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Balancing Strategic Missions

As noted in Chapter 1, NASA’s Science Mission Directorate (SMD) is divided into four divisions. Each division has different requirements and approaches to large strategic missions and to how their large strategic missions compare to smaller missions within their portfolio.

Generally, the Astrophysics Science Division and Planetary Science Division have similar approaches toward large strategic missions. For these divisions, large strategic missions are a major portion of their budgets and science observations, and their scientific communities believe that such missions are vital to making the greatest advances within their fields. The Heliophysics Science Division and Earth Science Division are more similar to each other in their approaches to and requirements for large strategic missions, although this has evolved over time. For heliophysics the spacecraft have tended to be small and medium size, in part because a major goal for the discipline is to make simultaneous multiple measurements of many different phenomena over a long period of time, and heliophysics instruments can be smaller and less expensive. For Earth science, most scientific observations are conducted from low Earth orbit and the spacecraft have similar operating environments. However, although both heliophysics and Earth science generally rely more on smaller missions than large strategic missions than do astrophysics and planetary science, they have operated large strategic missions, and these missions, as well as future ones, are still considered by some members of their communities to be vital for some major scientific breakthroughs in their respective fields. Furthermore, both Earth science and heliophysics are likely to operate constellations of multiple smaller spacecraft pursuing strategic science goals and having the cost and complexity common to large strategic missions.

This chapter discusses the role of large strategic science missions in each of the four divisions, examines a specific example for each, and discusses the respective roles played by smaller missions in the divisions. (Note that a list of acronyms is included as Appendix G.)

THE IMPORTANCE OF BALANCING LARGE, MEDIUM-SIZE, AND SMALLER MISSIONS WITHIN THE DIVISIONS

The concept of “balance” has evolved as a vital part of the recommended programs within each NASA science discipline.¹ “Balance” is a subjective term that can have many nuances, but the committee has adapted and

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¹ National Academies of Sciences, Engineering, and Medicine (NASEM). The Space Science Decadal Surveys: Lessons Learned and Best Practices, The National Academies Press, Washington, D.C., 2015, pp. 58ff.

expanded the definition as used in the recent report New Worlds, New Horizons: A Midterm Assessment,² which “interprets balance to refer to a viable mix of small, medium, and large initiatives on the ground and in space that optimizes the overall scientific return of the entire U.S. astronomy enterprise viewed collectively. It does not refer to a balance of wavelengths, nor of astronomy subtopics” (p. 42).

As the 2015 report stated:

The committee fully agrees with this statement. This committee similarly interprets “balance” to mean a viable mix of small, medium-size, and large missions that optimizes the overall scientific return of the entire science program within each respective NASA SMD division.

A portfolio that contains only large initiatives is not as sustainable scientifically or economically. Large strategic missions may directly impact the ability to maintain a balanced portfolio because single missions may place high demands on personnel resources, facilities, or funds. “Balance” is a multifaceted term that includes cost, number of missions, priority, and cadence. Thus, a balanced portfolio reflects how these factors form a successful observational program, providing mission flexibility for program managers. In order to understand balance, it is necessary to look over a period of time, because implementation of new projects can make a program appear to be unbalanced for short periods (i.e., less than 5 years). Small and medium-size missions are also essential elements of a balanced portfolio. As noted in Chapter 1, they can provide vital observations that guide the development, and the science observations, of larger missions.

Such considerations of balance have played a prominent role in all of the recent decadal surveys and have served community and other stakeholder interests and NASA planning well. Key to such implementation is an adherence to budgets during project and program development—a subject that is further discussed in Chapter 3.

ASTROPHYSICS SCIENCE DIVISION

Astrophysics has traditionally depended on large strategic missions for most of its major scientific advances. NASA’s Great Observatories program established in the 1980s was centered on the development of several large strategic missions: the Hubble Space Telescope (HST), the Compton Gamma Ray Observatory (CGRO), the Spitzer Observatory, and the Chandra X-Ray Observatory. (Spitzer’s original development cost was over $2 billion, but because of redesign, it was reduced to approximately $720 million, making it a medium-size strategic mission.) Large strategic missions in astrophysics have made many of the biggest scientific discoveries.

For the Astrophysics Science Division, the current decadal survey science priorities for large missions include the following: searching for the first stars, galaxies, and black holes; seeking nearby, habitable planets; and understanding scientific principles of the physics of the universe. Larger, general-purpose observatories can be used by the general observer community in ways that were not envisioned by the designers or captured in the original science requirements, and they drive development of new capabilities that can be infused later into smaller missions without further technical development.

Currently, NASA’s Astrophysics Science Division operates two large strategic missions, both in extended mission operation: the HST and the Chandra X-Ray Observatory, as well as the Stratospheric Observatory for Infrared Astronomy (SOFIA) airborne observatory. Hubble is one of the most well-known scientific instruments ever developed. In many ways it is unique, having been designed for updating over a long period of time, unlike any

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² National Research Council (NRC), New Worlds, New Horizons in Astronomy and Astrophics, The National Academies Press, Washington, D.C., p. 42.

³ NASEM, Space Science Decadal Surveys, 2015, pp. 58-59.

other robotic spacecraft. But this uniqueness in itself demonstrates the high visibility such large strategic missions can attract. Hubble has become a well-known, positive symbol of U.S. technological and scientific achievement. It has also become symbolic for both NASA and the astrophysics field.

The Hubble Space Telescope

The Hubble Space Telescope (HST) has been in orbit for more than 26 years and has been upgraded five times with improved instruments. Hubble was launched on April 24, 1990, on the space shuttle Discovery. Hubble was designed with eight instrument bays. The original instruments included three fine guidance sensors used for pointing, the Wide-Field Planetary Camera (WFPC) 1, the Faint Object Spectrograph (FOS), the Goddard High Resolution Spectrograph, the Faint Object Camera, and the High Speed Photometer (HSP). Since its inception, Hubble was designed to be serviceable via the space shuttle. In order to keep the observatory at the forefront of scientific capabilities, new instruments replaced the originals, and broken or outdated hardware has been replaced over the course of five servicing missions from 1993 to 2009.

The fifth and final servicing mission (SM4—two previous missions were designated SM3A and SM3B) almost did not happen, because its initially planned 2004 launch was canceled in the aftermath of the 2003 Columbia space shuttle accident. The high-profile nature of the Hubble mission resulted in considerable controversy over its planned demise and led to several studies, including a 2005 National Research Council report: Assessment of Options for Extending the Life of the Hubble Space Telescope. Ultimately, NASA’s leadership reinstated the servicing missions leading to a May 2009 launch. Two major instruments were replaced, with the Wide-field Camera (WFC) 3 replacing WFPC2 and the Cosmic Origins Spectrograph (COS) replacing the no longer needed Corrective Optics Space Telescope Axial Replacement (COSTAR) system. In addition, repairs were made to the Space Telescope Imaging Spectrograph (STIS) and to the Advanced Camera for Surveys (ACS), which had replaced FOS. To ensure the longevity of the telescope, astronauts replaced all six gyroscopes, all six of the original batteries, and another fine guidance system. (See Figure 2.1.) These actions have illustrated, perhaps most clearly, how the human and robotic aspects of NASA can interact to the betterment of the agency overall.

Hubble currently carries a suite of six complementary instruments:

1. ACS, or the Advanced Camera for Surveys, is responsible for Hubble’s profound images in deep space. Since its installation in 2002, it has doubled Hubble’s field of view of the universe and improved its ability to see the far ultraviolet to visible light, making Hubble capable of studying some of the earliest activity in the universe.
2. COS, or the Cosmic Origins Spectrograph, is excellent at observing points of light throughout the universe, breaking the light into colors, and measuring the intensity of that color. This allows scientists to detect the object’s temperature, density, velocity, and chemical composition. Examples of points of light include stars and quasars.
3. FGS, or the Fine Guidance Sensor, assists in accurately conducting celestial measurements. There are three sensors total: two point to an astronomical target and hold the target in Hubble’s field of view, and one performs scientific observations to measure the position of the telescope relative to the object being viewed.
4. NICMOS, or the Near Infrared Camera and Multi-Object Spectrometer, acts as Hubble’s heat sensor. The instrument is designed to see objects in near-infrared wavelengths, making it a crucial tool to observe objects in the deepest parts of the universe. This is because light from the most distant objects moving away from Earth in our universe appears red by a process known as redshift.
5. STIS, or the Space Telescope Imaging Spectrograph, is best at observing large areas of light and separating the light into its component colors. This instrument is especially useful for observing galaxies and black holes throughout the universe.
6. WFC3, or the Wide-Field Camera 3, brought new depth and range to Hubble’s image production since its installation in 2009. This instrument has the ability to see multiple wavelengths at a higher resolution than any other previous Hubble cameras. It is paired with the ACS to produce a complete and complex view of the universe.

