Priorities in Space Science Enabled by Nuclear Power and Propulsion (2006)

How did the Sun’s retinue of planets originate and evolve? How did life develop in the solar system? How do basic physical and chemical processes determine the main characteristics of the planets? These are some of the fundamental questions of solar system exploration that motivate planetary science, a discipline that encompasses the study of objects in our solar neighborhood, including planets similar to Earth and Jupiter; smaller bodies like asteroids, comets, and Kuiper Belt objects; and larger features such as planetary magnetic fields and magnetospheres. Increasingly, the field also includes the characterization of extrasolar planets. Planetary science tries to understand not only the basic physical properties of planetary bodies, but also the processes responsible for the formation and evolution of planets.

Solar system exploration pushes the frontiers of human knowledge to worlds beyond our planet, revealing a diversity of nature and processes that challenge current comprehension. The excitement of discovery and the challenge of exploration fuel the human spirit and inspire our imagination. We are born to be explorers. Our successful exploration answers old questions and raises new ones, challenging us to probe farther and deeper.

In 2001, the U.S. planetary science community initiated a major study to outline pressing scientific questions and prioritize future solar system exploration missions and programs. The results of their efforts are embodied in two volumes, New Frontiers in the Solar System: An Integrated Exploration Strategy (hereafter, the solar system exploration [SSE] decadal survey)¹ and a compilation of contributed papers published under the title The Future of Solar System Exploration.² As explained in the SSE decadal survey, the key scientific questions to be addressed in the coming decade are as follows:

• What processes marked the initial stages of planet formation?

• Over what period did the gas giants form, and how did the birth of the ice giants (Uranus, Neptune) differ from that of Jupiter and its gas-giant sibling, Saturn?

• How did the flux of objects impacting planetary bodies decay during the solar system’s youth, and in what ways(s) did this decline influence the timing of life’s emergence on Earth?

• What is the history of volatile compounds, especially water, across our solar system?

• What is the nature of organic material in our solar system, and how has this matter evolved?

• What global mechanisms affect the evolution of volatiles on planetary bodies?

• What planetary processes are responsible for generating and sustaining habitable worlds, and where are the habitable zones in the solar system?

• Does (or did) life exist in the solar system, beyond Earth?

• Why have the terrestrial planets differed so dramatically in their evolutions?

• What hazards do solar system objects present to Earth’s biosphere?

• How do the processes that shape the contemporary character of planetary bodies operate and interact?

• What does our solar system tell us about the development and evolution of extrasolar planetary systems, and vice versa?

Addressing these key questions will, as discussed in the SSE decadal survey, require a combination of large, medium, and small space- and ground-based projects backed up by theoretical and laboratory studies, and related research and data-analysis programs.

In response to ground rules set by NASA,^a the SSE decadal survey prioritized spacecraft missions in large (>$650 million), medium (between $325 million and $650 million), and small (<$325 million) cost categories^b and ranked non-Mars and Mars missions separately. Thus, the highest-priority large, non-Mars and Mars missions were, respectively:

• Europa Geophysical Explorer. An orbiter of Jupiter’s ice-encrusted satellite to assess the nature and depth of its putative ocean; and

• Mars Sample Return. A program to return several samples of the Red Planet to Earth for studies to search for life, develop chronology, and define ground truth.

Priorities in the SSE decadal survey for medium-cost missions to destinations other than Mars were, in priority order, as follows:

1. Kuiper Belt-Pluto Explorer. A flyby mission of several Kuiper Belt objects, including Pluto/Charon, to discover their physical nature and understand their endowment of volatiles;^c

2. South Pole-Aitken Basin Sample Return. A mission to collect and return to Earth samples from the solar system’s largest and deepest impact basin, which pierces the Moon’s crust and may expose the lunar mantle;^d

3. Jupiter Polar Orbiter with Probes. A mission consisting of a close-orbiting polar spacecraft equipped with various instruments that also acts as a relay for three probes to make in situ measurements of the jovian atmosphere below the 100+ bar level;^e

4. Venus In Situ Explorer. A mission to acquire and lift a core sample of Venus into the atmosphere for compositional analysis and to make simultaneous atmospheric measurements; and

5. Comet Surface Sample Return. A mission to return several pieces of a comet’s surface to Earth for organic analysis.

Priorities for medium-cost missions to Mars in the SSE decadal survey were, in priority order, as follows:

1. Mars Science Laboratory. A lander to carry out sophisticated surface observations and to validate sample-return technologies;^f and

2. Mars Long-Lived Lander Network. A globally distributed suite of landers equipped to make comprehensive measurements of the planet’s interior, surface, and atmosphere.

