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

How did the universe begin and evolve? How did we get here? Where are we going? Are we alone? The big questions of astronomy and the majesty of the universe capture the imagination of the public and excite young people’s interest in science and technology. The public’s enthusiasm for the images returned by the Hubble Space Telescope and people’s perennial interest in black holes and cosmology are just two examples.

The fundamental goal of astronomy and astrophysics is to understand how the universe and all its constituents formed, how they evolved, and what their destiny will be. To determine the optimum strategy for addressing these big questions, the astronomy and astrophysics community initiated the fifth of its so-called decadal surveys in 1999.¹ The results of the survey committee’s deliberations, published in 2001 as Astronomy and Astrophysics in the New Millennium (hereafter the AAp decadal survey), advised that astronomers do the following:²

• Map the galaxies, gas, and dark matter and dark energy in the universe as these evolve through cosmic time, and survey the gas, stars, and planets in the galaxy;

• Use the universe as a unique laboratory for probing the laws of physics in regimes not accessible on Earth, such as the very early universe or near the event horizon of a black hole;

• Search for life beyond Earth, and if it is found, determine its nature and distribution in the Milky Way galaxy; and

• Develop a conceptual framework that accounts for all that astronomers have observed.

The AAp decadal survey concluded that the key problems for astronomers and astrophysicists to address in the coming decade were the following:

• Determine the large-scale properties of the universe—the amount, distribution, and nature of its matter and energy; its age; and the history of its expansion;

• Study the dawn of the modern universe when the first stars and galaxies formed;

• Understand the formation and evolution of black holes of all sizes;

• Study the formation of stars and their planetary systems, and the birth and evolution of giant and terrestrial planets; and

• Understand how the astronomical environment affects Earth.

As discussed below, new discoveries since the publication of the AAp decadal survey motivated the development of a subsequent report on the interface between physics and astrophysics, Connecting Quarks with the Cosmos.³

To achieve the priority goals listed above, the AAp decadal survey selected as its highest priorities a number of ground- and space-based initiatives, grouped according to cost as major, moderate, and small.^a

The major space-based initiatives recommended are, in priority order, as follows:

1. Next Generation Space Telescope. A large telescope optimized for near-mid-infrared imaging and spectroscopy;^b

2. Constellation-X. A suite of four x-ray telescopes optimized for high-throughput spectroscopic observations;

3. Terrestrial Planet Finder (TPF). A telescope system intended to image faint planets orbiting nearby Sun-like stars;^c and

4. Single Aperture Far Infrared Observatory. A large telescope designed to study the important but relatively unexplored spectral region between 30 and 300 microns (μm).

The moderate space-based initiatives include, in priority order, the following:

1. Gamma-ray Large Area Space Telescope. A joint NASA-Department of Energy mission to study gamma rays with energies from 10 MeV to 300 GeV;

2. Laser Interferometer Space Antenna. A gravitational wave detector sensitive to radiation in the 0.1- to 100-mHz band likely to be emitted by merging supermassive black holes and close binary stars;

3. Solar Dynamics Observatory. A telescope to study the Sun’s outer convection zone and the structure of the solar corona;

4. Energetic X-ray Imaging Survey Telescope. An instrument designed to map the highly variable, hard x-ray sky; and

5. Advanced Radio Interferometry between Space and Earth. An orbiting radio antenna designed to work in conjunction with ground-based radio arrays to provide high-resolution observations of active galactic nuclei.

In addition, the AAp decadal survey strongly encouraged the continued development of small space missions (e.g., sounding rockets, Explorer-class and Discovery-class principal investigator (PI)-led missions). These provide low-cost opportunities to test new ideas or to use groundbreaking new technologies, and also serve to give personnel experience in mission development and implementation.

The two most startling observational discoveries of recent years in astronomy and astrophysics are as follows:

• Super-massive black holes are present at the center of virtually all galaxies; and

• The familiar forms of matter (e.g., gas, stars, and planets) represent only about 4 percent of the mass-energy in the local universe, and most of the content of the universe is dark energy and dark matter.