With the retirement of the space shuttle program in 2011, Hubble can no longer be serviced, but due to the efforts of SM4, NASA is hoping to keep it operational until at least 2020 to allow for at least one year of overlap with the James Webb Space Telescope (JWST). When it is finally retired, Hubble will have been in operation for three decades, a feat made possible because of multiple servicing missions. It is possible that future large strategic missions may be serviced as well, either by humans or robotically. The decision to design such missions to be serviced, and the decision to actually conduct servicing missions, will require substantial discussion and evaluation of their costs and benefits. Certainly the Hubble experience will offer important insight and perspective.

The scientific success of Hubble is unparalleled. With a 2.4-meter mirror and ultrasensitive instruments spanning from the ultraviolet to near-infrared, Hubble continues to transform most areas of modern-day astrophysics and remains the most popular telescope in the world for doing astronomical research. Recent science highlights include the imaging and spectroscopic discovery of possible geysers on Jupiter’s moon Europa, indicating plumes from a subsurface ocean; the discovery of a redshift 11.1 galaxy, providing a glimpse of the earliest stellar systems that the universe formed; a refined measurement of the Hubble constant that shows that the universe is expanding faster than expected; and the detection of molecules in the atmospheres of nearby exoplanets. Hubble’s efficiency is at an all-time high, and its operation now includes several new observing modes and other improvements (spatial scanning, rapid tiling, enhanced point spread functions, etc.) that are motivating entirely new research themes.

Astronomers envisioned the importance of implementing a space telescope beyond Earth’s atmosphere to examine infrared and ultraviolet light—phenomena difficult to observe from Earth because this light is heavily absorbed by Earth’s atmosphere. Hubble’s goal of producing detailed images of galaxies beyond our own was deemed critical for the advancement of our understanding of the universe for scientific knowledge and societal benefit. The lack of a consistent, extensive telescope for data collection was hindering our ability to understand

how distant stars, planets, and galaxies are formed and interact with one another throughout the universe. Hubble has produced thousands of observations specifically designed to meet its objectives. The science highlights of Hubble include the following:

• Cosmology: Hubble paved the way for the discovery of the age and expansion of our universe. The development of Hubble allowed scientists to see many more and much fainter stars, many of which are some type of variable star. This revelation helped pinpoint the age of the universe to approximately 13.7 billion years old. In turn, this established the foundation for creating future models of how the universe and all of its elements formed. Similarly, the rate at which the universe is expanding was also refined by the HST. Hubble was used to detect past deceleration of the expansion of the universe. This fundamental discovery about the evolution of the universe and the role of dark energy has generated over 3,000 citations.

  Observing distant galaxy clusters and quasars through the breakdown of light received by Hubble illustrates that the luminous light centers are produced by black holes. Additionally, discoveries in cosmology include dark matter and dark energy. Long-term observations resulted in the Hubble Deep Field. This discovery launched two decades’ worth of deep-field studies to explore the universe at the earliest times. This image from Hubble has transformed the study of galaxies and has generated over 1,000 citations to date.

• Planetary science and exoplanets: Hubble’s capabilities allowed scientists to observe the formation of stars and planets, discover exoplanets, and identify comet impacts on other planets. Hubble was the pioneer for taking the first light-visible image of distant planets. Hubble observations revealed the first exoplanet atmosphere. Other planetary discoveries include the uncovering of icy objects in the Kuiper Belt and underground oceans on Jupiter’s moons Ganymede and Europa.
• Galactic science: Hubble introduced a new approach to identifying the lifetime progression of supernovas and stars. The light observed from a supernova in one of the Milk Way satellite galaxies paved the way to better explain star and galaxy formation. The discovery of how supernovas, neutron stars, and black holes relate to one another has advanced our understanding of how the universe was formed and continues to interact. Likewise, the interaction of neighboring galaxies to the Milky Way, such as Andromeda, has depicted the history of each system of stars and promotes the further advancement of future discoveries and predictions in this field.

As a large strategic mission, Hubble has set the bar that future astrophysics missions such as JWST will have to clear. The Hubble program has produced over 14,000 refereed science publications (at a current rate of over 800 publications per year), with more than 600,000 citations. Similar to other Great Observatories such as Chandra and Spitzer and as expected for JWST and the Wide-Field Infrared Survey Telescope (WFIRST), within the Astrophysics Science Division large strategic missions support a range of science through a vast user community. Each year, there are hundreds of programs allocated to small principal investigator (PI) projects at U.S. and international universities, and yet there are also a few large collaborative programs involving many dozens of U.S. and international community members. Only large strategic missions can support such a large distribution of programs. This is the foundation of U.S. astronomy research in most American universities. Each year there are more than 1,000 research proposals for Hubble observations submitted by the community at an oversubscription rate of 5:1. There have been more than 15,000 users of the observatory to date.

Hubble’s unique capabilities (high ultraviolet [UV] and visible sensitivity) will not be reproduced by any current or planned ground or space telescopes, and after Hubble is retired there will be no equivalent coverage of these observing modes in the JWST era.

Hubble’s science operations model included significant investments in workforce development. Hubble has provided over $750 million of research grants to the science community to analyze and publish observations from the observatory, and this investment continues today at a rate of $30 million per year (one-third of the operating budget). With this investment, Hubble has trained nearly 1,000 graduate students, and there are 600 Ph.D. theses based on Hubble data (40 to 50 per year). Hubble has also supported a vibrant postdoctoral fellowship community, including prized postdoctoral fellowships (Hubble Fellows).

Alternatives to Large Strategic Missions in Astrophysics Science

The astrophysics branch has been assessing the scientific value of missions in the $0.5-$1 billion range, referred to as “Probe class” concepts. A recent Announcement of Opportunity (AO) for astrophysics probe concepts has met with considerable enthusiasm, and proposal evaluation in the first half of 2017 will provide further insight on their potential value. This is an area in which a committee external to NASA may provide advice and recommendations with regard to the advantages of such intermediate-cost missions, appropriate cost sizes, and cadence.

The history of astronomy and astrophysics has involved the development of larger and more powerful telescopes. This is due to the requirement to gather more photons to study—aperture has determined capability. But many astrophysics questions do not require increasingly larger apertures. Whereas the Hubble Space Telescope’s 2.4-meter-diameter mirror remains the largest ultraviolet/optical/infrared (UVOIR) space telescope ever placed in orbit, there have been many smaller space observatories that have answered vital questions. For example, the Cosmic Background Explorer (COBE) spacecraft launched in 1989 provided data vital to confirming the Big Bang theory of the origin of the universe. COBE was an Explorer-class spacecraft, with a launch mass of only 2,270 kilograms (compared to Hubble’s mass of 11,110 kilograms). The Nobel Prize committee stated that “the COBE-project can also be regarded as the starting point for cosmology as a precision science.”⁴ Another astrophysics mission, Kepler, originally started in NASA’s Planetary Science Division but has operated as part of the astrophysics program. Kepler has discovered many new planets around stars, refining scientific understanding of solar system formation and contributing to the dramatic growth in the study of exoplanets. Kepler has been the sole source of Earth-size planets, which cannot be found in the other programs. Many of the observations conducted with smaller astrophysics spacecraft ultimately lead to more focused investigations with larger spacecraft—for example, Kepler’s discoveries of exoplanets is informing plans for use of the JWST.

