Outer Solar System Beckons, but Moon/Mars Focus Could Slow Exploration There
Aviation Week & Space Technology
12/13/2004, page 56
Frank Morring, Jr.
Washington
Deep Impact, Cassini/Huygens push science in the outer Solar System, but Moon/Mars focus, nuclear issues could slow follow-ons
DISTANT DESTINATIONS
An upcoming NASA selection between very different robotic spacecraft for a $700-million mission scheduled to fly by 2010 is emblematic of the strains that are likely to beset the U.S. agency--and its international partners--for years to come as they struggle to map the new pay-as-you-go U.S. exploration program.
In choosing between a vehicle to return lunar samples from the Aitkin Basin at the Moon's south pole, and an orbiter that would circle Jupiter from pole to pole to see if the gas giant has a solid core, NASA managers must strike a balance between human space spectaculars close to home and the equally spectacular science possible deeper in space.
DUBBED MOONRISE, the Aitkin Basin mission would play right into President Bush's call for a return to the Moon as a nearby proving ground for eventual human missions to Mars. The "Juno" polar orbiter at Jupiter would provide valuable data about both the formation of our own solar system and what astronomers are seeing when they detect gas giants around other stars.
Each was selected for more study from a group of seven proposals vying to be NASA's next "New Frontiers" mission. Both were high on the list of 10-year priorities for planetary exploration set by the National Research Council (NRC) in 2002. NASA says both are getting equal treatment in the evaluation process that should lead to a selection next spring.
Even so, Moonrise would appear to have the edge. There may be water ice at the lunar south pole that humans working there could use for life support and propellant. Juno would be solar-powered at a time when NASA wants to build advanced nuclear power and propulsion systems for exploration, and has already started a major effort to explore the icy moons of Jupiter with a nuclear-powered orbiter.
That doesn't mean selection of Moonrise is a lock. Ongoing work on the nature and origins of the outer planets ensures there will be a significant scientific constituency for more missions there in parallel with the Moon/Mars push.
"Outer planet research will remain a priority item in Solar System exploration," says Orlando Figueroa, who oversees planetary exploration at NASA headquarters as deputy associate administrator for programs in the Science Mission Directorate. "The [President's] vision speaks of the Moon, Mars and beyond. If there is ever going to be hope for a 'beyond,' investigations and research beyond Mars need to be a strong component of our strategic plan."
The U.S. space agency has enlisted the aid of academia and industry in developing new strategic plans for 13 areas, including Solar System exploration, in light of the new space initiative. As the Solar System planners work, they may be guided by some new science from the "beyond" that Figueroa mentioned.
On Jan. 14 Europe's Huygens probe is scheduled to parachute into the murky atmosphere of Saturn's moon Titan. Delivered to the ringed giant's system by NASA's nuclear-powered Cassini spacecraft, Huygens may be able to penetrate the mysteries hidden by the hydrocarbon haze that shrouds the planet-sized moon to answer such basic questions as whether the surface is solid or liquid (see p. 63).
Half a world away from the Huygens control center in Darmstadt, Germany, controllers at California's Jet Propulsion Laboratory should be guiding the Deep Impact spacecraft toward its explosive rendezvous next summer with the comet Tempel-1, provided it makes its current Jan. 8, 2005, launch date. In July when it reaches the comet--believed to come from the Oort Cloud at the very edge of the Solar System--Deep Impact is designed to blast a sample of the primordial material inside Tempel-1 out where it can be analyzed by remote-sensing instruments in space and on Earth (see p. 64).
Although the Huygens mission will last a few hours at most, NASA is already laying plans to continue the mission of its Cassini mother ship beyond its four-year baseline. That extension was also a priority recommended by the NRC panel in 2002, as was the mission to Pluto and the Kuiper Belt set to launch in 2006 if a number of technical problems can be surmounted.
At the head of the NRC list for the most expensive class of exploration spacecraft was a return to Europa, the moon of Jupiter where the Galileo probe's cameras may have detected the sort of plate tectonics in its water-ice surface that could mean a liquid-water ocean lies below. The NRC-recommended Europa orbiter remains at least nominally at the head of NASA's list too, but now it is one stop on a three-moon visit planned for the Jupiter Icy Moons Orbiter (JIMO), the centerpiece of NASA's effort to develop a space-rated fission reactor for power and propulsion.
