetary defense awareness. Apophis is 370 m in diameter and its flyby will be the closest approach of an asteroid that size ever recorded, making for a truly exceptional event. At closest approach—occurring on April 13, 2029, at 21:46 UT—the asteroid will be visible with the naked eye or binoculars over Europe, Africa, and Western Asia. The flyby will alter Apophis’s heliocentric orbit, transitioning it from an Aten-class to an Apollo-class asteroid (Giorgini et al. 2008). While Apophis remains a potentially hazardous object, any impact with Earth has been ruled out for more than 100 years into the future by recent planetary radar observations (NASA 2021a).

During the flyby, the spin state of Apophis is also expected to undergo a relatively large change owing to Earth’s gravitational forces, which will torque the body (Scheeres et al. 2005). Despite these significant effects, the flyby is not expected to cause widespread shifts of surface or interior material, which would require a flyby a few Earth radii closer than will occur in 2029 (Hirabayashi et al. 2021).

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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The extremely precise Apophis flyby prediction provides an unprecedented opportunity for multiple observatories to coordinate simultaneous pre- and post-encounter observations of the NEO from multiple viewing geometries and at wavelengths from optical to radar. Owing to its inherent interest, Apophis is already a target of opportunity for astronomers, with every apparition leading to significant observations focused on determining its orbit, shape, spin state, and thermal and spectral properties. These have created a precise baseline for the physical state of Apophis prior to its 2029 close approach (Pravec et al. 2014; Brozovic et al. 2018). Precision observations of Apophis leading up to and through its 2029 close approach to Earth offer a unique opportunity to better understand potential changes in surface morphology, track changes in spin state through close approach, and compare pre- and post-flyby spectra to ascertain if the close passage has disturbed space weathered materials on the asteroid’s surface.

Finding: Obtaining the best measurements during Apophis’s close flyby of Earth in 2029 requires a coordinated, international observational response.

Apophis’s close approach has also stimulated discussion of visiting the body through either a flyby or rendezvous spacecraft mission. As one example of a flyby mission, the precise prediction of its 2029 trajectory would enable a spacecraft to be placed on an orbit with apogee beyond the GEO belt with only a small ΔV (i.e., velocity change). The spacecraft could be positioned to take observations of the asteroid as it passes by on either its inbound or outbound leg. This type of flyby space mission would not require the observing spacecraft to leave Earth’s gravitational influence. If equipped with an appropriate imaging suite, the craft could observe Apophis under complementary viewing geometries to ground-based observatories and at much higher resolution.

Requiring significantly greater capability would be a spacecraft rendezvous arriving before Apophis’s close approach to Earth. A rendezvous well in advance of the Earth flyby could map the asteroid in detail before and after its Earth passage (Binzel et al. 2020). Although significant physical changes in the body (except for its spin state) are not expected, this prediction could be tested by scrutinizing the surface for changes in regolith placement and distribution. If the spacecraft could station-keep with Apophis through the close approach (Scheeres 2019), it would provide novel opportunities for bi-static radar tomography around closest approach, utilizing ground and space-based radio telescopes (Cheng et al. 2020).

A spacecraft arriving at Apophis after its close Earth approach could still carry out significant and scientifically noteworthy observations. Apophis has had numerous close approaches to Earth, which may explain its current complex rotation state (Pravec et al. 2014). A rendezvous mission could map the asteroid in detail to determine if there are signatures of past, closer Earth flybys that may have caused more dramatic changes to the body’s morphology. In addition, precise determination and monitoring of Apophis’s highly excited spin state over an extended period of time could constrain, or possibly detect, the expected energy dissipation experienced by such a tumbling body.

Finding: The Apophis flyby of Earth creates an opportunity to observe a potentially hazardous asteroid via a coordinated ground-based campaign, potentially supplemented via space-based observations from flyby or rendezvous missions.

A flyby mission to Apophis could be carried out with a SIMPLEx-class mission. A rendezvous mission that could acquire more detailed observations would need a spacecraft with greater ΔV capability, likely requiring a Discovery or medium-class mission. Of particular utility would be a spacecraft capable of performing detailed mapping and characterization, followed by long-term monitoring. It is thus significant to note the effect of solar illumination on the Apophis orbit and spin state to better constrain its future impact potential. These measurements would have overlapping scientific and planetary defense implications. One option would be the OSIRIS-REx spacecraft, which could a rendezvous with Apophis following the delivery of its Bennu sample canister to Earth, although it would only encounter Apophis shortly after its 2029 Earth flyby (Dellaguistina et al. 2021).

Recommendation: NASA should study all relevant observing opportunities surrounding the unique Apophis encounter, using both ground and space-based assets. To maximize the scientific and planetary defense return, NASA should develop plans for making the best use of these identified assets during the Apophis encounter and support international cooperation in carrying out these valuable observations.

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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NEO MODELING, PREDICTION, AND INFORMATION INTEGRATON

Goal 2 of the National Near-Earth Object Preparedness Strategy and Action Plan focuses on advancement of modeling and analysis capabilities, including the assessment of impact probabilities, location, consequences, and mitigation options. The initial conditions and attendant uncertainties for these modeling efforts depend heavily on the detection, tracking, and characterization of NEOs (see NEO Detection, Tracking, and Characterization section). Corresponding modeling expertise is distributed among many communities, including NASA centers, DOE’s National Nuclear Security Administration (NNSA) laboratories, the National Oceanic and Atmospheric Administration (NOAA), and academic institutions. Hence, the first objective to meet Goal 2 is to “establish an interagency NEO impact modeling group” (NSTC 2018). The NEO Action Plan Modeling Working Group (MWG) was established in November 2019. The second and third objectives, “Establish an integrated suite of computational tools for modeling NEO impact risks and mitigation techniques” and “Exercise, evaluate, and continually improve modeling and analysis capabilities” are currently in development within the MWG.

NEO Consequence Assessment Modeling

Consequence assessment calculations can help minimize loss of life and property when an incoming NEO cannot be deflected by advising appropriate emergency responses, such as evacuations, safety precautions, and infrastructure protection. For a smaller NEO, these assessment calculations may predict details of atmospheric entry and breakup, and accompanying ground blast effects. For larger NEOs, which deliver most of their energy to the surface, assessment calculations may address ocean wave generation and subsequent coastal flooding, or on land, the expected fireball radiation and blast damage. NEO impact casualties can be caused by blast, ignition, seismic, cratering, or fireball effects. Longer term atmospheric perturbations can be estimated by handing off to a global circulation model, tracking the effects of ejected volatiles and dust.