Other currently operating NASA astrophysics missions include Nuclear Spectroscopic Telescope Array (NuSTAR), Fermi, Swift, and Spitzer. Spitzer is a medium-size mission, whereas NuSTAR, Fermi, Kepler, and Swift are smaller.

Balance in Astrophysics Science

NASA’s experience with the James Webb Space Telescope, which increased in cost substantially over initial estimates, has created the impression that astrophysics is conducted only with large-aperture spacecraft. More significantly, the need to cover these cost increases reduced resources available for other NASA astrophysics missions. This has created the perception within the astrophysics community that the NASA astrophysics portfolio was unbalanced and required redress. Recent National Academies astrophysics reports have addressed this topic.

The question of balance in astrophysics concerns the relative resources devoted to small, medium-size, and large missions. The small and medium-size Explorer mission lines have cost caps that differ by about a factor of 2, but the large strategic missions like HST and JWST have costs more like five powers of 2 higher than a medium-size Explorer. This large gap has led to concerns about balance.

A Probe class line with a cost cap more like the planetary division New Frontiers line is an option to bridge this gap, and the inclusion of this class of mission within Astrophysics will be considered by the next decadal survey. There are also important factors that already mitigate the concern over balance: the other Great Observatories like Spitzer and Chandra have costs that are much lower than JWST and HST. Also, the Great Observatories support a very wide range of small research projects through their guest observer programs. Certainly, a balanced program in astrophysics will always have a role for large strategic missions.

EARTH SCIENCE DIVISION

In the Earth science field, large strategic missions provide a centralizing and organizing force around which research and development activities can coalesce. Guided by the science community, the concentration of resources

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⁴ The Nobel Prize in physics, 2006, http://www.nobelprize.org/nobel_prizes/physics/laureates/2006/.

dedicated to the mission goals provides a wide reach both inside NASA as well as in industry and academia. As an example, the Terra mission, within the Earth Observing System (EOS) suite of missions, funded science team PIs inside NASA and in academia, internships, postdoctorates, and graduate and undergraduate students on a decadal timescale throughout all phases of the mission.

Having resources centered on a large strategic mission provides both continuity and critical mass supporting the science and engineering communities as well as providing science data needed to accomplish the goals and objectives of the science community. In Earth science the ability to collect consistent, well-calibrated, validated data over decadal timescales is critical for understanding Earth’s climate, physical, and biogeochemical systems. This remains a primary requirement for Earth science and the large strategic missions that generate hundreds of community-defined science products, and continuously receive very high marks in the senior review process.⁵

Currently, NASA’s Earth Science Division operates three large strategic missions: Terra, Aqua, and Aura. (Figure 1.1, earlier, demonstrates the emergence of these programs in the 1990s.) The experience of the Terra mission demonstrates the way NASA has approached large strategic missions in the past.

Terra

NASA launched the Earth Observing System AM-1 mission in late 1999, after almost a decade of planning and development. This was NASA’s largest Earth science mission. The spacecraft was renamed Terra and the mission was directed out of NASA’s Goddard Space Flight Center (GSFC). Terra included both facility and PI-class instruments from multiple NASA centers, academia, and significant international participation (facility-class instruments are also “directed,” meaning that instead of a PI, the instrument science team is led by an assigned project scientist). Terra was still operational at the time of this report, over 18 years after launch. (See Figure 2.2.)

As the first in the EOS series, Terra was meant to collect a rigorous multidisciplinary set of measurements required to identify and quantify the physical and biogeochemical interactions between Earth’s atmosphere, biosphere (terrestrial and marine), cryosphere, hydrosphere, and interior. The community-defined measurement requirements were deemed critical for the advancement of our understanding of Earth’s complex systems for scientific knowledge and societal benefit. The lack of data was hindering scientists’ ability to understand how the Earth system functions, delaying understanding historic trends in climate, weather, and the functioning of the biosphere; to model important interactions among processes; and to use improved models to project future trends. Most importantly, data were needed of sufficient quantity and quality to separate potential anthropogenic influences from the complex natural system. In the 1980s and 1990s the ability to identify and quantify the possible human component in global change was deemed to be time critical, and only a very large mission or suite of missions like EOS could provide the enormous amount of data required. Terra’s initial science mission objectives were as follows:

• Detect human impacts on climate and the ability to distinguish them from natural variability;
• Measure the effects of clouds on Earth’s energy balance;
• Measure the effect of aerosols and greenhouse gases (including CO) on Earth’s energy balance and air quality;
• Measure global carbon storage due to terrestrial and marine productivity and changes in surface characteristics;
• Measure and monitor global land and sea surface temperatures, albedo, snow cover, and so on;
• Improve medium- and long-range weather forecasts; and
• Improve prediction, characterization, and risk reduction for wildfires, volcanoes, floods, and droughts.

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⁵ The senior review process is the method by which NASA determines if missions that are completing their primary phase of operations should enter into an extended mission phase, and by which missions already in extended phase are further extended. For over a decade NASA has conducted senior reviews in all four science divisions every 2 years as required by law—recently extended to 3 years—and each mission is evaluated according to its scientific productivity.

Terra carries a suite of five complementary instruments:

1. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), contributed by the Japanese Ministry of Economy, Trade, and Industry with a U.S.-led science team at the Jet Propulsion Laboratory (JPL), is a multispectral imager that provides a unique benefit to Terra’s mission as a stereoscopic and high-resolution instrument required to measure and verify processes at fine spatial scales.
2. Clouds and the Earth’s Radiant Energy System (CERES), originally a PI instrument at Langley Research Center and now a facility-class instrument with similar sensors on several platforms, investigates the critical role clouds, aerosols, water vapor, and surface properties play in modulating the radiative energy flow within the Earth–atmosphere system.
3. Multi-Angle Imaging Spectroradiometer (MISR), a PI-led instrument from JPL, characterizes physical structure from microscopic scales (aerosol particle sizes and shapes) to the landscape (ice and vegetation roughness and texture) to the mesoscale (cloud and plume heights and 3-D morphologies).
4. Moderate Resolution Imaging Spectroradiometer (MODIS), a facility-class instrument directed by GSFC, acquires daily, global, and comprehensive measurements of a broad spectrum of atmospheric, oceanic, and land properties, improving and supplementing heritage measurements needed for processes and climate change studies.

5. Measurements of Pollution in the Troposphere (MOPITT), sponsored by the Canadian Space Agency (CSA) with a science team at the University Corporation for Atmospheric Research (UCAR), retrieves carbon monoxide total column amounts as well as mixing ratios for seven pressure levels, and its gas correlation approach still produces the best data for studies of horizontal and vertical transport of this important trace gas.

With five instruments colocated on one platform, Terra produces 81 community-defined, well-calibrated, validated, temporally and geospatially overlapping Earth observations specifically designed to meet the mission objectives. Many of these objectives were met or surpassed by the merger of data from the instruments on the Terra platform taking advantage of temporal and spatial simultaneity as well as using data from other satellites and sources to exploit differences in perspective. Also, most objectives require many years of observations to accomplish, and the science community benefited greatly by the longevity of Terra (and other missions).