The agency has already hired Northrop Grumman Space Technology to work with its own engineers on the preliminary design of the spacecraft, a $400-million contract scheduled to run through mid-2008. NASA and Northrop Grumman will design only the non-nuclear portion of the spacecraft, allowing scientists to develop their own specialized instruments and leaving the nuclear fission reactor that will power JIMO's electric engines and the instruments to the Energy Dept.'s naval reactors office.
In addition to Europa, JIMO would visit Callisto and Ganymede, using the steady thrust of its nuclear-electric propulsion to go into orbit around all three planet-sized moons. There, its reactor would drive power-hungry instruments such as synthetic aperture radars more typical of Sun-bathed Earth orbit than the dim light of the Jovian system. But the reactor and related hardware, such as the big radiators needed to dissipate the reactor's excess heat, are heavy (fuel weight was estimated at 28,600 lb.) and at present there is no way to get JIMO off the ground.
"That one is still in very early phases," Figueroa says. "We have a contractor working with the program office at JPL looking at how we can best implement it. They have tremendous technological challenges, as we knew from the get-go."
JIMO is the first application under consideration for NASA's Project Prometheus, as the space nuclear power initiative is known. Started before Bush outlined his space exploration program, Prometheus is a step toward the fulfillment of a long-held wish by space scientists for the nuclear power sources needed to explore beyond the range of practical solar power. But like Bush's plan itself, JIMO has critics elsewhere in the U.S. scientific community.
"The costs and technical feasibility of this multimoon mission to the Jupiter system (now said to be in the $8-billion-and-up range) are still highly questionable, and there is a high risk that large expenditures will be followed by endless delays, increasing costs and ultimate cancellation," stated a committee of the American Physical Society (APS). "So whether there will be any scientific exploration of the highest priority target, Europa, is very uncertain, and whether JIMO is the best way to carry out the highest priority science has not really been considered by the NRC."
The APS worries that Bush's space plans will consume the U.S. science budget at the expense of "the more global context of a systematic study of nature," and wants a wider review of the issue by the NRC. The council's Space Studies Board, which set the planetary exploration priorities in 2002, met in California last month to begin considering the impact of Bush's Moon/Mars push on those priorities. The work ultimately will play into the strategy NASA is working out to meet Bush's plan.
ONE ISSUE INVOLVES just how much nuclear power is needed to explore deep space. Not all missions to the outer planets necessarily require the high power levels and propulsion capabilities provided by a fission reactor. Some might be more cost-effective if powered by radioisotope thermoelectric generators (RTGs), low-power devices that produce electricity from the natural decay heat of its nuclear fuel. Project Prometheus includes an RTG component, and NASA is getting advice on which missions will need which power source. That could lead to a dispute on where best to place NASA's scarce research dollars.
If Project Prometheus succeeds in developing a space-rated nuclear reactor and demonstrates it at Jupiter with JIMO, there would be strong pressure to use the fission approach for both propulsion and power for other outer-planet missions, and for human power needs on planetary surfaces like the Moon and Mars. NASA already has studied different missions for a JIMO-type spacecraft as a way to amortize the cost of developing the basic reactor hardware.
"From the early days of us pursuing the initiative for fission and next-generation radioisotope systems, we didn't want to stop on JIMO," Figueroa says. "With such an investment, we envision a bus that allowed us to go to other destinations. One of the challenges is to bring the recurring cost of this system to a level where we can have follow-ons for roughly the flagship mission cost range [roughly $3 billion] and visit places that right now are not accessible."
At those prices, some scientists worry that reactor-based exploration might preempt missions that could go sooner with RTGs. And while the strategic planning for Bush's exploration initiative is strictly a U.S. exercise, there is strong international interest in using nuclear power for deep-space missions.
Bush included a call for cooperation with other spacefaring nations to achieve his exploration goals. NASA attracted 17 international space agencies representing 30 nations, including China, India and Argentina, to a workshop on the overall Bush plan last month, and is slating a second international workshop devoted to space science in March.
The European Space Agency has its own "Aurora" robotic planetary exploration program aimed at Mars. ESA may find a logical niche in NASA's new exploration program if Project Prometheus manages to deliver the space-rated nuclear power infrastructure for outer-planet exploration that Europe lacks.
"When we started with the initial call for ideas for Aurora for exploration, Europa was one of the key targets of interest from the scientific community," says Bruno Gardini, ESA's Aurora project manager. "But the problem is that we are limited in what we are doing by the availability of electrical power. You go far away from the Sun, and you get around Jupiter, that's the limit. Rosetta is at the limit of what you can do with the solar array. To have the Rosetta spacecraft working under these conditions at that far distance was a tremendous struggle."
