The Space Review

 
JIMO
Artist’s rendition of the nuclear-powered Jupiter Icy Moons Orbiter, a flagship mission of Project Prometheus planned for next decade. (credit: NASA/JPL)

Space science gets big at NASA

The future of NASA’s planetary exploration plans may rest on larger missions and nuclear technologies

For most of the 1990s—at least from 1992, when Dan Goldin became administrator—the guiding phrase for NASA’s space science programs was the now-famous “faster, better, cheaper.” After a disastrous drought of missions in the 1980s, when NASA focused its energies, and its limited funding, on a few large missions like Magellan and Galileo, the agency changed course and began to support a larger number of smaller missions. The 1990s brought us the Discovery program of planetary science missions, like NEAR and Mars Pathfinder; the Small and Medium-class Explorer programs, including IMAGE and FUSE; and the New Millennium program of technology-demonstration missions, most notably Deep Space One. The Nineties were not without large missions—Cassini was launched in 1997—but that mission dated back to the 1980s, and looked out of place among the fleets of smaller missions.

By the end of the decade, though, there was evidence that the pendulum had swung too far in favor of small spacecraft. The failures of Mars Climate Orbiter, Mars Polar Lander, and Deep Space Two, all in late 1999, were essentially blamed on trying to do missions too quickly and too cheaply. CONTOUR, a Discovery-class comet mission launched last year, was lost either because of a faulty rocket motor or a design flaw. NASA cancelled one mission, the Full-sky Astrometric Explorer (FAME), last January when it appeared the spacecraft could not meet its cost and schedule requirements; it cancelled another, the Spectroscopy and Photometry of the IGM’s Diffuse Radiation (SPIDR), earlier this year because the spacecraft could not achieve its stated goals.

The pendulum now seems to be swinging back in the direction of larger, more capable, and costlier missions. In 2002 NASA inaugurated New Frontiers, a program of planetary science missions with a maximum price roughly double that of Discovery. New Horizons, the Pluto flyby mission that NASA finally accepted after Congress rebuffed several attempts by the agency to cancel it, will be grandfathered into the program as the first such mission; NASA released a draft announcement of opportunity earlier this year for the next New Frontiers mission. Beyond New Frontiers NASA is looking into every larger, more expensive missions, including the James Webb Space Telescope, whose price tag has grown even as the size of its mirror shrinks; Laser Interferometry Space Antenna (LISA), a spaceborne observatory for detecting gravitational waves; and various concepts for spacecraft to detect and eventually photograph terrestrial exoplanets.

The pendulum now seems to be swinging back in the direction of larger, more capable, and costlier missions.

None of those planned or proposed missions, though, have attracted as much attention as Project Prometheus. The project was unveiled earlier this year as a multi-year, multi-billion effort to develop nuclear power and propulsion technologies, and put those technologies to use on space science missions. The cornerstone mission for Prometheus is a spacecraft descriptively, if unpoetically, called Jupiter Icy Moons Orbiter (JIMO). JIMO would putter among the three large icy moons of Jupiter—Callisto, Europa, and Ganymede—studying them in detail not possible before by Voyager or Galileo.

It’s not surprising, then, that when Women in Aerospace convened a panel discussion about NASA space science policy, the focus was not on the small missions but on larger missions, most notably Prometheus. The consensus of the panel, composed of NASA officials, Congressional staffers, and outside experts, is that Prometheus is a worthwhile and achievable project for NASA, but also one with its share of titanic hurdles to overcome.

The promise of Prometheus

What Project Prometheus offers to scientists and spacecraft designers can be summarized in a single word: power. It sounds simple, but power, along with mass, cost, and schedule, are the key variables that shape spacecraft design. The amount of power available determines what and how many scientific instruments a spacecraft can carry, as well as the rate at which those instruments can transmit their data back to Earth. Depending on the propulsion system used, power can also determine where and how fast a spacecraft can travel.

“Power is the wellspring for everything else required on a spacecraft,” said Hartman. “If you’re going to go to multiple targets in the outer solar system, you have to use fission for power.”

If successful, Prometheus can provide JIMO and future missions with a nuclear reactor that can generate far more power than the radioisotope thermoelectric generators (RTGs) that have been standard issue on outer solar system missions since Pioneer 10. “We’re not talking about a couple of light bulbs to power a spacecraft, we’re talking about stadium-style lighting,” said Colleen Hartman, director of the solar system exploration dividision of NASA’s office of space science.

That additional power can make all the difference in planning a mission to Jupiter’s moons. As currently envisioned JIMO will carry a large suite of instruments, likely including a radar sounder to measure the depth of the ice crusts on the three moons, a laser altimeter, and a camera, among others. Most of these instruments typically have large power and/or data transmission requirements that make them difficult to include on power-constrained budgets. “Power is the wellspring for everything else required on a spacecraft,” said Hartman. “If you’re going to go to multiple targets in the outer solar system, you have to use fission for power.”

JIMO is the successor to Europa Orbiter, a mission NASA was planning in the late 1990s but eventually cancelled because of cost overruns. The RTG-powered spacecraft would have performed only a handful of flybys of Callisto and Ganymede before entering orbit around Europa, where it would be able to spend, at most, a month or two studying the moon with a radar sounder and a few other instruments before radiation irreversibly damaged the spacecraft. JIMO will be able to spend months orbiting Callisto and Ganymede, studying them in detail before venturing into the harsher radiation environment around Europa. A goal of JIMO, Hartman said, is to provide 10-meter resolution images of virtually the entire surfaces of all three moons, something that would not have been possible with Europa Orbiter.

page 2: Promethean challenges >>

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