In addition to creating a very large library of measurements supporting the objectives listed earlier, highlights of the Terra mission (shared with other Earth science missions) organized by NASA’s Earth Science Focus Areas (NASA’s 2014 Science Plan) include the following:

• Climate variability and change (CERES including Aqua CERES, MISR, MODIS including MODIS): Enabling better understanding of the role of clouds (height, density, composition, and diurnal variation), aerosols (size, density and injection heights), and Earth surface characteristics (land, sea, and ice) and their interactions on Earth’s radiation budget and climate. This was accomplished through the creation of radiance data sets with the accuracy, precision, and stability required to show trends and establish the physical bases for developing, testing, and improving climate models.
• Atmospheric composition (ASTER, MISR, MODIS including Aqua MODIS, MOPITT, Ozone Measuring Instrument [OMI], Microwave Limb Sounder [MLS], Tropospheric Emission Spectrometer [TES], High Resolution Dynamics Limb Sounder [HIRDLS]): A greatly enhanced understanding of how fire, aerosols, clouds, and human activities and natural phenomena (such as ENSO—El Niño–Southern Oscillation) interact to affect atmospheric composition and human health. This includes an archive of volcanoes, glaciers, aerosol sizes and injection heights, CO maps, and a better understanding of global atmospheric circulation through tracking CO as related to fires and industrial activity.
• Carbon and ecosystems (ASTER, CERES, MISR, MODIS including Aqua MODIS, and MOPITT): First multispectral retrievals of CO elucidating the global atmospheric circulation and transport of CO and sources from fires and industrial activity. Greatly enhanced understanding of the impact of weather, climate, and human activity on ecosystem state and function and the carbon cycle. First global sets of vegetation and other surface parameters such as land surface temperature, emissivity, primary production, and so on, required for understanding the physical and biogeochemical interactions between the biosphere and atmosphere. These data have allowed the development of new models of the fluxes of carbon, water, nitrogen, and other important elements and compounds as well as the assessment of the impacts of changes on agriculture, weather, climate, and human health.
• Energy and water cycle (ASTER, CERES including Aqua CERES, MISR, MODIS including Aqua MODIS, MOPITT): Related to the previous focus areas, Terra data have created a decade-plus record of sea surface temperatures, land surface temperatures, land emissivity, flooding (inland and coastal), snow and ice cover (land and sea), and cloud and aerosol data, enabling a new era of modeling Earth’s hydrological cycle at many scales.
• Weather (ASTER, MISR, MODIS): MODIS-derived polar winds have had a positive impact on numerical weather forecasts in high latitudes and in the tropics, as have the combination of MODIS and MISR wind data at different levels in the atmosphere.
• Earth’s surface and interior (ASTER, MISR, MODIS, MOPITT): Data from ASTER have created a massive library of Earth’s topography and volcanoes and glaciers, including the extent, surface temperatures, and rate of change over time. These data combined with MISR injection heights and MODIS surface area maps have enabled a better understanding of volcanoes and their impact on both the surrounding landscape

In 2005 Terra entered extended mission phase and was one of the first Earth science missions to undergo the senior review process. During multiple subsequent senior reviews Terra has been consistently rated very highly for its contribution to both science and applications.

Terra is an example of a large strategic Earth science mission with international, multi-NASA center, and academic participation. Colocation of the instruments on one platform provides the opportunity to make simultaneous measurements required to elucidate important functional links between and among processes in the Earth system. Colocation also allows for each of the instruments to benefit from robust on-orbit services such as redundant power and communication systems and a dedicated mission operations team capable of coordinating on-orbit management and station-keeping activities benefiting all five instruments. While the overall mission was directed from Goddard Space Flight Center, Terra hosted both PI-class and facility-class instruments on one platform.

The Terra science team was commensurately quite large and composed of teams representing each instrument in proportion to their separate investment and science goals. MODIS, representing a very broad set of science disciplines and with a sister sensor on Aqua (the second in the series of EOS missions), has a science team of nearly 100 scientists.

In 2005 the EOS Project Office estimated that EOS (Terra, Aqua, and Aura collectively) was supporting more than 330 separate investigations with over 770 researchers. Of those, 260 were graduate students or postdoctorate-level staff. While just a snapshot, this approximate level of support persisted for these missions, providing a stable environment for learning and skill development across a meaningful period of time inside academia and the agency. In contrast, the science teams for PI instruments are much smaller and supported for a shorter period of time.

Historically, Terra’s mission budget was divided into two components: mission operations and research and analysis (R&A). Initially, the science or R&A portion of the Terra mission was closely tied to and managed by the mission itself. Announcements of opportunity issued through NASA/HQ were specifically designed to address the mission and its science requirements. In extended mission phase, efforts were made to reduce mission operations costs, and the R&A portion of the Terra mission not directly tied to mission science management or PI instruments was folded back into the Earth science budget.⁶

Alternatives to Large Strategic Missions in Earth Science

The Earth science community has grown out of a broad set of academic disciplines distributed over the focus areas in Earth science that include climate variability and change, atmospheric composition, carbon cycle and ecosystems, water and energy cycle, weather, and Earth surface and interior. Earth science as a singular discipline or academic entity has evolved to incorporate a wide range of what were once separate classic science disciplines, and therefore significant differences in perspective, methods, and vocabulary still exist within it. As a result it is difficult for a single or even several large strategic missions to satisfy all perspectives or goals, let alone distribute resources to such a large, varied community.

Even though the large strategic missions get very high ratings during mission extension senior reviews, the community has been calling for more opportunities via smaller, faster, and less-expensive missions. Smaller missions allow for new approaches, new measurements, and new players to get involved. Smaller missions can definitely provide opportunity for new approaches and measurements in shorter timescales over the large strategic scale missions, but they do not necessarily provide better or “more fair” access to funding resources for scientists. How funds are made available within either the large strategic or smaller mission approaches can be managed in such a way as to minimize the differences. The Earth Science Division has done this by maintaining the highly

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⁶ Senior reviews are designed to assess and justify mission continuation beyond the original scheduled budgetary lifetime of the mission. Senior reviews do not cover only the success of the mission for meeting its defined science goals and cost; the scientific value of both the goals and results of the mission is reassessed by the scientific community at that point. The cost versus value of continuing the mission must be justified, and a window of opportunity can be defined (if budget allows) to make use of the mission to create new measurements or data products called for by the science community.

rated large strategic missions in extended mission phase and separately managing the R&A components. Not having to build new missions to provide the data output by these missions and detaching a portion of the R&A budgets formerly allocated to them frees up funding for new analysis opportunities.

Keeping new missions smaller allows for a more rapid response to changing community-led science goals and measurement requirements. The potential pitfall in this approach is that with no centralizing large strategic missions, NASA may lose the opportunity to lead both scientifically and internationally.

As this study was concluding, the second Earth science decadal survey was still under way. This committee concluded that the issue of the future role of large strategic missions in Earth science was best left to the decadal survey to address. However, this committee notes that large strategic missions have served Earth science as a sort of laboratory facility that can be accessed by many members of the community, in some ways similar to the way NASA’s astrophysics Great Observatories have served their community. This may be harder to achieve with a series of smaller, more numerous spacecraft with shorter lifetimes.

Balance in Earth Science

Large strategic missions are important to maintaining a balanced program in Earth science, as they provide needed high-quality data across interdisciplinary boundaries on the timescales required, and create the conditions for leadership within a highly diverse set of disciplines. In the late 1980s NASA created the Earth Observing System (EOS), consisting of several large strategic Earth science missions. These EOS missions—Terra, Aqua, and Aura—with their multi-instrument suites enabled discoveries about Earth’s interconnected biogeochemical, climate, weather, and solid Earth systems. Collectively, the EOS missions, and the associated distributed active archive system for disseminating data, laid the groundwork for the evolution of Earth system science supported by satellite observation, and engendered the development of Earth system science research centers in government agencies and academia worldwide. Large strategic Earth science missions also benefit continued technological development. The instruments on the EOS platforms are widely emulated internationally, and the results from the research and development of EOS instruments are fueling the next generation of Earth science missions from large to small.

The A-Train, a constellation of spacecraft including large missions Aqua and Aura and smaller missions CloudSat and CALIPSO, demonstrates how Earth system science goals can be achieved with instruments on multiple spacecraft, as opposed to requiring a single platform for all instruments making complementary measurements.

The study of fundamental Earth system processes requires studying an enormous range of spatial and temporal scales. This can sometimes be accomplished only by using large strategic missions’ capabilities. However, the global coverage and multiple sensors requested in Earth sciences call for a variety of missions in different orbital inclinations and altitudes, with a large variety of instruments sometimes mutually exclusive, a goal that obviously cannot be fulfilled by a single large mission.

