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Introduction and Background

Large asteroid impacts have scarred our planet in the past and will likely do so again in the future. The consequences can sometimes be deadly. There is strong scientific evidence and consensus that the impact of asteroids and comets, or near Earth objects (NEOs),¹ played a major role in the mass extinctions documented in Earth’s fossil record. For example, during the Cretaceous-Paleogene (K-Pg formerly K-T) event 66 million years ago, an asteroid with a diameter currently estimated as 12-14 kilometers impacted what is now the Yucatan Peninsula and resulted in long-duration global climate change that famously caused, or contributed to, the extinction not only of the dinosaurs but also of more than 75 percent of all nonavian life on Earth.² Such devastating impacts are fortunately rare, but our highly interconnected modern society may be vulnerable to much smaller impacts. It is estimated that if Earth were struck by an approximately 1-kilometer-diameter NEO, the impact could trigger earthquakes, tsunamis, and other secondary effects—such as climate change sufficient to cause global crop failures for several years^3,4—that extend far beyond the immediate impact area.

As of January 2019, the number of known asteroids of all sizes that pass within 0.05 astronomical units (AUs) of the Earth’s orbit was 19,560.⁵ Of these, 897 are estimated to be larger than 1 kilometer in diameter. The frequency of NEO impacts rises in inverse proportion to their sizes (see Table 1.1), meaning that large NEO impacts such as the one that generated the K-T event (~15 kilometers diameter) are few and far between, occurring at most once every 100 million years.⁶ In comparison, the average time between impacts of 1-kilometer objects is around

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¹ A near Earth object (NEO) is an asteroid or a comet that has an orbit that brings it within 1.3 astronomical units (AU), approximately 125 million miles, of the Sun. They may also be referred to as either a near Earth asteroid (NEA) or an Earth approaching comet (EAC), as appropriate.

² A.S.P. Rae, G.S. Collins, M. Poelchau, U. Riller, T.M. Davison, R.A.F. Grieve, G.R. Osinski, J.V. Morgan, and IODP-ICDP Expedition Scientists, 2019, Stress-Strain Evolution During Peak-Ring Formation: A Case Study of the Chicxulub Impact Structure, Journal of Geophysical Research: Planets 5:33-22.

³ O.B. Toon, K. Zahnle, D. Morrison, R.P. Turco, and C. Covey, 1997, Environmental perturbations caused by the impacts of asteroids and comets, Review of Geophysics 35(1):41-78.

⁴ O.B. Toon, C. Bardeen, and R. Garcia, 2016, Designing global climate and atmospheric chemistry simulations for 1 and 10 km diameter asteroid impacts using the properties of ejecta from the K-Pg impact, Atmospheric Chemistry and Physics 16:13185-13212. See also: National Research Council, 2010, Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies, Washington, DC: The National Academies Press.

⁵ Center for Near Earth Object Studies, 2019, NEA Stats, JPL, https://cneos.jpl.nasa.gov/.

⁶ For comparison, at the smallest sizes, recently released U.S. Department of Defense (DoD) data show that between 1994 and 2013, 556 bolide events were observed in the atmosphere; these correspond to asteroids ranging from 1 m to 20 m in size entering Earth’s atmosphere. National Science and Technology Council, 2018, National Near-Earth Object Preparedness Strategy and Action Plan: A Report by the Interagency Working Group for Detecting and Mitigating the Impact of Earth-Bound Near-Earth Objects, p. 2.

500,000 years (see Table 1.1),⁷ and the greatest number of asteroids are small enough to burn up in the atmosphere, going completely undetected and doing no damage. However, there are many asteroids between these sizes that are able to cause damage—sometimes significant damage—and have impact frequencies on the time scales of human civilization. For example, objects approximately 25-30 meters in diameter can cause a fireball, airburst, shockwave, and minor damage. An object approximately 50 meters in diameter could cause local damage comparable to that of a large thermonuclear weapon. Objects 140 meters in diameter can cause destruction on a regional or national scale. Objects 300-500 meters in diameter can cause continental-scale destruction.