Recent theoretical predictions have also motivated new experimental work. The two most notable predictions are the following:

• Gravitational waves created during inflation when the universe was some 10⁻³⁷ s old can make a detectable signature in the polarization of the cosmic microwave background and may also be detectable directly; and

• “Cosmic censorship” can be violated, and black holes can have “quantum hair.” Cosmic censorship is the widely held speculation, originally espoused by Roger Penrose, that a gravitational singularity is always shrouded by an event horizon so that it can never be seen by a distant observer. “Quantum hair” refers to possible deviations from the conventional view developed by Stephen Hawking and Roger Penrose that the only external clues to the nature of material swallowed up by a black hole are manifested in terms of changes to the hole’s mass, angular momentum, and electric charge.

Some 26 percent of the mass-energy in the universe is in a mysterious “dark” form of matter, which might be exotic fundamental particles as yet not known from accelerator experiments, or might be black holes or some as-yet-undreamed-of objects. In addition, some 70 percent of the mass-energy is in an even more mysterious constituent, the so-called dark energy, which has exotic properties such as a sound speed of nearly the speed of light and exerts a negative pressure that is pulling the universe apart.

The fact that 96 percent of the universe is in the form of matter of an unknown nature has convinced most astronomers that a new fundamental physics is needed to understand the universe. But these frontiers of physics—grand unified theories of particles and their interactions, string theory, quantum gravity—have proved to be difficult to test in the conditions accessible to ground-based experiments. Much greater extremes of energy and gravity are reached in astronomical objects such as supernovas and black holes, as well as in the early universe. Many physicists have thus become convinced that astronomical studies hold keys to the future development of fundamental physics.

Recognition of the increasing importance of the close interplay among astronomy, astrophysics, and fundamental physics led to the publication in 2003 of Connecting Quarks with the Cosmos,⁴ a report that made recommendations and defined priorities for the field, informed by the developments since the release of the AAp decadal survey. The principal questions of interest identified were as follows:

• What is dark matter?

• What is the nature of dark energy?

• How did the universe begin?

• Did Einstein have the last word on gravity?

• What are the masses of the neutrinos and how have they shaped the evolution of the universe?

• How do cosmic accelerators work, and what are they accelerating?

• Are protons unstable?

• What are the new states of matter at exceedingly high density and temperature?

• Are there additional space-time dimensions?

• How were the elements from iron to uranium made?

• Is there a need for a new theory of matter and light at the highest energies?

Connecting Quarks with the Cosmos reaffirmed, with increased importance, two space missions—the Laser Interferometer Space Antenna (LISA) and Constellation-X—already listed as priorities of the AAp decadal survey. The report also recommended two new space-based missions that were not listed as priorities in the 2001 astronomy decadal survey:

• A mission to measure the polarization of the cosmic microwave background radiation; and

• A wide-field optical imaging telescope in space to investigate the properties of the dark energy.

NASA’s response to Connecting Quarks with the Cosmos was the 2003 Beyond Einstein roadmap.⁵ The latter document, which remains of vital interest and importance to the astronomical and physics communities,⁶ sought funding for LISA and Constellation-X (as facility-class missions) and for a line of moderate-cost Einstein Probes. The first three recommended Einstein Probes are as follows:

• Dark Energy Probe. A wide-field optical imaging telescope in space;

• Inflation Probe. A mission to measure the polarization of the cosmic microwave background radiation; and

• Black Hole Probe. A mission to survey the universe for hidden black holes.

Access to space uniquely enables astronomers and astrophysicists to achieve many of the goals of the AAp decadal survey and Connecting Quarks with the Cosmos. There is a great diversity of objects in the universe. Many of the important objects reveal themselves only through particular types of radiation. To discover the interactions between these diverse objects, and to thus gain an understanding of the workings of the universe, requires the study of both bright and faint objects at many different wavebands. Using a wide range of different detectors of electromagnetic and gravitational radiation, astronomers seek the diverse objects hiding in the universe.

Astronomy is inextricably tied to observations made from space because most of this radiation is absorbed and blurred by Earth’s atmosphere. X-rays, gamma rays, ultraviolet light, and most infrared radiation cannot penetrate Earth’s atmosphere: space-based instruments are the only way to discover what constituents of the universe emit this radiation. The changing gravitational fields from moving bodies on Earth’s surface make it impossible, on Earth, to discriminate gravitational waves with frequencies below 1 Hz. Thus, astronomers go to space to escape the atmosphere and the thermal, radiation, and seismic environment of Earth. With a very few exceptions discussed below, any space observatory close to but well separated from Earth (e.g., at the Sun-Earth L2 point, such as the Wilkinson Microwave Anisotropy Probe [WMAP], or in an Earth-trailing, 1-AU orbit, such as the Spitzer Space Telescope) can perform as well as one located anywhere else in the solar system or beyond.