The increase since 2013 in expertise and resources aimed at impact consequence modeling by NASA and its collaborators has significantly advanced U.S. capabilities to simulate NEO impact effects. That investment has also yielded a more nuanced yet quantitative understanding of impact risk and attendant uncertainties. The Asteroid Threat Assessment Project (ATAP) established in 2015 at NASA Ames Research Center has taken a multidisciplinary approach in developing higher-fidelity airburst modeling and probabilistic risk assessment approaches (Mathias et al. 2017).

An early focus of ATAP has been on smaller NEOs, such as those causing the Tunguska or Chelyabinsk events, which are statistically the most likely damaging objects to strike Earth. These types of events are especially demanding to simulate because these objects deposit most of their energy into the atmosphere and their break-up is sensitive to a bolide’s specific material properties. Future work will address impacts from NEOs in the hundreds-of-meters size range, where potential damage transitions from regional to global scales. Even though the probability of these larger events is lower than Chelyabinsk-like airbursts, the maximum probability for an impact with global cataclysmic effects is calculated to be in the 500 to 700 m size range (Reinhardt et al. 2016). Quantification of the damage from these large impacts requires engagement with experts on agriculture, supply chains, and other downstream effects.

Another critical area of impact consequences study are oceanic impacts and their generation of large water waves. This multiphysics problem requires a hand-off between a shock-physics code to an elastic-wave code that can simulate wave propagation over hundreds of kilometers and many hours (Ezzedine and Miller 2014). Coastal inundation can be further simulated by codes designed to handle fluid-structure interactions.

The Second International Workshop on Asteroid Threat Assessment (2016) focused on water impact modeling, with participation from many of the world’s experts (Boslough et al. 2016). Significant differences in scientific judgement of the ocean impact risk remain within the water impact modeling community, and will require continued collaborative engagement, including transparent presentation of the methods, assumptions, and limitations of each numerical approach necessary to make progress on ocean impact modeling and risk quantification of water-wave effects.

Finding: Significant differences in technical results and methodology remain within the consequence assessment simulation community. Although a diversity of approaches can provide helpful perspectives on impact problems,

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
×

more collaborative work is needed to understand the sources of these differences and to refine simulation capabilities.

The interagency NEO MWG, led by the NASA Ames Research Center, has begun working through representative impact studies. The MWG’s multidisciplinary activities include consequence assessment, mitigation, and information integration, with the goal of providing rapid assessments to enable decision making and to enhance public awareness in a NEO emergency. While early efforts have focused on code comparisons and probabilistic risk assessments for specific problems of interest, future work will include validation and verification of computational tools against experimental and observational data.

Ongoing support of NEO MWG is critical for meeting the next decade’s planetary defense objectives. The MWG currently exists as an “unfunded mandate”—without agency funding—yet is still expected to hold regular meetings to address strategic objectives for NEO modeling, predictions, and information integration. Given the national security value of rapid, trusted, and centralized assessments alongside the ever-present risk of misinformation during a NEO emergency, reliable funding for the MWG is a wise investment.

Finding: NASA’s leadership is vital for integrating consequence assessment calculations across the diverse communities capable of this work, particularly when their risk assessments differ. NASA-sponsored workshops and meetings provide an important venue for technical discussions of the assumptions and limitations underpinning various numerical approaches.

NEO Mitigation Modeling

If an asteroid or comet is calculated to be on a likely collision course with Earth, several mitigation techniques are available. Ideally, the NEO will be detected with sufficient warning time to conduct a deflection, in which a modest change in velocity (ΔV), is applied, either slowing down or speeding up the object in its orbit. The integrated change in position over many years will result in the NEO missing Earth entirely. In a successful deflection, the bulk of the asteroid stays intact (i.e., it is not fragmented) and misses Earth. Deflections can be applied using impulsive or “fast push” methods like a kinetic impactor or a standoff nuclear explosion,4 or via “slow push/pull” methods such as a gravity tractor or ion beam deflection (see NEO Deflection and Disruption Missions section below). For the most challenging scenarios, when warning times are less than a few decades, the kinetic and nuclear approaches are assessed to be the most effective (NRC 2010).

As warning time decreases, the required ΔV to achieve a successful deflection increases. Although nuclear standoff deflections can deliver momentum changes in a mass-efficient payload, once the required deflection ΔV increases to a significant fraction of the NEO’s escape velocity, any attempted deflection will risk unintentional disruption of the body, which could exacerbate the threat. Shorter warning times (less than a few years to a decade, depending on the details of the NEO) may require a disruption mission to prevent an Earth impact (Dearborn and Miller 2014). Disruption can be carried out by detonating a nuclear device near the NEO. Although a buried burst would couple a larger fraction of its energy to the body, burial would require increased mission complexity. A close-proximity standoff burst simplifies execution significantly, and the energy coupled into the NEO is sufficient to generate a strong shock wave which shatters the body and robustly disperses the fragments (King et al. 2021).

Both kinetic and nuclear deflection require numerical simulations to assess NEO response to these momentum impulses. In low-gravity environments, understanding late-time crater formation from a kinetic impactor deflection may also require discrete element method modeling approaches. Nuclear deflection or disruption simulation requires radiation transport and hydrodynamic modeling capabilities. Propagating the motion of fragments forward in time to ensure they miss Earth typically requires an N-body gravity code. All of these simulations are collectively represented by the term “multiphysics.”

Multiphysics simulations of NEO response to deflection/disruption efforts have matured significantly over the

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4 Discussion of the political, policy, and treaty implications of the deployment and use of nuclear explosive devices in space is beyond the scope of this report.

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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past decade, owing to advances in high performance computing, improved understanding of NEO initial conditions from spacecraft data, and focused research. Improvements include more realistic modeling of rubble pile structures, variable distributions of macro- and microporosity, improved strength and damage models representing asteroidal material, and simulation of a wider range of three-dimensional NEO shapes. Further, new and rapidly developing computational capabilities are now regularly incorporated into both kinetic and nuclear deflection modeling.