For example, an NEO with a diameter about 25 meters is expected to impact Earth about once every one or two centuries.⁸ According to current estimates, there are almost 10 million NEOs larger than 20 meters, and many are extremely difficult to detect prior to entering Earth’s atmosphere.⁹ An asteroid impact from a 20-meter-diameter object can have severe and costly effects. In early 2013, the air above the city of Chelyabinsk, Russia, was struck by a fireball and sound wave blast from a small asteroid about 20 meters (about the length of a bowling alley) wide that exploded approximately 25 kilometers above the town. The blast produced 20-30 times more energy than that released by the first atomic bombs (about 15 kilotons). There were more than 1,600 people injured by broken glass and approximately $30 million in property damages from the blast.¹⁰ Had the object had a steeper entry angle, the consequences would have been even more severe.

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⁷ National Research Council 2010, Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies, Washington, DC: The National Academies Press, https://doi.org/10.17226/12842.

⁸ P. Brown, R.E. Spalding, D.O. ReVelle, E. Tagliaferri, and S.P. Worden, 2002, The flux of small near-Earth objects colliding with the Earth, Nature 420:294-296.

⁹ National Science and Technology Council, 2018, National Near-Earth Object Preparedness Strategy and Action Plan: A Report by the Interagency Working Group for Detecting and Mitigating the Impact of Earth-Bound Near-Earth Objects, p. 3.

¹⁰ O.P. Popova, P. Jenniskens, V. Emel’yanenko, A. Kartashova, E. Biryukov, S. Khaibrakhmanov, V. Shuvalov, et al., 2013, Chelyabinsk airburst, damage assessment, meteorite recovery, and characterization, Science 342(6162):1069-1073.

In 1908, an object approximately 40-60 meters in size (the height of the Statue of Liberty) exploded over Tunguska, Russia, with the equivalent of 5-10 megatons of TNT (hundreds of times greater than the first atomic bombs and comparable to the most powerful hydrogen bombs), leveling more than 2,000 square kilometers of forest (10 times the area of Washington, D.C.). If a similar event occurred over a major metropolitan area, it could cause millions of casualties (see Figure 1.1). NASA estimates that there are some 500,000 objects larger than 40 meters that could pose an impact hazard. Many would be very challenging to detect more than a few days in advance.¹¹

NEOs larger than 140 meters (about the height of the Washington Monument) have the potential to inflict severe damage to entire regions. Such objects would strike Earth with a minimum energy of over 60 megatons of TNT, which is greater than the most powerful nuclear device ever tested. Although NASA is confident that it has discovered and catalogued nearly all NEOs large enough to cause damage on a global scale (objects greater than 10 kilometers in diameter) and those capable of causing global effects (objects greater than 1 kilometer in diameter) and has determined that they are not on collision courses with Earth,¹² after almost two decades of search, NASA and its partners have catalogued only about one-third of the estimated 24,000 NEOs that are at least 140 meters in diameter.¹³

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¹¹ National Science and Technology Council, 2018, National Near-Earth Object Preparedness Strategy and Action Plan: A Report by the Interagency Working Group for Detecting and Mitigating the Impact of Earth-Bound Near-Earth Objects, Washington, D.C., p. 3.

¹² Although they are not asteroids, there is still some chance that large comets from the outer solar system could appear and impact Earth with warning times as short as a few months.

¹³ National Science and Technology Council, 2018, National Near-Earth Object Preparedness Strategy and Action Plan: A Report by the Interagency Working Group for Detecting and Mitigating the Impact of Earth-Bound Near-Earth Objects, Washington, D.C., p. 4.

IMPACTS AND THE TSUNAMI THREAT

While the effects of impacts on land are reasonably well understood, there has been extensive uncertainty about impacts in the ocean. Early analyses suggested that tsunamis raised by even small impacts into the ocean would carry enormous amounts of energy and thus devastate coastal cities, greatly magnifying the hazard anticipated from small asteroid impacts. Later and more detailed analyses, including an older report by the Naval Research Laboratory, however, found that the impact waves would largely break near the impact itself or on continental shelves, dissipating the impact energy in turbulence and thus not substantially magnifying the hazard. At the present time, impact-generated tsunamis are not considered a serious global hazard, although they may create flooding near the impact site and should be considered in civil defense schemes.¹⁴