Priorities in the SSE decadal survey for low-cost missions to destinations other than Mars were, in priority order, as follows:

1. Discovery. A continuing line of innovative, principal-investigator-led exploration missions, to be launched every 18 months; and

2. Cassini Extended. An extension of the planned operational life of the comprehensive, multidisciplinary Cassini orbiter mission at Saturn.

Priorities for low-cost missions to Mars were, in priority order, as follows:

1. Mars Scout. A continuing line of missions similar in concept to Discovery, to be launched at a rate of one for every other Mars-launch opportunity; and

2. Mars Upper Atmosphere Orbiter. A spacecraft dedicated to studies of Mars’s upper atmosphere and plasma environment.

In the short time since the completion of the SSE decadal survey, ongoing discoveries have not resulted in major changes in the recommended priorities. Several areas of current research do, however, have the potential for generating priority-altering major discoveries. Indeed, results from Cassini in the Saturn system (including Titan), as well as ongoing results from the Spirit and Opportunity rovers on Mars, present particular opportunities for major discoveries that could shift priorities and drive programmatic decisions. These possibilities and a few other promising research directions are examined in subsequent sections.

The resilience of the rovers Spirit and Opportunity, and the wealth of data they have gathered on Mars, are opening a new chapter in scientists’ understanding of the Red Planet’s early history. Discoveries of stratigraphic layers, evaporite deposits, and mineral forms show clearly that Mars experienced a somewhat Earth-like warmer and wetter era.^3,4 Questions remain as to how this era came to be and how Mars changed to its current cold and dry climate.

Another significant set of results from Mars concerns the spectroscopic detection of methane in the planet’s atmosphere by ground-based telescopes^5,6 and the Mars Express spacecraft.⁷ Although the result obtained from Mars Express is still somewhat controversial, all three sets of observations indicate methane at concentrations of about 10 parts per billion (ppb). This is significant in that methane is unstable in the martian atmosphere and would disappear in ~300 years if not continuously replenished. Although the origin of the methane has not yet been determined, possible sources include volcanic activity, chemical reactions between water and iron-bearing minerals in a hydrothermal system, and biological activity.⁸

Cassini’s exploration of Saturn and its moons and rings has only just begun. Nevertheless, the successful descent of the Huygens probe through Titan’s atmosphere and the bonus of an unexpectedly long period of surface observations have confirmed some longstanding expectations and revealed some intriguing new characteristics of Saturn’s largest satellite.⁹ Images from the Huygens descent imager, for example, showed features highly reminiscent of drainage channels and shorelines. Similarly, images obtained on Titan’s surface appear to show icy pebbles rounded, perhaps, as a result of fluvial activity; standing bodies of liquid hydrocarbons, however, were not seen.

Data from the Huygens probe on the variations of temperature and pressure as a function of altitude were virtually indistinguishable from the values that were expected on the basis of models derived from observations made during Voyager 2’s flyby in 1981. However, the atmosphere appears to lack the expected argon (in the form of ³⁶Ar and ³⁸Ar), krypton, and xenon—a possible sign that Titan accreted at a somewhat higher temperature than previously expected. Huygens’ instruments did, though, detect ⁴⁰Ar, a daughter product of ⁴⁰K released from Titan’s interior, perhaps as a result of cryovolcanic activity. Chemical analyses performed by Huygens’ gas chromatograph/mass spectrometer revealed that methane became more abundant relative to nitrogen as the proximity to Titan’s surface increased. Huygens also registered a sharp increase in methane abundance soon after landing, possibly indicating the presence of liquid methane just below Titan’s surface.