Characterization data returned from small body missions like OSIRIS-REx (Lauretta et al. 2019) and Hayabusa2 (Watanabe et al. 2019), have improved the fidelity of simulations. However, mission results always challenge modeling techniques in surprising new ways. For example, the small carry-on impactor experiment on Hayabusa2 produced a crater size on asteroid Ryugu that exceeded predictions, suggesting that gravity, not strength, controlled crater formation (Arakawa et al. 2020). In the coming decade, additional insights from DART and other missions will further refine models, if support for such studies is available.

Mitigation modeling to support end-to-end NEO impact case studies has been a central part of interagency work between NASA and NNSA, such as the ongoing collaboration between NASA Goddard Space Flight Center and Lawrence Livermore National Laboratory, Los Alamos National Laboratory, and Sandia National Laboratories. These case studies are designed to stress the tools and methods used to recommend an optimal mitigation approach. An important element of these studies is the coupling of multiphysics simulations with mission design options for a given NEO. The efficacy and risks associated with a given mitigation depend on the details of an NEO’s orbit, the warning time, and derived launch opportunities (Barbee et al. 2017). Because this problem is so scenario-dependent, end-to-end case studies are a critical tool for preparedness. This collaboration has also enabled thorough code comparison efforts for kinetic and nuclear mitigation modeling (Dearborn et al. 2020).

There are scenarios in which a kinetic impactor would be unable to divert a NEO from Earth impact. In these cases, simulations and data from decades of nuclear tests indicate that a standoff nuclear explosion could successfully deflect or disrupt the NEO. Accurate modeling of nuclear deflection or disruption requires an understanding of nuclear device output, radiation transport, hydrodynamics, and material properties at extreme temperatures and pressures. The NNSA laboratories are positioned to provide this expertise, along with the computational resources required for large three-dimensional simulations.

Finding: The expertise to model nuclear mitigation techniques resides at DOE’s NNSA laboratories. Preparation for short-warning-time and/or larger-diameter asteroid threats requires effective partnership and open communication between NASA and the NNSA laboratories through joint activities under their interagency agreement.

Deflection and disruption modeling is also a key component of the NEO MWG. Similar to consequence assessment calculations to-date within the MWG, early work has focused on code comparisons for specific scenarios. Addressing the Goal 2 objective “Develop and validate a suite of computer simulation tools for assessing the outcome of deflection or disruption techniques applied to a NEO,” will require future work focusing on validation of tools against available experimental data.

The MWG work is addressing the Goal 2 objective, “Assess the sensitivities of these models to uncertainties in NEO dynamical and physical properties.” Some of this work is ongoing as part of DART impact preparations and general research on sensitivities and uncertainty quantification for kinetic and nuclear deflection. The MWG will play a key role in pulling state-of-the-art approaches together, to enable risk-informed decisions for asteroid mitigation. In particular, subject matter experts for both modeling and mission design need to work together to resolve difficult NEO scenarios, such as when it is unclear whether a kinetic impact approach may be sufficient, or when a required deflection velocity may inadvertently disrupt an asteroid. Future efforts may also benefit from closer partnership with mission design and technology development groups, who are addressing Goal 3.

Although simulations are an essential tool for predicting NEO response to impulsive deflection or disruption methods, validation of numerical simulations against experimental data provides an important measure of confidence for future mitigation missions. Validation can also illuminate sources of uncertainty (e.g., material model or numerical method shortcomings). For example, important validation of multiphysics X-ray ablation calculations can be achieved at laser facilities like the National Ignition Facility and the University of Rochester’s OMEGA laser. Experiments also frequently reveal fruitful new research directions. Without access to key experimental facilities, including NASA’s Ames Vertical Gun Range (AVGR) and NASA’s Johnson Space Center’s Experimental Impact

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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Laboratory (EIL), many planetary defense questions will remain unaddressed. Additional, complementary insights can be gained from field studies of Earth impact structures.

Over the past decade, R&A support for experimental impact work at NASA facilities has become more difficult to secure. Programs such as Yearly Opportunities for Research in Planetary Defense play an increasingly important role in supporting the continued operation of these laboratories and extraction of new impact results.

Finding: Although multiphysics modeling of impact events has advanced over the past decade, continued experimental validation of material models and numerical approaches is necessary. Natural experiments like the Chelyabinsk and Comet Shoemaker-Levy 9 collisions are relatively rare opportunities; in contrast, timely laboratory-scale experiments can offer fundamental insights and pathways to increase confidence in multiphysics modeling.

Although funding of numerical studies can be seen as logistically and financially simpler than funding of experiments, confidence in multiphysics simulations depends on validation against reality. The accessibility of NASA’s AVGR and EIL facilities to qualified researchers is an essential piece of a healthy and effective planetary defense program.

Finding: Establishing a credible and timely national capability for both consequence assessment and mitigation modeling requires enhanced support for NEO Modeling Working Group activities.

Recommendation: NASA should increase levels of support for multiphysics modeling and laboratory experiments necessary to meet the Goal 2 objectives described in the National Near-Earth Object Preparedness Strategy and Action Plan.

Information Integration for Planetary Defense Simulations

As emphasized in the NSTC 2018 document, the MWG will need to stand up a capability to provide verified data to decision-makers, using “an integrated suite of computational tools for modeling NEO impact risks and mitigation techniques.” Although modeling tools for NEO impact risks, consequences, and mitigation techniques currently exist, they are widely distributed between agencies and institutions, and interconnections between the various codes, models, and results are just beginning to form. Much of this nascent activity takes place within the MWG.

One way to provide verified and timely results to decision makers and the general public in advance of a real emergency is to build a national planetary defense assessment pipeline (Stickle et al. 2020). This pipeline requires a framework for sharing of characterization data, modeling parameters, validation data, calculation results, and analysis tools. Such a pipeline could enable version control, consistent updating of characterization data, sharing of geometry and material property files common to multiple NEO case studies, probabilistic treatment of risk, robust uncertainty quantification, and simpler hand-offs between tools designed to tackle each phase of an impact problem. The pipeline should also connect to international partners to enable common data exchange and formats, communication standards, modeling tools, and so on, while serving as a repository for a broad range of impact effects and mitigation calculation results.

Even with such a pipeline, it is important to obtain “multiple votes” on problems of interest by using different numerical approaches. Strengthened by diverse users and disciplines, the pipeline would streamline the process for validating multiple codes against common sets of benchmarking data, critical to developing a credible and responsive PD capability. The pipeline could also enable faster turnarounds for assessing model sensitivities to uncertainties in NEO properties.