Large strategic missions provide opportunities for advancing science around an organizing entity that have not commonly been achieved by smaller missions. A large centralizing mission focusing the community on a specific set of issues with resources sufficient to train and maintain a scientific workforce over longer timescales is a key element of leadership (e.g., EOS). A larger enterprise can bring stability to the community. A portfolio of constellation missions could provide similar focus and stability to the community, but only if sufficient resources were allocated and rapid replenishment of the small missions was ensured. While they provide excellent opportunities to make quick advances, weighting a portfolio toward many small missions runs the risk of becoming diffuse, difficult to manage, and leaderless. These risks can be and are, to some extent, addressed by expanding the definition of a “mission” to include an “enterprise” structure that integrates missions of various sizes, both at the same time (e.g., the A-Train) and between missions over decades to support long-term trending and satellite and algorithm intercalibration. This would require a structure and would be different from simple augmentation of the R&A budgets supporting small and medium-size missions. Whether the enterprise concept needs to be vetted by the decadal surveys can be debated, but the enterprise concept could provide a structure to a portfolio less constrained by mission size yet support both the agility and stability required to serve the goals of the science community and maintain a talented workforce.

HELIOPHYSICS SCIENCE DIVISION

Almost by definition heliophysics requires taking many different measurements in many different locations in order to build up a systematic understanding of heliophysical phenomena. Heliophysics instruments themselves are generally not large, but some measurements require that the instruments be taken to difficult locations, which can require more complicated and expensive spacecraft.

Not all NASA heliophysics is conducted within the Heliophysics Science Division or even within NASA. There is currently important heliophysics science being conducted on the MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft orbiting Mars, the Juno spacecraft orbiting Jupiter, and the New Horizons and Voyager spacecraft at the edge of the solar system. These spacecraft all started out as planetary missions that benefit the study of heliophysics overall. Voyager was reconfigured in 1990 as the Voyager Interstellar Mission to reflect its new role of exploring the edge of the solar system. In addition, the National Science Foundation (NSF’s) Geospace Section started work with solar and space physics CubeSats that is now being taken over by NASA. The NSF’s Astronomy Section has also funded ground-based solar observations.

Although heliophysics still requires large strategic missions to make important advances in the field, these are not necessarily large single spacecraft like the Parker Solar Probe currently under development. Scientists have proposed developing large constellations of small spacecraft to take many measurements across great distances. The development and integration of dozens to potentially hundreds of small spacecraft can result in a mission that is both strategic—vital to the development of the science discipline—and also large in terms of cost and complexity.

As in Earth sciences, the study of fundamental heliospheric processes requires studying an enormous range of spatial and temporal scales that can sometimes be accomplished only by using large strategic mission capabilities.⁷ To understand why the solar wind exists, for example, requires placing a spacecraft into the atmosphere of the Sun, within at least 11 solar radii of the Sun’s center. To understand the complex interaction of the solar wind with the interstellar medium requires measurements be made out to 200+ astronomical units (AU). Heliophysics processes can have natural multidimensional spatial gradients of 1 to several AU (e.g., cosmic ray modulation) and timescales that range from electron and ion inverse gyrofrequencies (magnetic reconnection over milliseconds) to the 11- and 22-year solar cycle timescales, meaning that spacecraft instrumental capabilities need to have very high cadences (spatial and temporal), the ability to make simultaneous multipoint measurements, as well as great longevity. To accomplish measurements and observations over such spatial and temporal heliophysical scales often demands complex instrument suites, clusters of spacecraft, multiple widely separated spacecraft, and challenging orbits, meaning that large strategic missions, defined in many different ways, are necessary to further advance the field of heliophysics.

Parker Solar Probe

NASA currently is operating the Magnetospheric Multiscale (MMS) mission, which the heliophysics decadal survey considered to be a “major space mission.” The previous large strategic mission in heliophysics was the joint European Space Agency (ESA)-NASA Ulysses mission to the Sun launched in 1990 and operated until 2009. In 2009 NASA began development of the Parker Solar Probe mission, which is scheduled to launch in 2018 and will send a spacecraft to within 3.67 million miles of the Sun’s photosphere. (Solar Probe Plus was renamed Parker Solar Probe in May 2017.) This will be the closest any spacecraft has traveled to the Sun and requires a spacecraft specifically designed to deal with the intense heat it will encounter. (See Figure 2.3.)

The ability to identify and quantify detailed information regarding the Sun is critical for the advancement of our understanding of stellar properties for scientific knowledge about distant stars in our universe. The lack of data on our Sun has created a gap in the knowledge that we have on stellar bodies throughout our universe. The Parker Solar Probe spacecraft is designed to withstand temperatures up to 1,370 degrees Celsius and solar radiation intensities 475 times higher than encountered on Earth. In order to gain enough momentum to reach a close

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⁷ NRC, Solar and Space Physics: A Science for a Technological Society, The National Academies Press, Washington, D.C., 2012.

distance from the Sun, the spacecraft will have to complete seven flybys of Venus and plans its first approach of the Sun is to be concluded in the winter of 2024.

The goals of the Parker Solar Probe mission include the following:

• Determining the structure and dynamics of the magnetic fields at the sources of solar wind,
• Tracing the flow of energy that heats the Sun’s corona and accelerates the solar wind, and
• Determining what mechanisms accelerate and transport energetic particles.

The payload on the Parker Solar Probe includes four instruments:

1. Field Measurement Experiment (FIELDS)—field measurements, led by the University of California, Berkeley.
2. Integrated Science Investigation of the Sun (ISIS)—energetic particle mass spectrometer, led by Princeton University.
3. Solar Wind Electrons Alphas and Protons (SWEAP)—plasma and solar wind particle counter, led by the University of Michigan.
4. Wide-Field Imager for Parker Solar Probe (WISPR)—coronal imager, led by the Naval Research Laboratory.

Alternatives to Large Strategic Missions in Heliophysics Science

NASA operates many smaller-size heliophysics missions, including Advanced Composition Explorer (ACE), Solar Terrestrial Relations Observatory (STEREO), Two Wide-angle Imaging Neutral-Atom Spectrometers (TWINS), Van Allen Probes, Wind Experiment (WIND), and others. Although many heliophysics spacecraft are physically small, some missions, such as the four-spacecraft MMS mission, involve multiple spacecraft gathering data in concert, thereby increasing the mission’s cost and complexity. The advent of CubeSat technology has already demonstrated the potential of this platform to support science measurements with small constellations of spacecraft for very targeted measurements. Larger small satellite constellations will continue to evolve to support specific multipoint large spatial-scale observations as the technology continues to mature.

Balance in Heliophysics Science

In the field of heliophysics, some of the most foundational discoveries of the Sun–Earth interaction have not been made and can be made only with large strategic missions.

Where heliophysics differs somewhat from the other space science fields is that the definition of what qualifies as a large strategic mission is changing. Rather than a single spacecraft, or a pair of spacecraft, in the future heliophysics may require the development of constellations of multiple spacecraft, perhaps even dozens of them. The overall cost and complexity of such missions will probably approach the level of the more traditional large strategic mission class.

PLANETARY SCIENCE DIVISION

Planetary science is conducted with large strategic as well as competed missions. Planetary science requires large strategic missions for several reasons: because the destinations/targets are difficult to reach and because of a need to carry a comprehensive instrument suite to a far or difficult destination to make multiple observations. For example, although Mars is relatively easier to reach in terms of distance, with travel times of less than a year, actually landing on the planet is difficult. Furthermore, the types of science goals NASA is pursuing for Mars are more complex than before. In general, outer-planet missions represent both of these qualities because they are distant and may require special power sources, and because after traveling all that distance it is more cost-effective to carry multiple instruments. Some outer-planet missions can encounter very harsh environments, such as high radiation, which increases mission costs.

Although both Mars landers and orbiters have been conducted as competed missions, Mars exploration also requires large strategic missions, which increase access to a wide range of important science objects at high-priority targets. For example, the Mars Science Laboratory (MSL)/Curiosity mission consists of a large rover with a substantial onboard scientific laboratory.