The instruments on Cassini itself are also returning important data, whose significance is still not entirely clear. Images of Titan’s surface show few craters, indicating a geologically active world. The surface does not, however, appear to exhibit any significant compositional variations. Data from initial radar investigations of Titan’s surface—covering just a few percent of the globe—are intriguing, and continued radar mapping, in conjunction with the ground truth provided by the Huygens landing, will improve researchers’ understanding of Titan as an active world.

Another highly unexpected finding was the detection of significant amounts of benzene in Titan’s upper atmosphere, as determined by in situ analysis performed during one of Cassini’s first close flybys. With so many tantalizing initial findings and with Cassini scheduled to make many dozens of additional Titan flybys during the next 3 years, it is clear that Titan will be a prime objective for additional studies long after Cassini itself has ceased operation.

The search for Earth-like planets and habitable environments around other stars is a key goal for planetary scientists as well as an important programmatic goal for NASA. The search for extrasolar planets is best addressed by a combination of ground- and space-based surveys and a better theoretical understanding of the processes leading to the formation of planetary systems. New observational results highlight the need for a systematic approach to better understanding the outer solar system, including in situ sampling of the giant planets. For example, a striking correlation has been discovered between the metallicity of a host star and the probability of its harboring one or more giant planets.¹⁰ One interpretation of these results is that the metallicity of a protostellar gas-dust disk is related to the mass and number of solid cores that can grow in the nebula to trigger the collapse of hydrogen-rich planets.¹¹

Studies of this type are emblematic of the natural synergy between astrophysics and planetary science. Indeed, extrasolar planetary research forms a continuum ranging from astrophysical studies relating to the search for, and astronomical characterization of, planets orbiting other stars to planetary studies concerning physical characteristics of extrasolar planets—e.g., their structure, atmospheric chemistry, and biological potential. Researchers need to understand the solar system’s giant planets more fully to place the new discoveries concerning extrasolar planets into context.

These new astrophysical results demonstrate the importance of carrying out in situ measurements of the metallicity of the solar system’s giant planets. Recent downward revisions of the estimates of carbon and oxygen abundances in the Sun indicate the importance of in situ measurements of these elements in the atmospheres of the

solar system’s four giant planets.¹² In the cases of Saturn, Uranus, and Neptune, in situ measurements of helium may yield results of great astrophysical significance.

In the last decade, NASA has dispatched robotic missions to Mars, the jovian and saturnian systems, asteroids, and comets. To the public, these have been successes and have both spurred increases in the breadth of the discipline (e.g., the development of astrobiology as a distinct, but related, scientific endeavor) and sustained a healthy planetary science community. Much of this program has been driven by science and discovery. NASA’s new exploration initiative,^13,14 initiated following President Bush’s January 14, 2004, policy speech on space, is built on these successes.

The new exploration initiative has already had an impact on the field of planetary science, and although it is too early to fully appreciate all of the ramifications, some general points are clear. Planetary science, as a field, is strongly supported in the new initiative. An increased emphasis on robotic missions to the Moon and Mars is, to the planetary science community, the most strongly supported component of the initiative articulated to date. For example, immediate responses to the President’s speech have been the initiation of the Lunar Reconnaissance Orbiter, scheduled for launch in 2008; the inclusion of a Mars Scout mission in 2011; and planning for the launch of additional missions to Mars starting in 2013. Top SSE decadal survey science priorities that are consistent with the new exploration initiative include the exploration of the Moon and Mars within the context of a program of comparative terrestrial planetology.

The boundaries are not yet clear regarding what science objectives for the Moon and Mars should be addressed by remote sensing and robotic exploration, and what should be addressed by human exploration. The most sensible strategy is to push remote sensing and robotic exploration to its limits, leaving human explorers to address those objectives that most strongly require a human presence.

