NASA's most successful planetary probe, Galileo, is due to end its mission on 21 September by crash-landing into Jupiter. There is just enough propellant on board to point its antenna towards Earth, so NASA scientists hope to retrieve more data in the craft's final days.

Galileo's lifetime has been extended three times by NASA since it arrived at Jupiter in December 1995, helping it to collect more information than any other planetary mission to date. Its observations led to the discovery that Jupiter's moon Europa probably has an ocean beneath its icy surface, and that Io harbours a giant volcano. In 1994, Galileo was close enough to the planet to see comet Shoemaker-Levy 9 break up and pummel the surface, giving scientists a better idea of what might happen in a similar collision with Earth.

If left alone, Galileo could have stayed in orbit around Jupiter for the next 60 years. But there was a risk that the craft would accidentally crash into and contaminate Europa, which planetary scientists think might harbour life. Intentionally crashing a spacecraft and getting data along the way isn't unprecedented — Magellan was smashed into Venus in 1994, for example.

In Galileo's final 24 hours, it will be trained on Amalthea, one of Jupiter's inner satellites. Researchers think that it may have a comet-like structure, shedding pieces that could make up one of Jupiter's faint rings.

If Galileo is still transmitting data in its final hour, scientists hope to determine where the outer edge of Jupiter's atmosphere begins. At present, no one knows how far the planet's gases extend beyond its surface, says Galileo's project manager, Claudia Alexander.

But it is not clear whether Galileo's instruments will still be functioning by then. Researchers expect the craft to be torn apart in the atmosphere on its speedy descent at 50 kilometres per second. Details of Galileo's fate will never be known, as its final few minutes will be spent on the far side of the planet, out of sight from Earth.