These missions also have an impact on current and future scientific communities through contributions to development and the demonstration of new technologies that are applicable to new missions and can answer fundamental science questions. There are different types of mission campaigns, and a series of competed missions launched over a long period of time (such as the numerous NASA Mars missions launched between 1998 and 2011) can serve a strategic purpose as well.

NASA currently has two large strategic missions under development, the Mars 2020 rover and the Europa Clipper. Mars 2020 is part of NASA’s ongoing study and exploration of Mars to seek to provide answers to critical questions such as: Did life ever arise on Mars and, if so, is it still there? Was the early climate more Earth-like and why did it change? Is Mars an appropriate destination for human exploration?⁸ Europa Clipper is the current iteration of a Europa mission that was prioritized in the 2011 planetary science decadal survey. Europa has one of the youngest surfaces in the solar system, with copious cryovolcanism, Earth-like tectonic activity, and potentially

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⁸ See, for example, NRC, Vision and Voyages for Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C., 2011, p. 76.

geysers and plumes. The subsurface ocean on Europa may provide the best chance for extant life beyond Earth. The Europa mission’s science goal and objective are primarily to investigate Europa’s habitability.

The complexity needed for a large strategic mission depends on the maturity of exploration of the target, and the level of detail necessary to move forward on answering the most fundamental questions of the mission. As the information and observation become more sophisticated, so do the questions. For example, the observation that Mars may be habitable would lead to the question of whether it was ever inhabited. Deciding between a large mission with many instruments or several smaller missions with more focused science depends on the risks and accessibility, in addition to cost variances, surrounding the missions. Larger missions can provide both the infrastructure and framework for smaller missions to fill strategic gaps.

The value and breadth of large strategic missions for planetary science is demonstrated by NASA’s Cassini mission to Saturn.

Cassini

At the time this report was being written, NASA was preparing the end of life for the Cassini spacecraft. The mission is supported through international collaboration, with contributions of hardware and scientists from over 27 countries. A major aspect of the mission was the Huygens probe, which was funded by the European Space Agency (ESA) and descended to the surface of Titan in 2005, providing unprecedented information about the moon.

The Cassini-Huygens mission comprised a Saturn orbiter from NASA and the Huygens Titan probe from ESA, with 18 instruments (12 for the orbiter, and 6 for the probe). With 27 investigations, there were 10 interdisciplinary scientists, and over 300 scientists on investigation teams, half in Europe. (See Figure 2.4.)

Cassini carries a suite of 12 complementary instruments:

1. CIRS, or the Composite Infrared Spectrometer, captures infrared light, breaks it down into component wavelengths, and measures the strength of the light. This instrument assists in determining the celestial object’s temperature and composition.
2. ISS, or the Imaging Science Subsystem, serves as the main camera for observing the Saturn system. The instrument consists of two digital cameras: one of a wide-angle range and one of a narrow-angle range to primarily observe visible light.
3. UVIS, or the Ultraviolet Imaging Spectrograph, creates images by capturing ultraviolet light, breaking it down into component wavelengths, and measuring the strength of the light. Scientists use this instrument to study Saturn’s rings, atmosphere, and multiple moons.
4. VIMS, or the Visible and Infrared Mapping Spectrometer, captures visible and infrared light and separates the two to determine characteristics in the Saturn system such as the content and temperature of the atmospheres, rings, and surfaces.
5. CAPS, or the Cassini Plasma Spectrometer, comprises three sensors: the electron sensor, the ion mass sensor, and the ion beam sensor, in which a particle travels and then is assessed on its kinetic energy and the direction in which it was traveling. Additionally, the ion mass spectrometer measures the particle’s mass. This instrument gives scientists insight on the composition, density, flow, velocity, and temperature of the ions and electrons in Saturn’s magnetosphere.
6. CDA, or the Cosmic Dust Analyzer, detects small dust particles and determines their charge, velocity, size, and composition. This instrument is helpful to examine the dust that orbits in Saturn’s system—some of which is from the surface of a celestial object in the system, whereas other particles are from beyond our solar system.
7. INMS, or the Ion and Neutral Mass Spectrometer, determines the composition and structure of positive ions and neutral molecules in Titan’s atmosphere, Saturn’s magnetosphere, and the ring environment.
8. MAG, or the Magnetometer, records the direction and strength of magnetic fields around Cassini. As the spacecraft orbits Saturn, this instrument collects data regarding the varying strength and direction of the planet’s magnetic field in different locations. This helps scientists learn about the composition and density of the celestial bodies in Saturn’s system.

9. MIMI, or the Magnetospheric Imaging Instrument, captures information regarding the charged particles of Saturn’s magnetosphere and how the magnetosphere interacts with the solar wind. This instrument comprises three sensors that detect protons, electrons, ions, and neutrons.
10. RPWS, or the Radio and Plasma Wave Science, uses a suite of antennas and sensors to detect radio and plasma waves through Cassini’s path. The instrument uses three types of sensors to study the intensity of waves across a broad frequency range in order to better understand the relationship between celestial objects in the Saturn system.
11. Radar, or the Radar instrument, has the ability to send radio waves at the surface and create pictures of the landscape by recording the differences in the signal’s arrival time and wavelength at the spacecraft. This instrument is primarily used to study Titan.
12. RSS, or the Radio Science Subsystem, sends radio signals to objects within the Saturn system in order to depict the composition of those objects. This instrument transmits the radio signals back to Earth, where scientists can determine the gravity fields, atmospheric structure, composition, ring structure, particle sizes, and surface properties within the Saturn system.

Cassini’s mission to Saturn studied many different aspects of the planet, including Saturn’s magnetic field, gravity field, ionosphere, radiation belts, and the rings. Huygens and Cassini had different prime mission science

objectives. Beyond the prime mission objectives, there were also multiple priorities for seasonal and temporal change, and new questions asked during the solstice mission that followed the prime mission. Cassini Saturn science also complements the Juno mission to Jupiter. Current study opportunities include analyzing the gravitational and magnetic fields of Saturn’s internal structure, age and mass of the main rings, and direct sampling of Saturn’s atmosphere.

• Rings: Cassini has captured extraordinary observations regarding Saturn’s ring–moon interactions, such as observing the lowest ring temperature ever recorded on Saturn, discovering that the moon Enceladus is embedded in Saturn’s E-ring, and viewing images of the rings at Saturn’s equinox when the Sun strikes the objects in orbit. The close-up interactions with Saturn’s rings have uncovered information regarding their composition—the rings are composed of particles ranging in size from micrometers to meters.
• Titan: Cassini was the first space mission to map Titan’s surface, study its atmosphere, discover liquid water beneath its surface, and send a probe to Titan’s surface. Cassini-Huygens revealed that Titan has lakes and seas of liquid methane and ethane that are replenished by rain from hydrocarbon clouds and that Titan has a liquid ocean beneath its surface likely composed of water and ammonia.
• Enceladus: Cassini’s discoveries uncovered the unsolved mysteries of the luminescent moon. Cassini revealed that both the coating on its surface and materials in the E-ring originate from a subsurface saltwater ocean. Cassini identified geyser-like jets that expose water vapor and ice particles beneath the surface of the moon. In turn this discovery has become a promising lead for life exploration in other worlds.
• Moons: Cassini uncovered knowledge regarding Saturn’s dozens of moons. These moons vary in size—they could range anywhere from the size of the planet Mercury to the size of a sports arena. This greatly enhanced our understanding of Saturn’s planetary and ring composition.
• Magnetosphere: The revelation of Saturn’s giant magnetosphere encompassing the gas giant has provided powerful insights to the planet’s atmosphere. This magnetosphere exerts a powerful influence on the space environment around Saturn, making it complicated to determine the planet’s rotation. However, this discovery has given scientists a great deal of insight on how the magnetic field around Earth operates.

Cassini provides significant contributions in advancing science:

• There are now extensive data on Saturn’s system.
• There is also more insight into other gas giant systems that could provide more questions and answers about Jupiter and the ice giants.
• With the calibration of exoplanet observations, there could be further observations into the use of ground theoretical models for exoplanet researchers.
• Science data and results are complementary in both cost and responsibility across international space organizations and countries.

