Like a champion in the final round of a prizefight, NASA's Galileo Jupiter orbiter is not going out without taking a few last shots of its own. Sent by NASA on an impending collision course with Jupiter's crushing atmosphere, Galileo took its closest and final flyby last week of one of Jupiter's four major moons as it neared a daring end to its long and successful mission.
Heated by the tremendous tidal flexing caused by Jupiter's gravitational pull, Io, one of Jupiter's moons, is the most volcanically active body in the solar system, with an estimated 200-300 volcanoes. Scientists hope to take measurements of charged particles and magnetic environments around the moon, as well as infrared and thermal image photography, looking for changes in Io's volcanoes since the moon was first photographed in 1979 by the Voyager spacecraft.
Jim Lattis, director of UW-Madison Space Place, is eager to compare the changes Galileo will detect.
\It's historically significant, the first time we've monitored a really alien planetary system in detail over a period of more than a few hours,"" said Lattis. ""We've now seen that system evolve, not just through optical imagery but [also]radiation fields and electromagnetic environment, for several years now.""
The sturdy spacecraft has been operating in Jupiter's orbit over three times as long as its mission was planned for, mainly due to its unforeseen ability to withstand Jupiter's tremendous radiation. In its 33 orbits around Jupiter, Galileo has photographed flybys of the four large inner moons 27 times. In recent missions, though, growing wear and tear on the spacecraft have been obvious, causing NASA to decide to end Galileo into its inevitable retirement.
""We're not totally surprised because Galileo has already outlived expectations and we knew that it might encounter additional difficulties from the high-radiation environment,"" said Eilene Thelig, Galileo's project manager.
UW-Madison planetary scientist Sanjay Limaye, director of the Office of Space Science Education, said that although Galileo could still continue for awhile longer, the time was right for NASA make the tough choice to ""put down"" one of its most productive creations.
The original two-year mission was to study Jupiter's atmosphere, satellites and surrounding magnetosphere, starting December 1995.
""At some point, you consider the rate of new discoveries [versus] the amount of effort it takes to maintain the spacecraft...[and] data-return issues, and the fact that the spacecraft is getting old and radiation-damaged,"" Limaye said. ""At some point, you have to decide if it is worth the effort and the money. You have only a limited budget to run these space missions, and new missions or other programs that also deserve support.""
Despite losing the main antenna, Galileo has still taken and returned over 10,000 images documenting planets, moons, asteroids and everything in between.
In October 1991 Galileo flew by the asteroid Gaspra, providing the first close-up images of an asteroid. In August 1993, Galileo made a close pass to another asteroid, Ida, and in the process discovered the first confirmed asteroid moon.
It was in late July 1994 that Galileo truly happened to be at the right place at the right time for an unprecedented view of the far side of Jupiter during its collision with 24 fragments of the Shoemaker-Levy 9 comet over a six-day span.
The ability to view the collision in real time was an unexpected but historic discovery, certainly one of Galileo's crowning moments. ""Galileo was the only spacecraft to actually watch the impact take place,"" said Limaye.
According to NASA, one of Galileo's most important discoveries has been the likelihood of a melted saltwater ocean ""possessing more water than the total sum found on Earth"" under Europa's icy crust, making the moon ""of great interest for future study of extraterrestrial life."" Photographs of Europa further showed an unusual lack of craters, suggesting a relatively young age. A thin oxygen atmosphere and ionosphere were also detected. Some suggest the possibilities may not end there.
Dr. Torrence Johnson, Galileo project scientist from NASA's Jet Propulsion Laboratory, said, ""Galileo has given us indications, primarily from magnetometer data, of the possibility that Europa, Ganymede and Callisto'three of Jupiter's four large moons'have liquid water layers. Liquid water is of interest not only for what it may tell us about the evolution of these bodies, but also for biological implications.""
Apart from a planned November flyby of Amalthea, one of Jupiter's small inner moons, this week's will be Galileo's final mission. Lattis said the bottom line is that Galileo should be remembered as a successful mission that adapted to the early problems and kept the spacecraft working and the data flowing.
""It's significant in the many technological challenges and problems that have been overcome. Galileo started life intended as a fast trip, but with a bad antenna became a slow, delayed trip.\