The implementation of NASA’s new exploration initiative should, at first glance, have a positive impact on astronomy and astrophysics. The report of the Aldridge Commission,⁷ charged to define a plan for implementing the initiative, proposes a notional science agenda that is consistent with the science goals outlined in NASA’s Beyond Einstein roadmap.⁸ Nevertheless, members of the astronomy and astrophysics community are concerned that current and future budgetary pressures will impact the implementation of the science priorities in the AAp decadal survey and in Connecting Quarks with the Cosmos.⁹

As indicated above, the diverse contents of the universe span a vast range of brightness and distance and emit an enormous diversity of radiation, only narrow bands of which can penetrate Earth’s atmosphere. Other objects of interest may not be visible at all and can be studied only through detection of their gravitational radiation or their influence on the surrounding medium.

Many of the important scientific priorities in astronomy and astrophysics can be addressed most easily through small, low-cost, focused space missions. An excellent example is the tremendous success of WMAP in revolutionizing understanding of the early evolution of our universe. Other investigations require medium-sized, space-based facilities. Only the most wide-reaching scientific problems should require development of large observatory-class spacecraft such as the James Webb Space Telescope (JWST), LISA, or Constellation-X. Telescope and detector technologies evolve and improve rapidly, making small-scale testing of new paradigm-breaking instruments and techniques vital. The AAp decadal survey recognized that regular access to a variety of mission opportunities in many wavebands is essential to the health of astrophysical science, stating that “NASA should continue to encourage the development of a diverse range of mission sizes, including small, moderate, and major, to ensure the most effective returns from the U.S. space program.”¹⁰

Unlike explorers of Earth and the solar system, astronomers cannot get better views of the distant reaches of the universe by moving their space-based telescopes closer to the object under study. Nor can they determine the characteristics of extrasolar planets, for example, by actively probing them with high-powered instruments. Astronomical discoveries are made simply by pointing telescopes in the appropriate directions and then looking farther, longer, or with a better resolution than ever before. Astronomers using space-based instruments accomplish this by doing one or more of the following:

• Building bigger and/or more telescopes. Space-based telescopes are limited by the size and mass that can be launched from Earth to Earth-escape, and also by the high cost of launch. The past few decades have not seen significant improvements in launch size, mass, or cost. If this trend continues, bigger telescopes will instead be enabled by technological advances in such areas as large lightweight mirrors and support structures, precision metrology and formation-flying capabilities, long-lifetime lasers, and in-space deployment and assembly.

• Pushing the limits of detector technology until the laws of physics prevent further improvements. New, more sensitive detectors with more pixels and better time or spectral resolution, and improved cryogenic techniques to reduce local backgrounds, are often low-cost substitutes for bigger telescopes.

• Designing detectors with wider fields of view to study more objects at once.

Astronomical observations typically involve passive, low-noise activities, which thrive with the least possible disturbance from local effects. For example, observatories seek to minimize the diffuse background of photons and cosmic rays, the thermal loading on the telescope from the Sun or Earth, and contamination of mirror surfaces. Similarly, most of the fundamental physics missions carried out so far have been either passive ranging experiments in near-Earth space (e.g., Lunar Laser Ranging and LAGEOS) or experiments on Earth-orbiting platforms (e.g., the Lambda Point and Confined Helium experiments, and Gravity Probe B).

These shared characteristics imply that the potential benefits of large nuclear power sources and propulsion systems are less apparent for astrophysics and fundamental physics missions than for planetary exploration. The following sections explore some relevant considerations and discuss some potential niche applications of nuclear systems.

Astrophysics and fundamental physics missions make very light demands on propulsion systems once the missions reach their observing orbits. It is possible, however, to conceive of a few very specific applications in which nuclear propulsion systems might allow researchers access to favorable observing locations that would otherwise be unattainable:

• Generating long baselines between two or more telescopes. Among the various possibilities are the following:

• Geometrical parallax. Telescopes separated by many tens to hundreds of AU can potentially improve the astronomical distance scale. (See “Geometrical Parallax Mapper” in the section “Generating Very Long Baselines” in Chapter 8.)