Gardini managed development of ESA's Rosetta comet-exploration mission, which will use solar power to explore and send a lander to the comet 67P/Churyumov-Gerasimenko. ESA also has studied missions to Europa and Pluto, only to reject them over the power issue. The agency has enjoyed close cooperation with NASA on the Cassini/Huygens mission to Saturn, in which the piggyback Huygens probe runs on batteries that draw their power from the U.S.-built radioisotope thermoelectric generator (RTG) in Cassini.
"IF WE WOULD HAVE the chance to contribute instruments to these distant missions, certainly we would be very happy to do so," says Gardini, who attended NASA's first international workshop. "I can only imagine that we would have a positive response to that. The availability of power is really the key."
Exploration in the dim light at Jupiter and beyond generally has a very different focus from the "follow-the-water" search for evidence of past or present life that motivates exploration of Mars. Aside from the possibilities beneath Europa's icy surface, much of the interest in the outer planets and regions like the Kuiper Belt and Oort Cloud, even farther from the Sun, centers on what they can teach us about how planets are formed.
Yet while it lacks the immediacy of a Martian fossil hunt, work in the outer Solar System shares the focus on finding extraterrestrial life that drives much of NASA's science. Astronomers are getting better at finding smaller planets circling other suns, where one day a future generation of space telescopes may find chemical signatures of life as we know it, such as ozone.
"Within the last six months we've actually had the first reports of bodies around other stars that are the size of Uranus and Neptune," says astronomer Heidi B. Hammel of the Space Science Institute. "So we're now probing planets around other stars at that size range, and yet we know very little about these planets in our own Solar System. So if we really want to understand how planets form and evolve in general, in our solar system and in other Solar Systems, it seems to me that understanding those bodies within our own Solar System where we can actually send spacecraft makes a whole lot of sense."
While scientists are still digesting the data collected by Galileo, which was sent to a fiery end in Jupiter's atmosphere on Sept. 21, 2003, to avoid possible contamination of Europa, and grappling with JIMO's technical complexities, the Cassini/Huygens mission has unleashed a flood of data about the next planet away from the Sun. After threading its way through Saturn's famous rings to enter orbit, the probe quickly found two more tiny moons and evidence that there is a lot more "dirt" in the rings than previously believed.
With the Huygens landing on Titan coming up in January, the mission has focused on reconnaissance work at the huge moon. In a flyby, its cameras and radars penetrated the murky atmosphere to provide surface-feature images of unprecedented clarity that suggest Titan is geologically active, and refined earlier estimates of its atmospheric composition. Analysis of Saturn's moon Phoebe, which Cassini encountered on June 11, suggests it was captured by Saturn's gravity and probably formed in the Kuiper Belt or some other region beyond Neptune. A dark material Cassini spotted on Phoebe could be duplicated in the particles found between the icy rings, suggesting to some scientists that the rings are the remains of a disintegrated moon.
CASSINI IS WORKING well, and NASA is indicating it will extend the mission beyond its scheduled end in mid-2008 as the NRC panel suggested. Titan will continue to draw interest even after the anticipated Huygens data set is thoroughly mined, and plans for a follow-on orbiter are already enticing some researchers.
"Clearly orbiting Titan is a good possibility, an orbiter that would not go around Saturn but around Titan, and which will drop at least a couple of long-duration probes, maybe some kind of balloon or other floating vehicle, which would stay in the atmosphere of Titan for several days and weeks," says Jean-Pierre Lebreton, ESA's Huygens mission manager and project scientist. "By floating and being transported on the winds, this [could] provide some mobility to explore from rather low altitude a good portion of the surface."
If Phoebe is indeed a captured Kuiper Belt Object (KBO), the New Horizons mission to Pluto planned for launch in 2006 could ultimately get a close look at KBOs in their home territory. Funded as NASA's first New Frontiers mission, the 465-kg. RTG-powered spacecraft would use an Atlas V launch and a gravity assist from Jupiter to reach the region of Pluto as early as July 2015.
The baseline mission would include a flyby of Pluto at a range of about 11,000 km., followed about 14 min. later by a flyby of its moon Charon at a distance of some 27,000 km. from the surface. Later, it would use its own instruments to home in on one or more KBOs at least 50 km. across as it proceeds into the Kuiper Belt.