The Cassini program scientist identified many advantages to the international collaborative approach used for Cassini. The Huygens probe included in the mission and built and paid for by European partners would probably not have been affordable. In addition, European funding prevented Cassini from being canceled in the 1990s, and later, U.S. support enabled extended mission funding for European scientists. Since 2004 the Cassini-Huygens mission has conducted research in five separate science disciplines with multiple targets and cross-disciplinary discoveries with international collaboration among 27 countries.

Cassini’s program scientist also stated that larger missions with higher overall life-cycle costs clearly have a much greater scientific impact than smaller missions. There have been thousands of published papers to date on various topics, and numerous Ph.D.s and postdoctorates produced and trained as a result of the mission. Also, many collaborative efforts and international relationships formed, while also increasing the depth and breadth of discoveries. In addition, longer duration missions provide better opportunities to develop effective processes, procedures, and tools that can be used for future missions. The wealth of data collected by Cassini will provide

study opportunities and materials for many years into the future, something previously demonstrated by prior large missions such as Voyager and Galileo.

Alternatives to Large Strategic Missions in Planetary Science

NASA’s Planetary Science Division has two other mission classes, both led by principal investigators: Discovery and New Frontiers. Discovery has a cost cap of approximately $425 million (excluding launch costs), and New Frontiers has a cost cap of approximately $850 million (excluding launch costs). Discovery mission targets are entirely at the discretion of teams proposing the missions and in the past have included asteroids, comets, and Moon and Mars missions. There have also been Discovery proposals for Venus and outer-planet missions that were not selected. In general the cost cap has limited Discovery missions to inner solar system targets. New Frontiers missions are competitively selected based on a list that has been developed by the decadal survey process. The most recent decadal survey recommended two New Frontiers AOs for the coming decade and a cadence of one new Discovery mission approximately every 36 months. Thus, generally speaking, the planetary decadal survey recommended a program of one large strategic mission, two New Frontiers, and at least three Discovery missions for the 2013-2023 period. To date the Discovery program cadence has been lower than recommended by the last decadal survey, with only two missions selected during the period. This will decrease the amount of science return. This subject is likely to be addressed by the planetary science midterm study, which was under way at the time this report was published.

An example of technology development occurring in smaller-size missions is NASA’s Dawn mission. Thanks to NASA’s prior investment in solar electric propulsion (SEP), which was tested extensively on the Deep Space 1 mission, this technology enables Discovery-class missions now to reach solar system destinations that previously would have been available only to large strategic missions, or even entirely unattainable. Dawn’s use of SEP allowed it to become the only spacecraft ever to have the capability to orbit and explore two distant destinations, Vesta and the dwarf planet Ceres, the two largest objects in the main asteroid belt. This mission would have been impossible for a single spacecraft with conventional chemical propulsion, even for a large strategic mission. The Discovery mission Psyche has recently been selected and will orbit the main belt asteroid of the same name to investigate whether it is the exposed metal core of a protoplanet that has broken up. These missions illustrate how goals that would have been very expensive or unaffordable can be accomplished by smaller missions with new technologies.

Recently, the Planetary Science Division implemented a program to explore the development of CubeSats for planetary missions. In March 2017 NASA selected 10 studies under the Planetary Science Deep Space SmallSat Studies (PSDS3) program. These studies involve mission concepts using small satellites at the Moon, asteroids, Mars, Venus, and the outer planets. These small satellites weigh less than 180 kilograms. CubeSats have many limitations for planetary missions. What tends to make them attractive for Earth-orbiting applications—their inexpensiveness and small size—comes at the price of short lifetimes and relatively low reliability (although both are improving). CubeSats, or even small satellites like those being evaluated as part of the PSDS3 program, are unlikely to conduct planetary science missions on their own, but will probably be carried to their destination aboard larger spacecraft.

Prior to 2011 NASA had a separate Mars Scout line of missions that was approximately equivalent to the Discovery program in size and scope. The Mars Scout missions were part of a coordinated Mars exploration campaign and demonstrated the benefits of a more rapid cadence of small missions, including the ability to adapt new missions to recent discoveries and to rapidly produce scientific results, creating a robust and dynamic scientific community. The Mars Scout line was discontinued after two missions (Phoenix and MAVEN) and Mars missions were allowed to compete as part of the Discovery program.

Balance in Planetary Science

Planetary science relies on large strategic missions both for accessing distant and unforgiving targets and also for carrying large suites of instruments to conduct synergistic observations in planetary environments. These

missions have provided a key component of scientific productivity across all aspects of the field. Cassini and the MSL/Curiosity rover both support hundreds of scientists and have produced thousands of publications during their lifetimes. In addition, these large missions host tens of participating scientists each, providing an on-ramp to junior scientists and others not involved in the design and development of the mission. These participating scientists infuse the missions with diverse insight and expertise while gaining important experience in leadership in mission planning and execution. New Frontiers and Discovery-class missions, as well as other Mars missions, are smaller in scale and scientific scope. Nevertheless, these smaller missions provide the community with more frequent mission opportunities with focused scientific investigations. All of these investigations provide the opportunity for technology development and advancement.

An unbalanced planetary science program could occur if either a single (or in rare cases, two) large strategic missions consume the majority of the planetary science budget and NASA cannot fund smaller missions. Alternatively, an unbalanced program would be one where there are no opportunities for large strategic missions during a decade.

ASSESSING BALANCE FOR NASA’S FOUR DIVISIONS AND THE ROLE OF THE DECADAL SURVEYS

Each scientific discipline—and the NASA division responsible for it—has different goals, interests, cultures, and history. This committee was tasked with addressing “general principles that SMD could use (e.g., a figure-of-merit approach) to trade off within limited budget between development and operation of large strategic missions and the cadence and or/cost caps of medium-size and small principal investigator (PI)-led mission lines.”

After much deliberation, the committee concluded that there is no single figure-of-merit approach that could be developed to apply to all four scientific disciplines, nor was it appropriate for this committee to seek to supersede the guidance that is already provided to NASA by the decadal surveys. Balance can be decided only by the decadal surveys themselves. Their definition of balance is likely to change over time, and therefore has to be revisited over subsequent decadal surveys and assessed during the relevant decadal midterm reviews. Furthermore, the definition of balance provided by the decadal surveys is likely the only one that will satisfy a diverse community.

The committee also considered whether it was possible to develop different figures of merit for different disciplines. Although that might be possible, it would require substantial expertise in each of the disciplines, which a cross-disciplinary committee cannot possess but discipline-focused committees could possibly provide. The committee notes that if SMD seeks “general principles” to trade off within limited budget between development and operation of large strategic missions and its medium and smaller mission lines, any such principles cannot be too general or they will not be very useful. In addition, those principles will be most helpful if they are timely and targeted to the areas most in need of help. The committee notes that NASA’s advisory structure at the National Academies was recently revised to enable discipline committees for each of the space science disciplines to respond to the needs of NASA’s science divisions in a more timely manner. This and other advisory structure changes could greatly assist NASA in making those decisions.

Although the committee did not think that a “figure-of-merit” approach was wise or possible, the committee did conclude that there are many general principles that can be applied to all of the NASA science mission divisions. The committee was reminded of both the importance and the strength of the decadal survey process for each division and impressed that NASA has previously sought to have the decadal surveys learn from each other. The committee’s recommendations encompass providing better inputs into the decadal surveys and noting that when a decadal survey is insufficient, NASA has other advisory paths to seek specific input—for instance, for reprioritization or redirection of a program during a midterm review.

This report cannot have the credibility that specifically tailored advice will have for unique problems that arise, and NASA benefits by relying on its advisory structure where the relevant scientific communities will have the most input.

Having examined the data that the committee requested from NASA, the committee did conclude that there were approaches (discussed later) that could be adopted by the decadal surveys and adapted to their specific requirements that could be of benefit to the Science Mission Directorate in achieving or maintaining balance. Each of the four scientific disciplines approaches its investigations differently. Nevertheless, there are some common characteristics to all of the disciplines.

