September 21, 2003, will mark the end of an important era in space exploration, when the robotic spacecraft Galileo dives into Jupiters atmosphere, ending its 8-year mission in the Jovian system.

Galileo arrived at Jupiter in 1995, but its journey started long before that. Originally planned in the 1970s, the spacecraft was the first outer solar system orbiter; a follow-up to the flybys of Jupiter conducted by the Pioneer and Voyager spacecraft. Even before the wildly successful flybys of Jupiter conducted in 1979 and 1980 by the two Voyager spacecraft, scientists knew that Jupiter and its large Galilean satellites were likely to be interesting destinations, worthy of future scientific study. However, its unlikely that the scientists originally designing Galileo knew just how exciting Jupiters moons would turn out to be.

Fortunately, the scientists and engineers at NASAs Jet Propulsion Laboratory (in Pasadena, CA) had four years before Galileos arrival at Jupiter to come up with a solution. While attempts to un-stick the antenna failed, their backup plan, which involved reprogramming the 1970s-era computer onboard Galileo, succeeded. They were able to persuade the aging piece of hardware to accept state-of-the-art image compression techniques, which, coupled with the on-board tape recorder, allowed the spacecraft to take images during close fly-bys of the satellites, compress them and store them on the tape recorder, and then play them back slowly during the long periods of time when Galileo was not collecting data.

After flybys of two asteroids, the Galileo spacecraft triumphantly entered orbit around Jupiter in 1995, and launched an atmospheric probe into Jupiter, which returned data about the temperature and composition of the huge planets thick atmosphere. While sending the probes data back to Earth, Galileo began the first of its eventual 34 orbits of Jupiter. These orbits looped around Jupiter like petals on a daisy, with each orbit involving a close encounter with one of the large Galilean satellites before or after a close swing around Jupiter on a carefully-designed trajectory planned to set up the next orbit.

On its first close encounter with Europa, in early 1996, Galileo took the first images of this icy moon since those taken by Voyager 2 in 1980. These images, coupled with those taken on subsequent orbits, began to reveal one of the most exciting discoveries of the Galileo mission a cracked, broken surface covered by ridges and other features, including iceberg-like areas where the surface seemed to have broken, moved around, and then refrozen with the pieces in different locations, like a jigsaw puzzle. This, and other geological evidence, began to suggest that Europa could currently possess an ocean of liquid water beneath its icy exterior.

The puzzle of whether or not Europa currently had liquid water found its most compelling piece of evidence from an unlikely source the magnetic field instrument onboard Galileo. This instrument measured evidence of an induced magnetic field on Europa that changed with Europas movement through Jupiters immense magnetic field. Though Europa does not generate its own magnetic field, the instrument measured changes that could only come from a conducting body moving through Jupiters field. The measurements were consistent with a large layer of liquid water similar in composition to terrestrial seawater located beneath Europas surface. Other configurations are possible (such as a large layer of a conducting metal or other mineral) however geologists consider these unlikely.

As the possibility of liquid water on Europa began to look more and more likely, astrobiologists began to get excited by the prospects for life on Europa. This idea had been considered based on Voyager images, but received much more widespread scientific notice as the presence of a subsurface ocean (which would have more water than all the Earths oceans combined) began to look like a reality. While it is unlikely that higher forms of life could exist on Europa, it is possible that microscopic organisms could have evolved and thrived beneath Europas surface, and endured until the present day.

The scientific excitement over Europa, however, would lead to Galileos ultimate demise. Originally scheduled for only 10 orbits of Jupiter, the incredibly successful spacecraft was granted an extended mission to study Europa, and then another extended mission to focus primarily on Jupiters magnetosphere, but which would also include a number of risky flybys of Jupiters inner volcanic moon, Io. Io had lost out on the earlier flybys because of the risk of damage to the delicate spacecraft electronics, and indeed a number of these later flybys were completely lost because the spacecraft essentially shut down due to radiation during the close approach period of its orbit. Fortunately, the spacecraft was successfully rebooted and fully recovered in each of these instances.

The radiation environment at Jupiter began to take its toll, however, and it became clear that the instruments on board Galileo were suffering gradual degradation as the spacecraft absorbed many times more radiation than it had been designed for. Instruments began to fail, and propellant started to run short. NASA had to make a decision as to what the eventual fate of Galileo would be.

Unlike the Pioneer and Voyager flyby missions, which are now on their way out of the Solar System headed for an encounter with the stars in many thousands of years, orbiting spacecraft do not have such a glorious legacy. Even if they are put on a stable orbit, the orbit will decay over time, and without propellant to do small course corrections, the spacecraft will eventually crash into either Jupiter or one of its moons. And it was one of those moons, Europa, which scientists were worried about. If there is the possibility of liquid water and life on Europa, then the impact of a spacecraft like Galileo could be a very bad thing. The remnant radiation from Galileos RTG power supply would be bad enough, but scientists are most worried about the possibility of contaminating Europa with terrestrial microbes. Even though Galileo has been in space since 1989, and in Jupiters huge magnetic field since 1995, it is still possible that some hitchhiking microbes could have been sealed inside the spacecraft during its construction on Earth, and survived its 14-year journey. Unlike the Viking spacecraft, the first Mars landers, which were carefully sterilized to significantly lower the chances of terrestrial microbes reaching Mars, Galileo was never subjected to any such precautions.

So just in case two remote possibilities prove true, that Europa has a hospitable environment for life and that terrestrial microbes have managed to survive aboard Galileo, NASA is making sure that there is no chance of a damaged spacecraft crashing into Europa and contaminating its potential biosphere. Instead, while the spacecraft still has enough propellant for a course correction and its computer is still responding to commands from Earth, Galileo will be sent on a forced impact with the giant planet Jupiter.

In a move which echoes Galileos entry into the Jovian system, when the probe entered Jupiters atmosphere, Galileos sojourn at Jupiter will end on September 21 at about 3 PM Eastern Daylight time when the spacecraft itself impacts Jupiter. The Galileo spacecraft was never intended to encounter an atmosphere, and while some instruments are expected to return data as Galileo approaches Jupiter, it is also expected that the spacecraft will break apart quickly then vaporize as it descends into Jupiters atmosphere. It will be a quick and fiery end for this spacecraft, one of NASAs most successful missions.

It will also be a sad day for the many scientists and engineers who helped make the Galileo mission such a success. A follow-up mission to study Europa, called Europa Orbiter, was recently cancelled by NASA, but it was replaced by an ambitious project called Jupiter Icy Moons Orbiter (JIMO) that would be a huge nuclear-powered spacecraft that would have enough power and propulsion to orbit each of the icy moons of Jupiter in turn. It will be well into the next decade before JIMO can be launched, however until then, scientists will be left with Galileos legacy as they study Jupiter and its moons.