Finding: Integrated modeling assessment across all aspects of planetary defense, including characterization data, modeling parameters, validation data, calculation results, and analysis tools, using uncertainty-aware methods, is needed to establish an operations-ready suite of computational tools for evaluating NEO impact risks and mitigation techniques.

Recommendation: To achieve the modeling, prediction, and information integration objectives listed under Goal 2, NASA should allocate resources for the establishment of a planetary defense modeling pipeline, including support for collaboration between modeling teams and software developers to establish initial requirements.

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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NEO DEFLECTION AND DISRUPTION MISSIONS

One of the overarching strategic goals of the National Near-Earth Object Preparedness Strategy and Action Plan is developing capabilities for NEO impact prevention. Goal 3 of the action plan outlines the activities that NASA will lead to strengthen the U.S. response to NEO impacts, and enable activities to prevent or minimize the damage inflicted by future, NEO-caused natural disasters. Specifically, the emphasis for NASA is to focus on the development and design of rapid reconnaissance mission technologies for NEO characterization, and deflection/disruption mission technologies for NEO mitigation.

The operational threat spectrum also includes potential impacts from long-period comets (e.g., C/Hale-Bopp) and inter-stellar objects (ISOs), which allow little warning time for mitigation efforts. In both cases, the statistical probability of Earth impact by a comet or ISO (e.g., the recently discovered 1I/Oumuamua and 2I/Borisov) is much lower (~1 percent) than from NEOs (NASA 2017). Thus, for the next decade, PD efforts focused on NEO impact prevention are the priority. However, as technologies continue to improve, defense capabilities from ISOs and long-period comets will also need to be developed and matured. Studies of rapid response mission architectures may be useful for helping identify such capabilities for future development (see the Planetary Defense Rapid Mission Response Strategies section).

Planetary Defense Missions Within NASA

NEO characterization and mitigation efforts are essential to understand the range of potential impact scenarios posed by NEOs approximately 50 m in diameter and larger, the size of object determined to potentially invoke spacecraft mission activities in the Report on Near-Earth Object Impact Threat Emergency Protocols (NSTC 2021). Most NEOs of this size range have yet to be discovered, and little is known about their dynamical and physical characteristics. Knowledge of orbits, masses, sizes, and other physical attributes of these NEOs will help bound the range of dynamical and physical characteristics to be considered in formulating appropriate NEO mitigation options. Some of this information is provided by ongoing surveys and will be supplemented by more capable NEO search facilities becoming operational in the mid-2020s. However, much of the required information can only be collected in situ via spacecraft missions dedicated to high-priority PD objectives.

NASA has significant scientific expertise and institutional knowledge regarding NEOs that are applicable to PD characterization and mitigation objectives. The agency also has detailed experience developing and operating spacecraft missions to investigate NEOs. Thus, NASA has been identified by the U.S. government’s National Near-Earth Object Preparedness Strategy and Action Plan (NSTC 2018) as best suited to lead the development, testing, and flight of technology demonstration missions aimed at proving NEO mitigation techniques. Such missions are, by definition, not focused on science return, but rather utilize NASA’s experience and expertise to accomplish high-priority PD objectives. Although scientific exploration of the solar system and PD share common measurements and spacecraft implementations, missions focused on accomplishing PD objectives are a valid priority for NASA.

Finding: The recommendation of the 2019 NASEM report Finding Hazardous Asteroids Using Infrared and Visible Wavelength Telescopes remains valid and important to follow for the next decade and beyond: “Missions meeting high-priority planetary defense objectives should not be required to compete against missions meeting high-priority science objectives.”

Double Asteroid Redirection Test Mission

NASA’s DART mission is scheduled to impact the 160 m asteroid Dimorphos in 2022, demonstrating kinetic impact technology as one approach to accomplish asteroid deflection. ESA’s Hera mission is scheduled to rendezvous with the Didymos-Dimorphos system in 2026, providing further insight into the results of DART’s kinetic impact demonstration. NASA’s DART mission is an essential first test of asteroid deflection technology and will significantly increase U.S. and international preparedness for future NEO impacts. However, given the diversity of possible NEO characteristics, this kinetic impact demonstration is simply the initial step in expanding NASA’s abilities to divert a threatening object. NASA and the extended PD community need to take advantage

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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of the DART results and apply them to broader mitigation technology efforts to achieve an effective and versatile PD capability. Similarly, the launch of NASA’s NEO Surveyor in 2026 will greatly expand our knowledge of the NEO population over the next decade, which will help inform the PD activities that follow.

Finding: Owing to the diversity of possible NEO threats, including variation across individual object characteristics and differences in warning time before Earth impact, significant deflection technology questions will remain even after a full analysis of a successful DART experiment in 2022.

Finding: Sustained investment in planetary defense mission technology and development of additional demonstration missions beyond DART would enable NASA to accomplish critical planetary defense characterization and mitigation objectives for a variety of impact scenarios and build on the lessons learned from the DART mission.

Next Steps for Planetary Defense Demonstration Missions

The committee commissioned a rapid mission architecture (RMA) study to examine a range of spacecraft concepts that would adequately address the needs for PD technology demonstration missions focused on NEO characterization and mitigation objectives (NASA 2021c). The goal of the RMA study was to identify and prioritize PD demonstration missions to be flown in the upcoming decade. More than 30 representative demonstration missions were examined that included a variety of characterization and mitigation objectives designed to advance development of PD mitigation capabilities. This section summarizes the main results of the RMA study, while the RMA study report provides many more details.

The RMA study focused on missions that demonstrated critical techniques for risk reduction, operational readiness, and expanding the knowledge base of NEO characteristics. The study assumed the desired mission cost to be <$500 million (including the launch vehicle and operations but excluding foreign contributions). Other assumptions included the successful launch of the DART and Hera missions to Didymos, the successful launch and operation of NEO Surveyor, completion of VCRO, and the continued operation of ground-based NEO discovery assets (e.g., Catalina Sky Survey and Pan-STARRS).

Two basic mission types were examined: first, characterization missions, designed to obtain key information about the dynamical and physical characteristics of NEOs necessary to inform mitigation approaches; and second, mitigation missions, which would demonstrate mitigation technologies and improve operational readiness to prevent a NEO impact.

