TCS Daily

Galileo's Last Ride

By Kenneth Silber - November 4, 2002 12:00 AM

The spacecraft Galileo, explorer of Jupiter and its moons, is entering the final phases of its mission. On November 5, the probe will conduct a flyby of Amalthea, a potato-shaped moon deep within Jupiter's magnetic and radiation environment. Since the spacecraft is running out of the propellant that keeps its antenna pointed toward Earth, its controllers at NASA's Jet Propulsion Laboratory have put it on a collision course with Jupiter. In September 2003, it will plunge into the giant planet's atmosphere and cease to exist.

Galileo, however, has lasted much longer than had been expected when it arrived in the Jupiter system in late 1995 for a two-year primary mission. (Unmanned space projects are often extended until instruments are decrepit and funding has dried up.) Moreover, Galileo has produced an enormous wealth of scientific data and images. The craft has detected, for instance, lightning on Jupiter, a magnetic field around the moon Ganymede, intense volcanic activity on the moon Io, and - most importantly - evidence of an ocean of liquid water beneath the ice-covered surface of the moon Europa. The latter makes Europa the most plausible abode for any life that may exist in the outer solar system.

The Galileo project has provided not only a vast scientific payoff but also some glimpses into the politics of space exploration. For one thing, Galileo was an early target of antinuclear and environmental activists worried about "nukes in space." This is because the spacecraft, similar to the later Cassini probe to Saturn, carries generators that draw on the heat from decaying plutonium to power its scientific instruments. Prior to Galileo's launch in October 1989, activists sounded the alarm that a launch accident could scatter plutonium throughout the atmosphere. The opponents also raised the far more improbable scenario of the spacecraft hitting Earth during a later flyby.

Such arguments shrugged off the formidable precautions that are taken to minimize any possible release of radioactive material. (For instance, the plutonium is in a ceramic-like form that's extremely difficult to vaporize, and is encased in high-strength, heat-resistant materials.) Interestingly, when Galileo was set to launch aboard the space shuttle in October 1989, opponents denounced the shuttle as entailing much greater risks than an unmanned rocket. When Cassini was set to launch aboard a Titan rocket in 1997, the protest movement decried the supposedly extreme risks of that unmanned launch vehicle.

The Galileo mission also highlighted some divisions that exist among space experts and enthusiasts. The scientific community has long tended to support unmanned space probes and to regard human space exploration with wariness or opposition. At the same time, various space advocacy groups have made human exploration their priority (but generally do not oppose unmanned projects). Planetary scientists have been vocal in celebrating Galileo's achievements and calling for extensions of its mission. However, broad, grass-roots interest, such as attends the possibility of a human mission to Mars, has been less evident.

Even among scientists and enthusiasts of unmanned planetary exploration, Galileo underscored significant differences over priorities and objectives. A substantial constituency exists for near-term exploration of Europa (probably via an orbiter followed by a lander) to follow up on Galileo's evidence for an ocean and to search for possible life. But a similarly dedicated constituency, also possessing strong scientific arguments, emphasizes near-term exploration of Pluto (the only planet that has not yet been visited by a space probe; and one whose orbit presents difficulties if a mission is delayed). Given the limitations of plausible funding, NASA will have difficulty satisfying both groups.

Even Galileo's final mission has been the subject of some contention. During its November flyby of Amalthea, the spacecraft will collect data about the satellite's composition and about the intense radiation and magnetism in the vicinity. But there has been criticism, from Aviation Week magazine and some scientists, of NASA's decision not to take any further pictures of the 154-mile-long moon. Such imaging was deemed too expensive and of lesser scientific importance. There may be further controversy over plans to shut off the probe's instruments months before the plunge into Jupiter. The spacecraft is running out of fuel and money, and has been battered by politics and radiation. But any final glitches notwithstanding, Galileo's performance has been heroic.



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