Decadal surveys provide community-consensus science priorities and recommendations for space and Earth science, principally to NASA and NSF, but also to the U.S. Department of Energy, the National Oceanic and Atmospheric Administration (NOAA), the U.S. Geological Survey, the White House, and Congress.⁹ The National Academies have conducted decadal surveys for more than 50 years, since astronomers first developed a strategic plan for ground-based astronomy in 1964. The committees and panels that carry out the decadal surveys are drawn from the broad community associated with the discipline in review, and involve many of the leading scientists and engineers in that discipline. The National Academies’ decadal surveys are notable in their ability to thoroughly sample the research interests, aspirations, and needs of a scientific community. Decadal survey reports to agencies and other government entities play a critical role in defining the nation’s agenda in that science area for the following 10 years, and often beyond. In particular, decadal surveys have become the accepted guidance documents to the divisions within NASA’s Science Mission Directorate.¹⁰ The decadal surveys are now mandated by law in the NASA authorization acts. For example, the National Aeronautics and Space Administration Transition Authorization Act of 2017 states that “the Administrator should set science priorities by following the guidance provided by the scientific community through the National Academies of Sciences, Engineering, and Medicine’s decadal surveys.” Each division is charged with advancing the state of knowledge in the relevant fields of astrophysics, Earth science, heliophysics, and planetary science. To accomplish that goal, each division is also charged with advancing the state of technology, as appropriate, which may include both hardware (e.g., instrumentation and techniques) and software (e.g., modeling and processing techniques and implementations and simulations).

The decadal surveys’ recommended science programs are constructed by their relevant committees based on assumptions about available budgets in the forthcoming decade. To be successfully implemented, programs advocated in decadal surveys also have to be flexible to budgetary realities, but also based on sound, adequate cost

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⁹ See, for example, NASEM, Space Science Decadal Surveys, 2015.

¹⁰ National Aeronautics and Space Administration Transition Authorization Act of 2017 (P.L. 115-110).

estimates. Such realities can take the form both of external forcings such as evolving current and out-year available budgets, as well as internal forcings, such as cost overruns or unexpected inefficiencies in a given program line or mission implementation. To minimize the latter, it is crucial that the responsible NASA division adequately fund and carry out preliminary (pre-Phase A and Phase A) studies for such missions in a way comparable to those executed by proposal teams working on competed missions. At the same time, in order to enable progress on a wide variety of fronts in a timely fashion and maintain programmatic flexibility, each division has historically implemented a variety of missions, projects, and initiatives, which can roughly be divided into small, medium-size, and large cost categories. Exact definitions of these categories in terms of absolute costs and kinds of missions have varied over the decades (from the establishment of NASA), and they vary across the SMD divisions and the corresponding decadal surveys.

The nature of decadal surveys is that the budget projections that are provided to the decadal survey steering groups are developed by the executive branch, not by Congress. Thus, if decadal surveys are tasked by the agencies to narrowly confine their recommendations to fit within the budget projections, they could be prone to excluding mission implementations that might offer revolutionary science results by the application of advanced technologies that could have been developed if the scientific community had made a convincing case for them and budgets are provided. Decadal surveys thus have competing objectives both to be realistic and to have “a license to dream.”

In order to enable decadal committees to propose new and potentially revolutionary missions that might not fit within existing budget projections, the recent decadal surveys have been tasked to project different possible budget levels and what can be accomplished for each. These budget levels can accommodate different numbers and mixes of missions, but they could also accommodate different implementations of large strategic missions if those missions have design and implementation flexibility.

At the same time, budget projections provided to the surveys can turn out to be optimistic, in which case it is also imperative for there to be in place decision rules that minimize any potential damage to a decadal survey’s science plan. Such scenarios must be discussed with extreme caution in order that they not become self-fulfilling. Nonetheless, simply ignoring such possibilities can lead to even more harm to the affected disciplines.

All of the recent decadal surveys have described in detail the highest priority science questions and frontiers, and science is the primary focus of the decadal surveys. However, although the science questions and opportunities change over time, technology also changes over time, sometimes providing opportunities that were not apparent or available when a decadal survey was initiated.

In general terms, missions prioritized in the decadal surveys aim to be aspirational and visionary while still being feasible and affordable. Several of the last round of decadal surveys were created under challenging budget constraints, as required by their statements of task. Looking forward, when developing statements of task for future surveys it will be necessary to ensure that the charges to the decadal committees do not lead to their being overly cautious and conservative in the missions they prioritize.

The use of decision rules by recent decadal surveys provides an opportunity to consider more ambitious science goals and associated missions. For example, the planetary science decadal survey provided decision rules that articulated science goals associated with Mars, Europa, and Uranus with a decision tree where missions were prioritized contingent upon reaching certain budget targets.¹¹ In some cases these were difficult challenges—such as directing that the proposed baseline mission cost half as much as the mission concept that was originally evaluated using the survey’s cost and technical evaluation (CATE) process. The objective of the CATE process is to perform a cost and technical risk analysis for a set of concepts that may have a broad range of maturity, and to ensure that the analysis is consistent, fair, and informed by historical data. Typically, concepts evaluated via the CATE process are early in their life cycles, and therefore are likely to undergo significant subsequent design changes. Historically, such changes have resulted in cost growth. Therefore, a robust process is required that fairly treats a concept of low maturity relative to one that has undergone several iterations and review. CATEs take into account several components of risk assessment. Because the CATE is best suited to the comparative evaluation of a family of pre-Phase A concepts, it was the methodology used in the decadal surveys and is best suited to the early-phase analysis of strategic missions. After the release of the planetary science decadal survey report,¹² several project teams were able to redesign or de-scope their mission concepts to fit a cost goal. (This is discussed further in Chapter 3.)

These kinds of opportunities for de-scoping are best accomplished if the decadal survey stipulates the boundary conditions such as minimum science and maximum budget, and what science may be accomplished at different levels of technical capability (which can translate to cost), allowing NASA and the scientific community to make trade-offs within the provided guidelines. There is also the possibility that new technology or a less-costly approach may emerge within a decade, and therefore providing the decision makers with an “on-ramp” would be beneficial for the pursuit of the scientific program. Allowing for opportunities to de-scope large strategic missions would result in the opportunity for more new mission concepts to be initiated within the decade within the boundaries of the recommended scientific program from the decadal survey.

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¹¹ The decision rules included flying the MAX-C rover only if the mission could be conducted at a cost to NASA of less than or equal to $2.5 billion FY2015, and flying the Jupiter Europa Orbiter mission only if changes to both the mission and the NASA planetary budget make it affordable without eliminating other recommended missions. If less overall funding was available, NASA was advised to de-scope or delay “flagship” missions, slip New Frontiers and/or Discovery missions only if adjustments to “flagship” missions could not solve the problem, and place high priority on preserving R&A and technology development funding.

¹² NRC, Vision and Voyages, 2011.

Flexibility in mission implementation may identify opportunities where small and medium-size missions can provide measurements as effective as large strategic missions—for example, where constellations of simultaneous (i.e., multipoint) measurements are desired in solar and space physics as well as in Earth science. Small and medium-size missions offer the capability to make high-priority measurements where metrics such as cost, development time, observation cadence, and ability to integrate new technology can be optimized. Smaller missions can also accept greater development and operational risk. Furthermore, they can also provide benefits when compared to large strategic missions where sustained/continuity measurements are needed and enabled by rapid deployment/replacements of spacecraft as well as to scientific objectives that are more easily reached.¹³

CONCLUSION

Large strategic missions continue to have value for all of NASA’s space science disciplines. It is not possible for NASA to abandon large strategic missions simply because they can be challenging and still maintain world leadership in the space sciences. The primary argument that has emerged against such large strategic missions is their cost, particularly when costs run far over the original estimates. However, large cost overruns on large missions are not inevitable. There are methods both to better predict and plan for a mission’s cost and to better manage missions so that costs can be controlled. NASA has achieved success at both of these efforts in the past decade. That is the subject of the next chapter.

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¹³ For further information, see NASEM, Space Science Decadal Surveys, 2015.