Both mission types would collect information designed to fulfill planetary defense objectives, particularly to inform the development and implementation of mitigation strategies and techniques. Significantly, the RMA study concluded that increased detection and characterization of the NEO population is critical to reducing risks and ensuring successful mitigation, given the diversity of physical characteristics among these objects. The study also recognized that the most likely object to pose an impact risk was from the undiscovered population of objects greater than approximately 50 m in diameter.

Data from ground-based planetary radars, ground-based telescopes, and the few in situ missions conducted to date (e.g., NEAR, Hayabusa, Hayabusa2, and OSIRIS-REx) suggest that the NEO population is very heterogeneous, varying from unconsolidated “rubble piles” to heavily fractured bodies with some degree of physical integrity. Knowing a NEO’s characteristics is crucial because the efficacy of any deflection method depends on the body’s mass, cohesiveness, and associated physical properties. For example, an intended deflection may disrupt a loosely bound NEO into multiple objects, and inadvertently increase the probability of impact (albeit with smaller pieces).

In addition, it is estimated that approximately 15 percent of the NEO population are binary systems—that is, they contain two gravitationally bound objects (Pravec et al. 2006)—further complicating any mitigation scenario. Without a sufficiently broad data base of NEO characteristics, it would be difficult to predict the properties of a newly discovered threat without actually observing the object in situ. The best strategy is a two-step process: first, characterization; and second, mitigation efforts appropriate for that specific NEO and its attendant warning time.

Finding: There is much to be learned about the physical characteristics of the NEO population. Only a handful of NEOs have been observed in situ and there are many unknowns concerning the range of physical properties that may be relevant for planetary defense. In addition, smaller NEOs (>50 m in diameter) are challenging to detect and

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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characterize via ground-based methods, and represent the least understood, but statistically the most likely subset of the NEO population to require mitigation actions.

For NEOs with short warning times, rapid characterization may be required in order to implement appropriate mitigation measures. A rendezvous reconnaissance mission would be preferred because it could provide critical knowledge of the NEO’s physical properties and monitor it post-mitigation. But if the warning time is short, and there is not enough time to implement a rendezvous, a fast flyby mission would be highly useful to provide at least rough information on the object’s key properties and to refine impact probability estimates. Note that in most instances, flyby opportunities are more prevalent than rendezvous mission opportunities to the same NEO. However, such flyby reconnaissance missions are challenging given the high encounter velocities involved, limiting the time available for collecting detailed characterization data.

Finding: Prior characterization of a hazardous NEO via an in situ reconnaissance mission is advisable to determine its physical characteristics and to develop an appropriate mitigation response based on the available warning time. Although rendezvous missions are preferred, fast flyby missions may be required to obtain timely characterization data for short warning time scenarios.

Recommendation: The highest priority planetary defense demonstration mission to follow DART and NEO Surveyor should be a rapid-response, flyby reconnaissance mission targeted to a challenging NEO, representative of the population (~50–100 m in diameter) of objects posing the highest probability of a destructive Earth impact. Such a mission should assess the capabilities and limitations of flyby characterization methods to better prepare for a short-warning-time NEO threat.

For any given impact scenario, selection of appropriate mitigation technologies depends on knowledge of the NEO’s physical characteristics, precise trajectory, and available warning time. The choice of mitigation technology requires a balance between generating an adequate amount of deflection/disruption without causing deleterious results (e.g., unwanted disruption or ineffective deflection). Several mitigation techniques have been proposed based on current technologies. The most technically feasible of these are, in no particular order, kinetic impact, nuclear, ion beam deflection, and gravity tractor. All have specific advantages and disadvantages for NEO mitigation and are briefly described below:

Kinetic Impact: This is a relatively straightforward, high-impulse technique that transfers momentum to a NEO, altering its trajectory via a direct hypervelocity impact. However, the effectiveness of the deflection varies with the amount of momentum enhancement generated by the post-impact ejection of material from the NEO. The amount and direction of material ejected is dependent on the physical properties of the NEO and the intercept geometry. Kinetic impact is not effective against larger, more massive NEOs, and for some smaller objects may result in unwanted disruption with subsequent unpredictable outcomes (Figure 18-4). However, kinetic impact may be useful for deflecting NEOs with short warning times, and in cases (very short warning times and/or relatively small NEOs) where deliberate disruption may be desired (see Figure 18-4).

Nuclear: This high-impulse method relies on the radiation from a detonated nuclear explosive device (NED) to deliver an impulse that deflects the NEO and is effective over a wide range of NEO physical characteristics. The NED is triggered near the object’s surface; the explosion generates X-rays and neutrons which vaporize the exposed surface layer of the NEO. The burst of vaporized material imparts momentum to the NEO, altering its orbit. Because the detonation of the NED is timed to occur at an optimum distance from the NEO, the NED is best deployed via a rendezvous, which also enables precise directional control of the deflection. However, in cases of short warning times, hypervelocity intercepts could be employed, but attaining the optimum stand-off distance requires precise detonation timing. Nuclear explosives can transfer significant momentum instantaneously, which may be required to deflect large NEOs. When very short warning times make deflection impractical, nuclear disruption of the object may be the only option to prevent Earth impact.

Figure 18-4 demonstrates the effective parameter space of the various mitigation techniques with respect to NEO diameter and warning time. High impulse techniques such as kinetic impact and nuclear are broadly useful in situations with short warning times and/or when disruption of a NEO is desired. However, nuclear methods may be the only suitable option for deflecting large NEOs. Slow, controlled methods such as ion beam deflection

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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Image
FIGURE 18-4 Numerical results from simulating deflection capabilities of various techniques across a variety of asteroid sizes following fifteen different Earth-impacting orbits. Kinetic impact (KI) techniques (yellow) largely overlap the region where ion beam deflection (IBD) is effective (green). If the warning time is very short and/or the asteroid relatively small, deliberate KI disruption may be the only viable nonnuclear technique (yellow dots). Gravity Tractor not shown. See RMA study for additional details. Note that Civil Defense is a mitigation technique designed to reduce the number of casualties in the projected impact affected area. This can be achieved either through evacuation of the population or by issuing warnings to shelter-in-place and seek cover. SOURCE: Modified from National Research Council, 2010, Defending Planet Earth, Washington, DC: The National Academies Press.

and gravity tractor are more applicable given longer warning times and may offer sufficient control to deflect a NEO in an optimal direction.

