From: LARRY KLAES (ljk4_at_msn.com)
Date: Tue Dec 09 2003 - 10:58:31 PST
In search of life on Jupiter's moons
Nuclear-powered spacecraft to scope oceans for organic molecules
David Perlman, Chronicle Science Editor
No sooner had the Galileo spacecraft fascinated the world by discovering an icy crust covering deep oceans on one of Jupiter's moons than scientists began planning a far more ambitious interplanetary voyage to see if the oceans harbor life.
It will take years and billions of dollars to build and fly their new mission, but the impact on humanity of discovering life beyond Earth would be incalculable. On Monday, a team of astronomers, geologists and astrobiologists described for the first time the spacecraft they envision and the flight plan they hope to follow.
Their report came at the opening of the annual fall meeting of the American Geophysical Union in San Francisco, where some 10,000 scientists have gathered for five days of technical sessions at the new Moscone West Center.
Scientists envision sending a huge, 300-foot-long, nuclear-powered craft -- called JIMO, for Jupiter Icy Moons Orbiter -- on a voyage to the Jovian neighborhood to spend up to five years circling the ice-encrusted moon called Europa, plus two others, Callisto and Ganymede, which also are covered with ice but are less likely to have inhabitable oceans.
Even though landing the spacecraft on Europa's surface would be extraordinarily difficult, mission scientists have not ruled it out.
Europa is JIMO's principal target. Images from the Galileo mission nearly seven years ago revealed its surface as studded with icebergs, frozen ridges and dark streaks that seemed to indicate organic molecules rising through gigantic cracks from a deep ocean beneath.
"It is a most exciting and ambitious undertaking," said Ronald Greeley of Arizona State University, who co-chaired the teams of mission planners. "It will demand new technologies, new instruments, new and more powerful ground stations to receive the enormous quantities of data that will be coming back, and new power to get the spacecraft to its destination."
According to Torrence Johnson, who was the Galileo mission's chief scientist, the JIMO mission is planned to fly no earlier than 2010 and could cost as much as $8 billion.
Because of the huge number of instruments the spacecraft must carry, it cannot depend even on the small generators powered by a plutonium isotope that provided electricity for the Galileo spacecraft and for the Cassini mission now flying to Saturn, Greeley said.
Instead, engineers will have to design a small and unconventional fission reactor built on the same principle as today's commercial power reactors Like the many other instruments on the spacecraft, it would represent a substantial step beyond current nuclear technology. But he added, "if we don't take these first tentative steps, it's not going to happen."
Europa, like all Jupiter's moons, is subject to great tidal forces from its huge parent planet. Its crust bulges and subsides nearly 100 feet every 1. 8 Earth days as it orbits Jupiter. That kind of stress must crack the crust deeply -- enough, scientists hope, to reveal the ocean beneath.
William B. Moore, a geophysicist at UCLA, hopes those cracks can be observed by telescopes aboard JIMO. The surest way to measure the crustal stresses would be to place one or more seismometers on Europa's surface, but that would require a landing, an extremely difficult mission, at best.
To determine whether Europa's ocean is habitable, and whether anything lives there, is perhaps the toughest problem of all, said Christopher McKay of the NASA Ames Research Center in Mountain View.
Chemical-sensing instruments mounted on the spacecraft could identify organic chemicals on or just beneath Europa's surface, McKay said. However, those organic chemicals would have been so badly degraded by Jupiter's intense radiation that it would be impossible to tell if they came from anything alive.
On Earth, proteins -- the basic building blocks of life -- are based on just 20 amino acids. But if there's life in the Europa ocean, it might well be based on far different building blocks, McKay said.
"Wouldn't it be fascinating to discover life on Europa that's based on amino acids and proteins entirely different than the stuff we know on Earth?" he said with a grin. Sensors aboard the spacecraft might not be able to detect such life at all, but an instrument landed directly on the ice could surely do the job.
With its heavy load of instruments, the spacecraft would have to be at least 300 feet long. The nuclear reactor and the instruments would have to be at opposite ends of the craft -- widely separated by scaffolding to protect the instruments from the reactor's radiation.
The planning funds for the Jovian mission are in the current Bush administration budget for NASA, Greeley said, but ultimately billions more will have to be approved by Congress.
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