Concerns remain, though. A balanced program of planetary exploration, whether science- or mission-driven, should include a portfolio of diverse exploration activities directed toward diverse planetary bodies. A major concern is that the rest of the solar system, exclusive of the Moon and Mars, may be inadequately represented in the current program sequence. To understand the differences among Earth, the Moon, and Mars, for example, will require additional exploration of Venus (Earth’s twin in size), and care will be needed early to identify and plan for key outer solar system missions. Some might argue that outer solar system science is robust because of Cassini’s ongoing exploration of the saturnian system, the recent launch of the New Horizons mission to Pluto, and the selection of the Juno mission to Jupiter. However, Cassini’s scope is finite, New Horizons will not reach Pluto until 2015, and Juno will not launch until 2010 at the earliest.

An especially troubling issue for the planetary science community is the relative scarcity of missions to the outer solar system. If launches occur only once every 10 to 15 years, then interest in this area will fade because of the absence of continuing activities and new results that attract established researchers and new students alike to the field. Even with the advanced propulsion systems that are currently in development, missions to the outer solar system will require flight times of a decade or more. If the interval between launches is factored in as well, then the time for such a mission from conception to data return rapidly approaches, and may exceed, the professional lifespan of the average researcher.

For the planetary science community, a particularly positive aspect of the new exploration initiative is the recognition that nuclear power sources—both fission reactors and radioisotope systems—are important enablers for future human and robotic voyages beyond low Earth orbit. Reaching distant destinations and exploring in new ways both depend on having adequate power for long-term operation and survival. However, when the SSE decadal survey was drafted, only one fully fueled RPS—a Cassini spare—remained in the inventory available for NASA’s use. Indeed, it was far from clear to the survey’s authors whether additional RPSs, let alone more exotic nuclear technologies, would be available for future high-priority missions. For this reason, the SSE decadal survey placed a very high priority on the development of advanced RPSs and nuclear-electric propulsion systems.¹⁵ With adequate power, once-future visions of exploration can become reality: whether roving along the winter ice cap of

Mars, exploring Titan’s methane playas, surviving in the long term on the surface of Venus, or sending long-lived probes into the atmospheres of the outer planets, just to name a few.

To address their scientific priorities planetary scientists use a combination of in situ, remote-sensing, laboratory, and theoretical studies. Of these approaches, the in situ and remote-sensing studies conducted by landers, orbiters, rovers, and other types of robotic spacecraft dispatched to diverse planetary bodies are the most apparent and are the focus of this chapter, which, together with Chapter 6, discusses how the capabilities of nuclear power and/or propulsion can significantly enhance or enable a broad range of solar system exploration missions.

A typical robotic planetary mission is likely to require many, if not all, of the following characteristics:

• Propulsion to get the spacecraft to its destination. All robotic spacecraft require propulsion systems to get them to their destinations. Current chemical systems are adequate to provide access to the Moon, Mars, and Venus, but missions to Jupiter and beyond are much more challenging in the demands they place on propulsion systems. Chemically propelled spacecraft, enhanced with gravity assists, can travel to the outer solar system. Once there, their instruments and communications systems can be powered by an RPS. However, the mass and power limits of such spacecraft in the outer solar system constrain the choice of the types of instruments that can be used, as well as their sequencing and data-transfer rates. In addition, it is challenging for these craft to make a detailed study of more than one body, even in the same planetary system. For example, while orbiting Jupiter, Galileo was only able to make fast flybys of the Galilean satellites. To travel expeditiously to the outer solar system and then investigate individual bodies for extended periods requires large propulsive capabilities, which may be provided by nuclear-electric or bimodal systems. The Titan Express/Interstellar Pioneer concept (see Box 6.4) is an example of such a propulsion-enabled mission.