Ion Beam Deflection: Deflection via use of ions discharged from solar electric propulsion engines is a promising technique. A spacecraft near a NEO delivers momentum by spraying its surface with thruster-generated ions, enabling a slow and controlled deflection. Optimal deflection requires that the spacecraft rendezvous with the NEO and be able to operate autonomously in its vicinity for an extended period. Hence, this technique is useful for objects with longer warning times. The rendezvous offers the potential for detailed NEO characterization and monitoring, while offering more time to reconfigure from critical flight system faults than during a high-speed flyby/intercept. Rendezvous is also more capable of dealing with unexpected NEO physical characteristics. Ion beam deflection is not effective against large NEOs and may not be suitable for deflection of NEOs with natural satellites.

Gravity Tractor: This technique uses the mutual gravitational attraction between the spacecraft and the NEO to slowly alter the latter’s trajectory. This technique is similar to ion beam deflection, offering fine deflectional control and extended opportunities for detailed characterization and monitoring. To be effective, however, it is necessary for the spacecraft to perform close, autonomous, and extended proximity operations to station keep at a predetermined distance from the NEO. The required guidance, navigation, and control capabilities are challenging, and the technique may not be suitable for NEOs with irregular shapes, chaotic rotations, or natural satellites. It requires long warning times and is also less tolerant of technical system faults and unexpected NEO physical characteristics.

The gravity tractor technique is not shown on Figure 18-4 since its performance overlaps regions of the ion beam deflection parameter space; its lower efficiency requires longer operation times. Note the overlap in the

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
×

parameter space between the kinetic impact and ion beam deflection techniques. Please see the RMA study for additional details (NASA 2021c).

Finding: Several proposed planetary defense mitigation techniques are technically feasible, but none has been demonstrated in practice. Each has its own advantages and disadvantages depending on the physical characteristics of the NEO and the available warning time to impact.

Recommendation: Following a rapid-response, flyby reconnaissance mission demonstration, the next highest priority planetary defense mission would be a characterization and/or mitigation mission.

The suggested candidates are, in no particular order:

  • A characterization tour mission to exercise characterization capabilities and gain characterization information required for future deflection/disruption missions across a range of NEO targets.
  • A kinetic impact mission on a smaller NEO and at a higher closing speed than DART to acquire experience needed for more challenging mitigation missions.
  • A slow-push/pull mitigation demonstration, such as ion beam deflection, to develop several different technologies that may be employed against future hazardous objects.

Which technique to be demonstrated will depend on an assessment of the current state of knowledge. The RMA study report contains additional engineering details to demonstrate that all these mission options are technically feasible at a total mission cost of ~$500 million (FY 2025) or less; a characterization tour may be feasible in the $200 million to $430 million (FY 2025) range. Additionally, the RMA study demonstrates that certain mission architectures have the potential to accomplish multiple planetary defense mission objectives at the same target(s) or with the same spacecraft. In particular, mission concepts that share fundamentally similar flyby/intercept or rendezvous implementations could be effectively and efficiently designed with capabilities that test both characterization and mitigation technologies at a relevant NEO.

Finding: Mission concepts that address multiple characterization and mitigation objectives in future planetary defense technology demonstrations would potentially maximize results.

Given the large number of important PD objectives that need to be addressed and are technically and financially feasible, funding and launching a series of modest missions demonstrating capabilities and technologies essential to implementing an effective planetary defense against a range of possible NEO impact scenarios is both important and achievable within the next decade. The RMA study provided cost estimates for the studied missions. Appraisal of those costs shows that it is realistic for the DART mission to be completed in 2023, NEO Surveyor to launch in 2026, and at least one new PD focused mission (a rapid-response, flyby reconnaissance mission) to start prior to the end of 2032. An additional new start from the above list of characterization and mitigation missions may also be possible, depending on the costs of the new missions. A regular cadence of launches will advance key PD technologies while also regularly exercising required capabilities.

Recommendation: NASA’s Planetary Defense Coordination Office should be funded at adequate levels to conduct a robust program of necessary planetary defense-related activities, technologies, and demonstration missions launching on a regular cadence.

NASA has a history of soliciting spacecraft mission concepts to increase knowledge of the solar system, explore new destinations, and ensure the safety of humanity. Such solicitations have supported open competition, which has been instrumental in providing a pool of promising cost-effective mission concepts for future development.

Finding: Making planetary defense demonstration mission opportunities, with well-defined objectives, open to industry, academia, U.S. government institutions, and NASA centers would ensure that the most promising and cost-effective concepts are considered and developed.

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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Promising Planetary Defense Mission Technologies

There are several promising technologies whose pursuit would help attain specific goals of PD characterization and mitigation missions. These technologies can be broadly grouped into several categories and are utilized in mission architectures for flyby/intercept and rendezvous PD concepts. These technology categories include: NEO reconnaissance; impulsive mitigation; slow-push mitigation; and, guidance, navigation, and control (GNC) systems.