• Localizing gamma-ray bursts (GRBs). A very small and low-power GRB detector on an interstellar-probe type mission (see Chapter 4) would provide GRB positions with arc-second accuracy even if there were no afterglow. Although these detectors cannot operate near a fission reactor, they have operated successfully on spacecraft powered by radioisotope power systems (RPSs). (See “Gamma-Ray Burst Locator” in the section “Generating Very Long Baselines” in Chapter 8.)

• Radio interferometry. Interferometers operating at long radio wavelengths require baselines of more than 1 AU to provide very high resolution observations. (See “Long-Baseline Radio Interferometer” in the section “Generating Very Long Baselines” in Chapter 8.)

• Microlensing parallax. Telescopes separated by distances of up to a few AU can be used to disentangle the effects of the tranverse-velocity and the mass of the lensing object. (See “Microlensing Parallax Mapper” in the section “Permitting More Favorable Observing Locations: Accessing Special Alignments” in Chapter 8.)

• Permitting observations from a more favorable location. To date, astronomers have made use of spacecraft in low and geosynchronous orbits about Earth (e.g., the Hubble Space Telescope and the International Ultraviolet Explorer, respectively), at the Sun-Earth L2 point (e.g., the Wilkinson Microwave Anisotropy Probe), and in 1-AU heliocentric orbits that gradually drift away from Earth (e.g., the Spitzer Space Telescope). But other locations can be more favorable in various ways, including the following:

  • Other locations can provide a lower diffuse background, rendering background-limited observations more sensitive, and can vastly reduce the level of uncertainty in cosmic background observations. Moving a telescope from 1 AU to between 3 and 5 AU from the Sun can reduce the background by a factor of up to 100. This improvement occurs only for optical through far-infrared wavelengths. For ultraviolet and shorter wavelengths, and for wavelengths of 100 μm and longer, the foreground from interplanetary matter is less prominent. At these greater distances from the Sun, RPSs are certainly competitive in cost and weight with solar panels as a way to provide power. (See “5-AU Optical/Near-Infrared Observatory” in the section “Permitting More Favorable Observing Locations: Beyond 3 AU” in Chapter 8.)

  • They can enable the use of a cooler telescope. Cooler telescopes make infrared observations much more sensitive, extend the lifetime of cryostats, and make the job of cryocoolers much easier. Since the outer skin of the Spitzer Space Telescope is passively cooled to 35 K at 1 AU from the Sun, it is clear that careful attention to using sunshades may be a more economical approach to creating a cooler telescope. (See “5-AU Far-Infrared Observatory” in the section “Permitting More Favorable Observing Locations: Beyond 3 AU” in Chapter 8.)

  • They can provide an environment free of manmade radio-frequency interference. This is extremely important for low-frequency radio astronomy. A radio observatory on the farside of the Moon could well be an application for an RPS, since operation through a lunar night relying on solar cells and batteries will be quite difficult. (See “Farside Radio Observatory” in the section “Permitting More Favorable Observing Locations: The Moon and Moons of Mars” in Chapter 8.)

• Accessing special alignments. By placing a telescope at carefully selected places in the solar system, it may be possible to make use of gravitational lensing to obtain ultrahigh-resolution images of extragalactic objects. Possibilities include the following:

  • Some of the lines connecting known active galactic nuclei (AGN) and known binary stars pass within tens of AU of the Sun. A telescope located on such a line could use the caustics from the binary star to undertake extremely high resolution observations of the AGN. (See “Binary-Star Gravitational Telescope” in the section “Permitting More Favorable Observing Locations: Accessing Special Alignments” in Chapter 8.)

  • The Sun itself can be used as a gravitational lens. The telescope would have to be located at least 550 AU from the Sun. Indeed, effective shielding against the light from the Sun would require placement at distances greater than 800 AU and very large occulting disks. (See “Solar Gravitational Telescope” in the section “Permitting More Favorable Observing Locations: Accessing Special Alignments” in Chapter 8.)