• Access to multiple observing locations. The scientific return from robotic spacecraft is greatly enhanced if, for example, a static lander can be replaced with a rover, or if an orbiter can change its orbit, or if a rendezvous mission can visit multiple objects. Conventional chemical propulsion, coupled with gravity assists, has been the mainstay of the solar system exploration program for decades. However, such propulsion systems allow for observation of more than one object only if a spacecraft’s trajectory happens to allow it to pass by additional objects, and even then the spacecraft can go into orbit only around one body, owing to limited fuel. As the era of simple flybys comes to an end and knowledge improves, the desire grows for increasingly sophisticated observations, requiring longer durations in orbit around target bodies. In addition, it is cost-effective and beneficial for comparative science to utilize a single spacecraft to rendezvous with more than one target. An example of such a mission is Dawn, which will use a solar-electric propulsion system to visit the two large, main-belt asteroids Vesta and Ceres. Multiobject rendezvous for many more than two objects or for destinations beyond the asteroid belt is likely to be practical only if nuclear propulsion systems are available. The Neptune-Triton System Explorer concept (see Box 6.5) is an example of a mission with enhanced capability to maneuver.

• Survival in hostile environments. The surfaces and atmospheres of many planets are far from benign, subjecting spacecraft to environmental extremes that challenge spacecraft designers. Spacecraft on the surface of Venus, for example, are subject to very high temperatures and pressures and to corrosive atmospheric constituents; it is not surprising, therefore, that the record for spacecraft operations on the surface of Venus is only about 2 hours. Most science investigations cannot be performed in such a short period. Additionally, sunlight cannot power surface or atmospheric probe experiments if the atmosphere is substantially opaque. An RPS may, however, be used to enable the operation of a refrigerated Venus lander capable of functioning for a month or more. With refrigeration, it is possible to envision a network of long-lived Venus landers equipped to measure seismic and geochemical activity, to study any volcanic emissions, and to quantify the interaction of the surface and the atmosphere. The Long-Lived Venus Lander concept (see Box 6.1) discussed in Chapter 6 is an example of a mission with enhanced capabilities for survival in a hostile environment.

• Power to ensure reliable operation of instruments. Whether a spacecraft is on a simple flyby mission or is a complex rover, it needs a reliable power system to ensure the long-term operation of its subsystems. Although

solar cells coupled with batteries yield enough power for many types of orbital and surface exploration missions, there are other experiments that require sustained power for months to years. When the solar flux is too low (e.g., beyond the asteroid belt), the Sun is not visible for long periods (e.g., during the lunar night), or solar cells are likely to deteriorate over time (e.g., from dust on Mars or radiation damage incurred close to the Sun), then experiments powered by solar power sources have limited utility. However, some experiments, such as monitoring the seismic properties of a planetary body or atmosphere/surface seasonal interactions, require the availability of power over a span of months to years. Such experiments often do not need large quantities of power, but rather need power that can be available continuously for long periods or periodically over long timescales. Advanced RPSs are a solution for such needs, providing moderate power outputs for extended periods. The Long-Lived Mars Network concept (see Box 6.2) is an example of such a power-enabled mission.

• Communications to return data to Earth. High-priority investigations discussed in the SSE decadal survey will generate very large datasets that must be transmitted to Earth. The data rates required for timely transfer of these datasets can outstrip the current capabilities of the Deep Space Network (DSN) as well as of spacecraft telecommunications and power systems. All other parameters such as communication distance and receiver performance being equal, telecommunication systems’ signal strengths (and thus data rates) for two-station systems (i.e., no intervening “repeater” stations) are proportional to three parameters: the transmitted power, the area of the transmitting antenna, and the area of the receiving antenna. Practical approaches to significantly increasing data rates from a given location in the solar system must involve increases in at least one of those three parameters. In the past, spacecraft power and launch constraints limited transmitted power and transmitting antenna size, which tied the limits of data-transmission rates to fixed DSN assets. Nuclear power sources, in particular fission, promise to greatly enhance data-transmission rates via large increases in transmitted power. Some increase in the transmitting aperture might also be possible. Another possibility being explored by NASA is to migrate from radio-frequency to optical communications systems. And a fourth option should be considered: a large increase in ground-based receiving apertures, possibly involving arrays of many antennas that would offer flexibility and simultaneous servicing of multiple missions and would obviate the requirement for high power on all serviced missions.