  • NEO Reconnaissance Technologies: Development of these technologies is needed to make necessary measurements during either flyby or rendezvous missions, and are focused on determining key NEO physical characteristics that will inform the required subsequent mitigation efforts given the available warning time. Such technologies include spacecraft systems (e.g., high-speed gimbals) to enable instruments to track NEOs during fast flybys and instruments/systems with the ability to determine the mass of the NEO during such high-speed encounters. Additional development of technologies that involve landed or deployed instruments to determine geophysical and geotechnical properties (e.g., strength, cohesion, and internal structure) would be useful to further enhance NEO characterization objectives and could be used during either flyby or rendezvous missions.
  • Impulsive Mitigation Technologies: Technologies for impulsive mitigation involve those needed for kinetic impact and nuclear methods. Such missions would benefit from improved visible and thermal-infrared imaging systems to help with targeting NEOs during hypervelocity intercepts. In addition, nuclear mitigation methods would benefit from improvement in sensor technologies (e.g., radar ranging systems) that at very high approach velocities enable accurate and reliable triggering of a NED at a precise time and distance from a NEO’s surface. Such sensors would ensure that detonation occurs at the optimum location for deflection or disruption. A radar ranging system, for example, can be flown as a ride-along payload on a future kinetic impact demonstration. Note that testing of a nuclear device in space is neither needed nor advised.
  • Slow-Push/Pull Mitigation Technologies: Slow-push/pull ion beam deflection and gravity tractor techniques would both benefit from improved solar electric propulsion (SEP) technologies. Both methods rely on low-thrust, high efficiency propulsion systems to deflect a NEO during precise station keeping maneuvers, ideally suited to SEP. Hence development of high-power, long-life SEP systems is an enabling technology for these concepts. Ion beam deflection would also benefit from new thruster technologies which would better focus the ion beam, minimizing the divergence angle from the thruster. This would reduce ion plume losses and increase the impulse imparted to the NEO.
  • Guidance, Navigation, and Control Technologies: Advancement of these technologies has the broadest impact for all future characterization and mitigation techniques and are applicable to both flyby/intercept and rendezvous missions. Improvement in precision terminal GNC algorithms and associated spacecraft systems for hypervelocity flybys/intercepts would enable accurate and reliable targeting of small NEOs (~50 m and larger) at closure speeds of up to ~15–20 km/s. These improvements would enable both rapid reconnaissance NEO flyby characterization missions, and kinetic impact/nuclear mitigation missions to reach their desired targets and achieve their PD objectives. Further improvement of autonomous GNC systems and associated algorithms for extended, long-duration proximity operations would enable both ion beam deflection and gravity tractor mitigation methods. These techniques require autonomous, real-time sensing of the spacecraft’s relative position and attitude with respect to the NEO for appropriate throttling of the SEP thrusters.

In addition to the technology categories discussed above, overall advancement in instrument designs would be beneficial to increase capability and reduce mission costs. Similarly, continued investment in smaller spacecraft and maturation of flight systems will incrementally help reduce costs for deep space missions with PD objectives.

Finding: Impact scenarios may vary widely given the diverse range of NEO physical characteristics and potential warning times. As such, it is important to have several mature technologies available and optimized for possible planetary defense characterization and mitigation situations before they arise.

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
×

Finding: Promising new technologies for both characterization and mitigation demonstration missions could be tested on relevant NEOs. Technologies such as those for guidance, navigation, and control (GNC) and sensor instrumentation appropriate for hypervelocity flybys/intercepts, as well as autonomous GNC for long-duration proximity rendezvous operations at NEOs, are fundamentally important for planetary defense objectives.

Planetary Defense Rapid Mission Response Strategies

The first goal of PD is to develop the means to detect and characterize a hazardous NEO with enough warning time to implement an effective response. The combined results from current and upcoming NEO survey systems will likely provide adequate warning for a majority of potentially hazardous NEOs well in advance of an impact. However, assuming that a newly discovered object among the NEO population could pose an impact risk, it would be prudent to consider scenarios where warning times may be relatively short (~5–7 years), and thus create challenging situations for characterization and/or mitigation. Current spacecraft development processes, hardware integration practices, and launch vehicle infrastructure are not optimized for such situations, and often require long lead times (e.g., more than 4 years) for successful mission launch and reliable systems operation. Additional time is required for spacecraft transit and implementation/assessment of necessary activities at the NEO. Hence a comprehensive PD response requires the capability to rapidly assess a potentially hazardous object and take appropriate action in the case of short impact warning times.

There are several rapid mission strategies that could be evaluated in development of PD demonstration missions. These strategies have been discussed in the survey’s RMA study (NASA 2021c) and include such options as the following:

  • Rolling Phase A/B Design, which would enable an advanced starting point for development and construction of a mission;
  • Build on Demand, which designates that a spacecraft be built as fast as possible via a streamlined process;
  • Repurposed/Commandeered, wherein a mission could repurpose/commandeer parts, components, or possibly entire spacecraft;
  • Build to Inventory, wherein entire spacecraft can be held ready in time of need, or modular components compatible for rapid assembly are placed on standby status; and,
  • Store in Space, which places assets on station, ready to be deployed on very short notice (e.g., GOES weather satellites).

All these proposed rapid response strategies have advantages and disadvantages in addressing key PD mission aspects. Of particular importance for evaluating each of the rapid response strategies is consideration of the specific NEO hazard, the spacecraft size and complexity (e.g., SmallSats to medium-class missions), the available time for integration and testing, the required mission response/deployment time, and the resulting overall mission cost.

NASA has previous experience with science missions to NEOs (e.g., NEAR-Shoemaker and OSIRIS-REx), and thus has some knowledge of PD characterization requirements, but currently has no detailed experience in conducting mitigation missions (e.g., DART will not impact Dimorphos until the Fall of 2022). Therefore, concepts for characterization missions are likely more mature than mitigation mission designs. In addition, the challenges of rapid reconnaissance (flyby or rendezvous) are better understood, and so tests of these rapid-response strategies may be implemented sooner in comparison to doing so for mitigation demonstration missions.

Finding: Current practices and procedures for spacecraft development and deployment are not optimized to address planetary defense mission needs if the available warning time is short. However, several rapid response strategies exist which could be tested on planetary defense demonstration missions.

Finding: A study of specific rapid-response strategies as part of planetary defense demonstration missions would help assess what preparations and resources would enable a launch 1, 2, 3, or 4 years from time of alert.

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
×

Special focus on examining the feasibility of a standardized rapid reconnaissance design, applicable to the greatest number of short-warning impact scenarios, would be beneficial in developing rapid-response planetary defense capabilities.

INTERNATIONAL COOPERATION ON NEO PREPARATION

With this decadal survey effort focused on NASA and NSF activities and support, this section provides a high-level overview of those NASA and NSF activities that will foster—and benefit from—greater international cooperation. The risk of a NEO impact is of worldwide concern, and thus international cooperation on PD efforts will pay dividends in both warning and response.

NASA continues its efforts to raise international awareness of the NEO hazard through a vigorous presence at international meetings (e.g., International Academy of Astronautics Planetary Defense Conference) and taking an active leadership role at the Scientific and Technical Subcommittee of the United Nations Committee on the Peaceful Uses of Outer Space (UNCOPUOS). NASA accomplishes this by supporting NEO response planning at the UNCOPUOS, by promoting education and outreach on NEO impact effects among international disaster management organizations, and by engaging with nongovernmental organizations. By cooperating on planetary defense demonstrations with international spacefaring partners, NASA will increase the global ability to respond to a future NEO threat.

