After 13 years in space and 34 orbits around Jupiter, the Galileo spacecraft has reached the end of its mission. On February 28th, NASA controllers turned off Galileo's memory-bank tape recorder and transmission of stored data to Earth stopped. Currently looping around in a highly elliptical orbit, the aged craft is set to crash into Jupiter's Equatorial Zone on September 21, 2003.
After suffering high doses of radiation during its flyby of Amalthea in November 2002, the spacecraft's computer chips began to malfunction. Although engineers partially solved the problem, a nearly depleted supply of onboard fuel and a lack of funds to continue the project contributed to Galileo's shutoff.
However, some of the craft's instruments will continue operation until September. "Galileo is continuously sending science data in real time, but we have no intent to look at the information," says Eilene Theilig, Galileo's Project Manager. "We've put the spacecraft into a relatively quiet mode." Aside from a weekly Earth-Galileo contact to determine the craft's exact position (except for the summer months, when Jupiter disappears behind the Sun), scientists have laid the spacecraft to rest.
Launched in October 1989 from the Space Shuttle Atlantis, Galileo successfully completed its primary mission to study Jupiter and its moons despite a near-catastrophic failure when its main antenna failed to deploy.
Among Galileo's diverse contributions:
· Documenting the first and only major planetary impact humanity has observed directly, Galileo stood 1.6 astronomical units from Jupiter when the fragments of Comet Shoemaker-Levy 9 smashed into the gas giant's surface in July 1994. The craft observed flashes caused by the cometary chunks hitting Jupiter's atmosphere, creating scorching fireballs at approximately 8000°K.
· The spacecraft studied the Galilean satellites — Ganymede, Callisto, Io, and Europa — in unprecedented detail. Galileo's observations led scientists to believe Europa may harbor sub-surface seas, and the craft's magnetometer also revealed magnetic-field disturbances akin to those expected from a body covered by a salty ocean. In addition, magnetometer results predicted sub-surface seas underneath the ice crusts of Callisto and Ganymede. Galileo also imaged highly active sulfuric volcanic regions on Io's surface.
· After releasing a probe from Galileo into Jupiter's atmosphere, scientists studied just how hot, dense, and pressurized Jupiter is. In its one-hour lifetime, the space probe revealed that Jupiter's atmosphere is much drier than expected and sustains blustery winds that increase with depth. This may help explain the longevity of Jupiter's Red Spot.
The mission was fraught with trouble at the start — when the high-gain antenna aboard the craft failed to open, forcing astronomers to send the craft's stored data to Earth at a frustratingly slow rate from the backup low-gain antenna. After assessing what science was possible without the high-gain antenna, "we concluded that we could plan on achieving at least 70 percent of the planned science objectives," says Galileo scientist Torrence V. Johnson (NASA / Jet Propulsion Laboratory).
"The atmospheric probe was always rated as the highest priority part of the mission, and that was accomplished at 100 percent."
But despite the hindrance, the overall mission was deemed a tremendous success. Galileo far outlasted its life expectancy and was so successful in its extended mission that it more than made up for the science opportunities lost with the high-gain antenna.
"We had a basic plan to address all the major science objectives by December 1997 — and we did," says Johnson. "As with most missions of discovery, we ended up surpassing [the planned science objectives] just due to things we had not anticipated, such as Ganymede's magnetosphere, magnetic field, and the signature of oceans on the icy satellites."
But the team will have one final reunion: "We will celebrate the life of Galileo and its many accomplishments on September 21st," says Theilig. Everyone will be on hand to watch the signal disappear as Galileo flies behind Jupiter and plunges into the planet's atmosphere.