Instruments are being built for the probe to provide imaging, spectral mapping in visible and infrared ranges, ultraviolet spectroscopy, radio science and plasma measurements over a period of 150 days at Pluto/Charon, and again in the Kuiper Belt. There is an urgency to start this mission because Pluto's orbit is taking it away from the Sun, which could cause the planet's atmosphere to precipitate out, and when the Jan. 11-Feb. 14, 2006, launch window closes, Jupiter will not be in position to slingshot the spacecraft toward the outermost planet.
Problems with instrument development and procurement of plutonium fuel for the New Horizons RTG threaten the 2006 launch date, although NASA managers say they are still trying to make it. If they can't, another opportunity for a direct trajectory launch exists Feb. 2-15, 2007. A launch then would delay arrival at Pluto until 2019-20.
The big Keck II 10-meter telescope on the Mauna Kea volcano in Hawaii has sparked renewed interest in Uranus, the next planet after Saturn. Although Voyager 2 visited the planet early in 1986, its flyby images provided only a snapshot of the planet and its complex system of moons and rings. Using Keck's adaptive optics and the Hubble Space Telescope for a clearer view of the planet, astronomers have started charting Uranus' weather patterns and learning more about the rings as the perspective of the planet relative to Earth changes.
In 1986, when the southern hemisphere of Uranus was in full summer, scientists analyzing the Voyager imagery saw a relatively monotonous atmosphere with only 10 visible clouds and some banding that was very faint when compared to the vivid atmospheres of Jupiter and Saturn. But Uranus spins on an axis almost in the same plane as its 84-year orbit around the Sun, so the Sun's rays are reaching parts of the atmosphere that were in shadow as Voyager 2 passed by, stirring up atmospheric convection that gives astronomers something to work with as they try to understand just what lies beneath its atmosphere of hydrogen, helium, methane, acetylene and other hydrocarbons.
As its adaptive optics have become better at compensating for atmospheric distortion, the Keck telescope has tantalized astronomers studying Uranus and Neptune. Now considered "ice giants" with a slushy mantle over a nickel core instead of gas giants like Jupiter and Saturn, as once was thought, the two planets are much more interesting than they seemed after the Voyager flybys.
"I THINK IT'S FAIR to say that our observations are [now] about 20 years ahead of the models for these two planets," says Hammel, who is using the Keck telescope to study Uranus. "That's because all the models were based on Voyager information, and we've learned a lot since then. We're seeing . . . dynamics in the atmosphere that people frankly didn't think could exist on Uranus, and didn't suspect were as dramatic on Neptune as they are appearing to be.
An outer-planets expert, Hammel has called for a mission to place an orbiter around Neptune, which lies beyond Uranus and actually trades places with Pluto as the outermost planet for 20 years in every 248 years as Pluto's elliptical orbit brings it closer to the Sun. In the past, NASA has studied a solar-powered Neptune orbiter concept that would use inflatable structures for large arrays and concentrators to focus solar energy. It is currently funding a small study of a nuclear-powered orbiter that would drop hardened probes into the planet's atmosphere to measure it down to pressures approaching 1,000 bars.
Scientific areas of interest at Neptune include atmospheric chemistry, dynamics and structure; the nature of magnetic fields around the gas giant, and whether the planet's rings are just forming or stable. Triton, the planet's largest moon, has a thin atmosphere and is almost certainly a captured KBO, which Hammel finds particularly intriguing.
"We've never seen a single one up close," she says. "Yet when we look at other stars and we look at dust disks around other stars, which are nascent solar systems, it's really the Kuiper Belt Objects and their progenitors that we're seeing around the other stars. So we want to understand them within our own Solar System, so that's why we've got to get out there and look at them."
Power is not the only limiting factor for exploration in the outer Solar System. The kinds of instruments that would be enabled with a fission reactor would generate volumes of data that would be very difficult to transmit back to Earth across the vast distances involved. Neptune is about 30 times as far from the Sun as Earth, and effective broadband data transmission over that range would probably require a system of lasers and telescopes. NASA has plans to test an optical communications system with a Mars orbiter to be launched in 2009.
"The biggest beneficiaries will be the outer-planet missions," says Figueroa, who lists Neptune, Titan, the Kuiper Belt and "the edge of interstellar space" among potential targets for advanced spacecraft using the new nuclear and optical technology.
Well, I think the whole thing boils down to the basic issue: we really need to have a heavy lift capability. Both future manned and unmanned expeditions depend upon this. To think that outer solar system exploration will 'slow' the progress of nearer term human exploration is preposterous. The absence of a robust heavy lift launcher will slow both!
A properly funded and conceptualized effort could do both with reasonable cost--because the experiences and technologies developed with one program can be directly applied to the other.