NASA is an active participant in two United Nations-endorsed groups addressing planetary defense. The first is the International Asteroid Warning Network (IAWN), a coordinated system of asteroid detection and warning activities that shares NEO discovery, orbit determination, and impact prediction information (IAWN 2021). IAWN’s 35 signatories share their validated NEO findings, hazard analyses, and impact predictions with UN member states through the UNCOPUOS. IAWN also coordinates NEO observation campaigns to help refine early warning protocols and expand PD characterization efforts.

NASA also is active in the UN-endorsed Space Mission Planning Advisory Group (SMPAG), comprised of several space agencies and international organizations (ESA 2022). SMPAG coordinates international efforts in NEO impact mitigation and response. SMPAG’s information exchange and NEO mitigation planning increases its members’ ability to respond to a NEO threat. Through SMPAG, NASA can propose and pursue significant, joint NEO technology efforts with spacefaring partners. International participation in U.S.-led technology R&D programs, and in-space demonstrations of PD techniques will avoid duplication and get maximum return from limited global resources.

Finding: NASA leadership and participation in the International Asteroid Warning Network and the Space Mission Planning Advisory Group has produced significant progress toward development of international planetary defense capabilities.

International collaboration expressed through NASA’s DART and ESA’s Hera missions will generate insights surpassing what either would produce on its own. Cooperation on these missions may be followed by further joint demonstration missions through the decade and beyond.

Finding: Knowledge obtained by planetary defense demonstration missions, such as DART and Hera, will advance understanding of NEO mitigation techniques and further international collaboration in planetary defense efforts.

NEO IMPACT EMERGENCY PROCEDURES AND ACTION PROTOCOLS

Devastating NEO impacts are low-probability, high-consequence events that may result in extensive loss of life, and warrant appropriate levels of preparedness. As in the case of hurricanes and other natural disasters, damage prevention depends on developing and exercising response protocols to support reliable communications, sound decision-making, and employment of effective mitigation measures. These efforts necessarily span the responsibilities of many U.S. government and international entities. Because the scope of this decadal survey

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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effort is focused on NASA and NSF planetary defense activities, this section outlines key activities that support NEO impact emergency procedures and action protocols.

NASA has developed a sound process for the collection, dissemination, and communication of information regarding specific NEO impact threats. If through data supplied by MPC an object is identified as a potential impact threat by CNEOS’s high-precision orbit determination, NASA will inform the relevant U.S. government entities (e.g., National Security Council, Office of Science Technology Policy, DHS, and Federal Emergency Management Agency [FEMA]) (NSTC 2021). Depending on the specific impact location, impact severity, and warning time, these entities will then pursue appropriate steps to assess the risk, prepare detailed communications, and implement necessary PD mitigation efforts.

Finding: The Minor Planet Center and the Center for Near-Earth Object Studies provide crucial data for identifying NEOs and evaluating their impact probabilities, and therefore, are vital components for an effective planetary defense response.

Specific actions related to Goal 5 are addressed in the Report on NEO Impact Threat Emergency Protocols (NSTC 2021). NASA coordinates NEO end-to-end observations campaigns, performs table-top PD exercises, and constructs hypothetical impact scenarios to exercise and refine NEO impact protocols.

As more capable surveys increase our knowledge of the NEO population, PD planners may take advantage of future close approaches by coordinating opportunistic observation campaigns as part of exercises simulating potential impact events. These campaigns may enlist space- and ground-based facilities for orbit determination and characterization, as has been done with previous IAWN observation campaigns (e.g., 1999 KW4 and Apophis) (IAWN 2021).

Another method to improve PD readiness is through continued collaborations between U.S. government agencies and international partners. This decade’s expected improvement in knowledge of the NEO population and characterization of relevant NEOs will add fidelity to NEO impact response exercises and increase national and international readiness for a potential impact. NASA, FEMA, DoD, and other U.S. government agencies have held several joint PD table-top exercises presenting realistic impact scenarios, and similar exercises involving hypothetical potentially hazardous NEOs have been conducted during biennial International Academy of Astronautics Planetary Defense Conferences; these provided useful information regarding impact consequences and disaster response preparations at the local, state, national, and international levels.

Finding: NASA’s continued commitment to propose, plan, and participate in intra- and intergovernmental planetary defense table-top response exercises and international observation campaigns, will enable NASA’s Planetary Defense Coordination Office to broaden and solidify connections to relevant U.S. and international agencies. These activities aid planetary defense preparation and strengthen global NEO impact emergency protocols.

CONCLUSIONS

NASA, NSF, and other government agencies play a leading role in developing the capacity to understand the NEO hazard and build a long-term ability to counter a potential impact threat. Society now possesses sufficiently mature telescope and space operations technologies to provide two of the three elements necessary to prevent a NEO impact.

First, NASA is expanding its NEO search abilities and, with its U.S. government and international partners, can issue warning information for any asteroid discovered on a threatening trajectory. New ground- and space-based search systems will increase the ability to provide impact warning for more numerous smaller asteroids.

Second, NASA and its partners possess spaceflight technology that makes NEO impact prevention practical. NEO deflection demonstrations, like DART, are essential to provide the technology building blocks of impact mitigation capability.

The third element for NEO impact prevention is the readiness and determination to respond to a future Earth impact. However, focused in-space efforts over the next decade and beyond are necessary to develop a suite of proven, practical technologies for safely deflecting or disrupting a threatening NEO.

Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
×

Without the development and testing of in-space mitigation technologies, the only possible response to a threatening impact would be evacuation of the impact area and subsequent disaster response. A robust program of activities in the coming decade will enable the U.S. planetary defense community to forge detection, warning, and mitigation capabilities that will stand as a global example of how to shield society from a destructive yet preventable natural disaster.

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Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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Suggested Citation:"18 Planetary Defense: Defending Earth Through Applied Planetary Science." National Academies of Sciences, Engineering, and Medicine. 2023. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. Washington, DC: The National Academies Press. doi: 10.17226/26522.
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The next decade of planetary science and astrobiology holds tremendous promise. New research will expand our understanding of our solar system's origins, how planets form and evolve, under what conditions life can survive, and where to find potentially habitable environments in our solar system and beyond. Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032 highlights key science questions, identifies priority missions, and presents a comprehensive research strategy that includes both planetary defense and human exploration. This report also recommends ways to support the profession as well as the technologies and infrastructure needed to carry out the science.

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