The NASA spacecraft Galileo is set to crash into Jupiter on Sunday, sacrificing itself for the sake of alien life. But there should be some rewards for astronomers, too. In its final hour, the spacecraft may discover whether a Jovian moon is disintegrating, and explore Jupiter鈥檚 outermost atmosphere for the first time.
Since its launch in 1989, Galileo has orbited Jupiter for nearly eight years 鈥� far longer than the two-year mission that was originally planned. It has made 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 have aimed it at Jupiter and at 0655 GMT on 21 September Galileo will hit the planet鈥檚 atmosphere and disintegrate.
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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鈥檚 moons. 鈥淭here had been talk of putting it in a 60-year orbit 鈥� parking it there to study comets, for example,鈥� says Claudia Alexander, head of the team at NASA鈥檚 Jet Propulsion Laboratory 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鈥檚 course.
鈥淭here was a not insignificant chance that the orbit would be perturbed,鈥� says Alexander. If so, the spacecraft would probably still have hit Jupiter in the end. But there is just a chance that it would hit Europa, Ganymede or Callisto, which probably hold liquid water, and possibly life.
Nuclear waste
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. 鈥淵ou don鈥檛 want to contaminate the environment with nuclear waste,鈥� 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. The possibility that terrestrial organisms could have arrived on a crashing spacecraft might confuse the results of any future mission searching for life.
But some scientists say these fears are overplayed. 鈥淚 am not too worried about contamination,鈥� says William McKinnon, a planetary scientist at Washington University in St Louis, Missouri.
Jupiter鈥檚 moons are subject to ionising radiation from cosmic rays and the planet鈥檚 radiation belts, he points out. 鈥淚 tend to think that Europa gets so zapped by radiation that bacteria wouldn鈥檛 survive.鈥�
Star scanner
During Galileo鈥檚 final plunge, Alexander hopes to make two unique measurements. 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 鈥� a string of rocks rather than the circlets of fine dust that constitute the known rings 鈥� or they may just be a local grouping around Amalthea. Either way, they may have originated on Amalthea itself.
On Sunday, JPL scientists will try to use the spacecraft鈥檚 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鈥檚 most intense radiation belt. If can survive for just half an hour or so past Amalthea, down to within 40,000 kilometres of Jupiter鈥檚 cloud tops, the radiation may start to decline.
鈥淎fter that it will be plain sailing 鈥� and we will learn some interesting things,鈥� says Alexander. Galileo鈥檚 last act could be to discover the transition between the Jovian atmosphere and space 鈥� an unexplored realm called the exosphere. No one knows what to expect here.
A full version of this story, plus a history of the mission, appears in New 杏吧原创 print edition, on sale from 18 September.