CHARACTERIZING ASTEROIDS

Asteroids emit no visible light of their own; their visible brightness depends on the amount of sunlight reflected/scattered from their surfaces. For a given location relative to the Sun and the observer, larger asteroids and those with lighter, more reflective, surfaces appear brighter. The brightness of an asteroid can therefore provide information on its size. Astronomers quantify the brightness of an object in the sky with a quantity called magnitude. Historically, the Greek astronomer Hipparchus categorized stars into six magnitude classes, 1 through 6. The brightest stars in the sky (e.g., Sirius, the brightest star visible from Earth) were assigned magnitude 1, the next brightest group magnitude 2, and so on, with the faintest stars visible to the human eye assigned magnitude 6. Somewhat paradoxically, the fainter a star is, the larger is its magnitude on this scale. In the nineteenth century, this system was placed on a more mathematical basis by defining a difference of five magnitudes as being exactly equal to a factor of 100 in brightness. A consequence of this was that magnitudes were no longer restricted to positive integers and some stars’ magnitudes were changed. Thus, following this change, Sirius is magnitude −1.46. Powerful telescopes extend this scale down to magnitudes as large (as faint) as 28.

Unlike stars, however, asteroids are sometimes brighter and sometimes fainter, depending on how close they are to Earth at the time of observation. For these objects, a brightness scale called absolute magnitude, denoted by the capital letter H, has been developed. The absolute magnitude (H) is defined as the magnitude the asteroid would appear to have at visible wavelengths if it were located 1 astronomical unit (AU; 1 AU is approximately 150,000,000 km) distant from both Earth and the Sun, and observed in a direction exactly opposite to the Sun, so that it is fully illuminated like the full moon (this is a hypothetical configuration, which is impossible to achieve when observing from Earth). Asteroids are never actually observed under these conditions, and so corrections must be made for their actual distance from both Earth and the Sun and their solar phase angle (i.e., the Sun-asteroid-observer angle) to obtain their absolute magnitude from the magnitude actually observed.

In addition to the phase correction, the absolute magnitude of an asteroid with a given diameter also depends on the fraction of the sunlight it reflects at visible wavelengths. A measure of this fraction is called the albedo, with the symbol p_V, and for known asteroids it varies between extremes of about 1 and 50 percent, with a typical range being between 2 and 35 percent. If the albedo is unknown, which is usually the case upon first discovery, it is common to assume a mean value of about 15 percent.

The absolute magnitude of an asteroid is related to its size because the amount of light reflected is proportional to its area, which in turn is proportional to the square of the diameter. In order to derive a diameter from H, an albedo must be known or a default value used; if the latter, the resulting diameter is uncertain, as discussed in detail elsewhere in this report. The George E. Brown, Jr. Near-Earth Object Survey Act’s minimum size of 140 meters

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¹⁴ G.S. Collins, H.J. Melosh, and R.A. Marcus, 2005, Earth Impact Effects Program: A Web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth, Meteoritics and Planetary Science 40:817-840, http://doi.org/10.1111/j.1945-5100.2005.tb00157.x; J.G. Hills, I.V. Nemchinov, S.P. Popov, and A.V. Teterev, 1994, “Tsunami Generation by Small Asteroid Impacts,” in Hazards from Comets and Asteroids, edited by T. Gehrels (Tucson: University of Arizona Press), pp. 779-789; W.G. van Dorn and B. Le Mehaute, 1968, Handbook of Explosion-Generated Water Waves, Rep. TC-130 (Pasadena, Calif.: Tetra Tech); K. Wünnemann, G.S. Collins, and R. Weiss, 2010, Impact of a cosmic body into Earth’s ocean and the generation of large tsunami waves: Insight from numerical modeling, Reviews of Geophysics 48(4):RG4006, http://doi.org/10.1029/2009RG000308.

corresponds to an absolute magnitude of 22 (a reasonable limiting faintness for telescopes envisaged in 2005). Accurate diameters, however, are very difficult to obtain from visible observations, because true albedos are not easily determined, whereas infrared observations give far more accurate estimates of diameters, as explained later in this report.

THE NATION’S RESPONSE TO THE NEO IMPACT THREAT

Like other natural disasters (e.g., tsunamis) and space weather events (e.g., solar storms), NEO impacts can be deadly to life and property. However, unlike most other natural disasters, NEO movements and impacts are predictable many years in advance and may be preventable if the impacting object is known, hence the requirement to search for them. Even if the impact were beyond U.S. territory, its environmental, economic, and geopolitical consequences would be detrimental to the United States. The U.S. government has therefore directed action in planetary defense—identifying and, if possible, preventing the hazard of NEO impacts.