• Transfer to Earth of samples collected for study. The collection of samples from the surfaces of planetary bodies for return to Earth is an important goal of solar system exploration. The capabilities of analytic instruments available in terrestrial laboratories far exceed what can conceivably be packaged to fit on a planetary spacecraft in the foreseeable future. Ices abound in the solar system. Many of these ices are highly evolved, but some are primitive, enabling studies of material left from the early solar nebula. Although in situ laboratories are useful for the initial studies of these ices, more can be learned by analyzing them in the superior laboratories on Earth. Careful collection and preservation of samples, then, would allow for more in-depth study of the structure of the ices, which would yield information about their deposition and evolution. Holding a sample of ice at low temperatures in space is within the bounds of current technology and can be accomplished with low-power refrigeration or radiators. However, returning these samples to Earth in their ice phase is a very difficult process that will require excellent refrigeration and protection, something that may be advantageously accomplished using RPSs. Cryogenic sample return is a technology that will have to be developed for future cometary and Mars polar-sample missions. The Cryogenic Comet Sample Return mission concept (see Box 6.3) discussed in Chapter 6 is an example of a sample-return-enabled mission.

1. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003.

2. Mark Sykes, ed., The Future of Solar System Exploration 2003–2013—Community Contributions to the NRC Solar System Exploration Decadal Survey, ASP Conference Proceedings 272, Astronomical Society of the Pacific, San Francisco, Calif., 2002.

3. S.W. Squyres et al., “The Spirit Rover’s Athena Science Investigation at Gusev Crater Planum, Mars,” Science 305: 794–799, 2004. Also see subsequent papers (pp. 800–845) in this issue of Science.

4. S.W. Squyres et al., “The Opportunity Rover’s Athena Science Investigation at Meridiani Planum, Mars,” Science 306: 1698–1703, 2004. Also see subsequent papers (pp. 1703–1756) in this issue of Science.

5. M.J. Mumma, R.E. Novak, M.A. DiSanti, and B.P. Bonev, “A Sensitive Search for Methane on Mars,” AAS/DPS 35th Meeting, September 1–6, 2003.

6. V.A. Krasnopolsky, J.P. Maillard, and T.C. Owen, “Detection of Methane in the Martian Atmosphere: Evidence for Life,” European Geophysical Union Meeting, Nice, May 2004.

7. V. Formisano, S. Atreya, T. Encrenaz, N. Ignatiev, and M. Giuranna, “Detection of Methane in the Atmosphere of Mars,” Science 306: 1758–1761, 2004.

8. J.S. Kargel, “Proof for Water, Hints of Life?” Science 306: 1689–1691, 2004.

9. See, for example, T. Owen, “Huygens Rediscovers Titan” and references therein, Nature 438: 756–757, 2005.

10. D.A. Fischer and J.A. Valenti, “Metallicities of Stars with Extrasolar Planets,” Scientific Frontiers in Research on Extrasolar Planets, D. Deming and S. Seager, eds., ASP Conference Series 294, pp. 117–128, 2003; available online at <http://exoplanets.org/iau_proc.pdf>.

11. S. Ida and D.N.C. Lin, “Toward a Deterministic Model of Planet Formation II: The Formation and Retention of Gas Giant Planets Around Stars with a Range of Metallicities,” Astrophysical Journal 616: 567–572, 2004.

12. The solar oxygen and carbon abundances were revised downward by a factor of 1.38 and 1.35, respectively, in: C.A. Prieto, D.L. Lambert, and M. Asplund, Astrophysical Journal Letters 556: 63–66, 2001; and C.A. Prieto and D.L. Lambert, Astrophysical Journal Letters 573: 137–140, 2002. For more details, see for example, W.B. Hubbard, “The Core Problem,” Nature 431: 32, 2004.

13. National Aeronautics and Space Administration, The Vision for Space Exploration: February 2004, National Aeronautics and Space Administration, Washington, D.C., 2004.

14. President’s Commission on Implementation of United States Space Exploration Policy, A Journey to Inspire, Innovate, and Discover, U.S. Government Printing Office, Washington, D.C., 2004.

15. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, pp. 203 and 205.