WHEN Galileo crashes into Jupiter this Sunday, it will be sacrificing itself for the sake of alien life, according to NASA and the Jet Propulsion Laboratory (JPL) in Pasadena, California. But there should be some rewards for humanity, too. In its final hour, the spacecraft may discover whether a Jovian moon is disintegrating, and explore Jupiter’s outermost atmosphere for the first time.
Galileo has orbited Jupiter for nearly eight years, far longer than the two-year mission that was originally planned, making discovery after discovery about the giant planet and its moons. One suggestion was to keep Galileo in orbit as a long-term observatory, but with little fuel left aboard the craft, NASA decided to make a quick end of it. So they aimed it at Jupiter, and at 0655 GMT on 21 September Galileo will hit the planet’s atmosphere and disintegrate.
The main reason the Galileo team gives for destroying the craft is to ensure that there is no chance of it contaminating any of Jupiter’s moons. “There had been talk of putting it in a 60-year orbit – parking it there to study comets, for example,” says Claudia Alexander, head of the JPL team that will oversee the final plunge. But no one could guarantee that the orbit would be stable. The complex gravitational effects of Jupiter and its moons are hard to predict, and the strong magnetic field around the planet could change a small spacecraft’s course.
Advertisement
“There was a not insignificant chance that the orbit would be perturbed,” says Alexander. With no fuel left to correct the orbit, that could have spelled disaster. In which case the spacecraft would probably still have hit Jupiter eventually. But there is just a chance that it could have hit Europa, Ganymede or Callisto, which probably hold liquid water, and possibly life.
Alexander points to two risks if Galileo had hit one of the moons. First, its instruments are powered by a generator that uses heat from the decay of plutonium to generate electricity. “You don’t want to contaminate the environment with nuclear waste,” she says. And it is just possible that terrestrial microbes could have survived on board, as they seem to have done on satellites and space probes. Although any life on these moons, if it exists, is most likely to be many kilometres under the ice, the mere possibility that terrestrial organisms could have arrived on a crashing spacecraft might confuse the results of any future mission searching for life.
Some scientists say these fears are overplayed. “I am not too worried about contamination,” says William McKinnon, a planetary scientist at Washington University in St Louis, Missouri. Jupiter’s moons are subject to ionising radiation from cosmic rays and the planet’s radiation belts, he points out. “I tend to think that Europa gets so zapped by radiation that bacteria wouldn’t survive.” This might also weaken the case for worrying about radioactive contamination. However, Alexander points out that Galileo would still be introducing a foreign substance into this pristine environment, as there is almost certainly no plutonium on Europa’s ice.
Whether or not the risk of contamination is serious, there are more positive reasons for sending Galileo to its doom. Alexander hopes to make two unique measurements on the way down. On its last pass by the inner moon Amalthea in November 2002, Galileo saw nine bright objects whose size and origin are still a mystery. They may be part of a new ring around Jupiter – a string of rocks rather than the circlets of fine dust that constitute the known rings – or they may just be a local grouping around Amalthea. Either way, they may have originated on Amalthea itself.
The mystery bodies could have been knocked off the surface by impacts. But researchers are intrigued by the possibility that a bizarre electromagnetic process is tearing the moon apart. Galileo has already found that Amalthea is a loose aggregation of rocks and ice, and that its gravity is weak. If charged particles striking the moon leave the boulders electrically charged, Jupiter’s magnetic field could then lift them off the surface, causing them to fly away.
On Sunday, JPL scientists will try to use the spacecraft’s star scanner to find out more about these rocks. It is a crude camera designed for navigation, but it might still be good enough to measure the locations, motions and even size of the rocks, and work out where they came from and where they are going.
After that, Galileo will press deep into Jupiter’s most intense radiation belt. In the past, the probe has sometimes temporarily switched itself off when it encountered intense radiation, but the hope is that it will stay switched on during its descent. If it survives for just half an hour or so past Amalthea, down to within 40,000 kilometres of Jupiter’s cloud tops, the radiation may start to decline. “After that it will be plain sailing – and we will learn some interesting things,” says Alexander. Galileo’s last act could be to discover the transition between the Jovian atmosphere and space – an unexplored realm called the exosphere. No one knows what to expect here.
But whatever the scientific benefits of Galileo’s dramatic demise, the public relations pay-off may have been a deciding factor in planning the crash. Even if engineers had managed to put the craft into a longer-term orbit, its electronics are increasingly damaged and its attitude control is degraded, so they might lose contact within a few years. Then no one would know where it was – until it hit something. “I don’t think the team wants to have to say that,” says McKinnon.
Far better, surely, to stay in control to the last. “This is a nice tidy end,” says McKinnon. “In a way it’s a noble end.”