Congressional interest in the subject started when Congress directed NASA to initiate a “Spaceguard Survey” to search for NEOs in the late 1990s and also officially established an NEO survey program. In 1998, Congress directed NASA to discover at least 90 percent of 1-kilometer-diameter or larger NEOs within 10 years; NASA met this mandate by the end of 2010, according to the best statistical models of the overall population.

The George E. Brown Act, included in NASA’s fiscal year 2005 authorization act, amended the National Air and Space Act of 1958 to declare that “the general welfare and security of the United States require that the unique competence of the Administration be directed to detecting, tracking, cataloguing, and characterizing near-Earth asteroids and comets in order to provide warning and mitigation of the potential hazard of such near-Earth objects to Earth.”¹⁵ Section 321 of the act provides the following specific guidance:

While NASA created the Planetary Defense Coordination Office (PDCO) in January 2016 to serve as a planetary defense coordination and communications node for the federal government and to achieve the George E. Brown Act objective to detect, track, and catalogue at least 90 percent of NEOs equal to or greater than 140 meters in size by 2020, this target will not be met, given the inadequate resources dedicated to the task since 2005.

Subsequent federal policies and directives have revisited this theme without specific thresholds or funding to carry out the goals stated in the George E. Brown Act. The 2010 National Space Policy of the United States underscored the general mandate, specifically directing the NASA Administrator to “pursue capabilities, in cooperation with other departments, agencies, and commercial partners, to detect, track, catalogue, and characterize near-Earth objects to reduce the risk of harm to humans from an unexpected impact on our planet and to identify potentially resource-rich planetary objects.”¹⁷ Most recently, in 2018, the government released a National NEO

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¹⁵ NASA Reauthorization Act of 2005, P.L. 109-155, 119 Stat. 2923, December 30, 2005.

¹⁶ NASA Reauthorization Act of 2005, P.L. 109-155, 119 Stat. 2922, December 30, 2005.

¹⁷ Executive Office of the President, 2010, Presidential Policy Directive 4: National Space Policy of the United States of America, p. 12.

Preparedness Strategy and Action Plan.¹⁸ The very first goal of the strategy is to “Enhance NEO Detection, Tracking, and Characterization Capabilities” (see Figure 1.2).

Box 1.1 lists the action items that are expected to address this goal. The actions do not select any particular solutions—such as a space-based infrared telescope—but simply lay out the process.

Although Congress added “detect[ing] asteroids, understand[ing] their composition, predict[ing] their paths, and provid[ing] timely and accurate communications about potentially hazardous objects” as the seventh of seven policy directives and purposes for NASA in 2010, and although “detect[ing] asteroids, understand[ing] their composition, predict[ing] their paths, and provid[ing] timely and accurate communications about potentially hazardous objects” is part of NASA’s strategic objective, these activities have remained under the umbrella of the Science Mission Directorate’s planetary science division. This has left NEO detection to compete with planetary science research for funding.

In many ways, NEO detection, tracking, and characterization is primarily an operational mission rather than one that pushes the frontier of science, though it does that as well.¹⁹

As the Chelyabinsk and Bering Sea fireballs demonstrate, objects smaller than 140 meters in diameter frequently reach Earth. And the Chelyabinsk event demonstrates that some of them can be dangerous. Although it is difficult, they can occasionally be detected.

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¹⁸ National Science and Technology Council, 2018, National Near-Earth Object Preparedness Strategy and Action Plan: A Report by the Interagency Working Group for Detecting and Mitigating the Impact of Earth-Bound Near-Earth Objects, Washington, D.C., p. 1.

¹⁹ See Title 51 US Code, http://uscode.house.gov/view.xhtml?path=/prelim@title51/subtitle2/chapter201&edition=prelim); NASA’s strategic plan 2018, https://smd-prod.s3.amazonaws.com/science-red/s3fs-public/atoms/files/nasa_2018_strategic_plan_0.pdf; 2020 budget request, https://www.nasa.gov/sites/default/files/atoms/files/fy_2020_congressional_justification.pdf.