Current and planned astrophysics missions are in general not constrained by the currently available power. Operating even the biggest telescopes and detectors does not require large amounts of electrical power. Although the capabilities of onboard computers have increased exponentially, electrical power requirements have remained nearly constant. Major missions have electrical power requirements in the range of 1 to 10 kWe (see Table 7.1), requirements that solar power meets with only a minor impact on the cost, weight, and risk of missions. Indeed, the current cost for solar-electric power on spacecraft is low enough that it is only a minor component of mission cost at 1 AU, whereas the cost estimated for nuclear-electric power is substantially higher (Figure 1.1). There may conceivably be electrical power needs in the range of 10 kilowatts or greater in the foreseeable future for massive onboard computation or for lasers on gravitational-wave detectors.

Although access to sources of electric power over a wide power range is important, it is likely that except for a few niche applications (see, e.g., “Lunar Astronomical Observatory” in the section “Permitting More Favorable Observing Locations: The Moon and Moons of Mars” in Chapter 8), solar power will remain more cost- and weight-effective than nuclear power for applications near 1 AU. If in fact the currently estimated kg/kWe for nuclear fission reactors, multiplied by the cost of launch (in $/kg) to an escape trajectory, is much higher than the cost of solar power, even a reactor supplied at zero cost to the mission would not be cost-effective.

Transmitting the increasing volumes of data from, for example, wide-field detectors back to Earth could consume large amounts of radio power, but Shannon’s theorem suggests that it is generally far more cost-effective to increase the bandwidth and collecting area of the ground-based receivers than to provide each mission with more powerful transmitters. As indicated in Table 7.1, the expected data-transmission rates of future major astronomy and astrophysics missions are, with a few notable exceptions, expected to be relatively modest.

Onboard detectors in nearly all experiments reach quantum-limited levels of sensitivity and hence tend to be quite sensitive to disturbance by local high-energy particles and photons. The effects of other environmental factors such as vibrations, stray magnetic fields, and outgassing of spacecraft components are also of concern. The radiation emanating from nuclear power systems, particularly fission reactors, is especially worrisome because it can significantly degrade the sensitivity of astrophysical observations. This issue is most acute for x-ray and gamma-ray observatories, which are by design exquisitely sensitive to exactly the types of radiation produced by fission reactors. Shielding of nuclear power systems is therefore a leading issue in their application to astrophysical missions.

1. The four previous decadal surveys are as follows: Committee on Science and Public Policy, National Academy of Sciences, Ground-Based Astronomy: A Ten-Year Program, National Academy of Sciences, Washington, D.C., 1964; Committee on Science and Public Policy, National Academy of Sciences, Astronomy and Astrophysics for the 1970’s, National Academy of Sciences, Washington, D.C., 1973; Assembly of Mathematical and Physical Sciences, National Research Council, Astronomy and Astrophysics for the 1980’s, National Academy Press, Washington, D.C., 1982; and Board on Physics and Astronomy, National Research Council, The Decade of Discovery in Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1991.

2. National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001.

3. National Research Council, Connecting Quarks with the Cosmos—Eleven Science Questions for the New Century, The National Academies Press, Washington, D.C., 2003.

4. National Research Council, Connecting Quarks with the Cosmos—Eleven Science Questions for the New Century, The National Academies Press, Washington, D.C., 2003.

5. The Structure and Evolution of the Universe Roadmap Team, Beyond Einstein: From the Big Bang to Black Holes, NP-2002-10-510-GSFC, National Aeronautics and Space Administration, Washington, D.C., 2003.

6. The February 11, 2005, letter report of the NRC’s Committee to Review Progress in Astronomy and Astrophysics Toward the Decadal Vision (The National Academies Press, Washington, D.C., 2005) commented that the Beyond Einstein roadmap “is an excellent implementation and synthesis of the [AAp decadal survey] and Connecting Quarks with the Cosmos.”

7. 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.

8. National Research Council, “Review of Progress in Astronomy and Astrophysics Toward the Decadal Vision: Letter Report,” The National Academies Press, Washington, D.C., 2005, p. 11.

9. National Research Council, “Review of Progress in Astronomy and Astrophysics Toward the Decadal Vision: Letter Report,” The National Academies Press, Washington, D.C., 2005, p. 10.

10. National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001, p. 28.