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Rocky start

Galileo has had a romantic history, often flirting with disaster but always scraping through. The craft was scheduled for launch in 1986, but the explosion of the Challenger shuttle meant it had to wait till 1989. By then, the Centaur booster rocket, originally thought essential for sending such a big spacecraft to Jupiter, had been banned from the shuttle. The solution was an arcane orbit that passed Venus once and Earth twice to gain gravitational boosts.

Disaster strikes

With the craft on its way, engineers tried to open its 5-metre main antenna. The antenna stuck, and it seemed we would get back only a fraction of the data that scientists were hoping for. But with data-compression software and an improved network of receiving satellites, scientists squeezed all the information they needed through the spacecraft’s smaller secondary antenna.

Asteroids and comets

Heading out through the asteroid belt, Galileo discovered the first known asteroid moon, Dactyl, which orbits Ida. And in July 1994, it became the first instrument to see a comet impact, when the fragments of comet Shoemaker-Levy 9 hit Jupiter. The impacts were out of sight of Earth, but Galileo saw the fireballs and measured their size and temperature.

Technical snafu

Just three months from Jupiter, the tape recorder, vital for storing data, got stuck on rewind for 15 hours. It eventually responded to commands, but fearing that one end of the tape had been weakened, engineers wound on the recorder so that this end would never again be used.

Final destination

In December 1995, Galileo finally arrived in orbit around Jupiter. A small probe, released a few months earlier, hit the planet and radioed out observations for nearly an hour as it fell through the dense Jovian atmosphere. The probe hit rare weather – a kind of hot, dry downdraft that covers only 1 per cent of the planet. It found that Jupiter’s hurricane-force winds extend far deeper than thought, implying they are driven by heat from the planet’s interior, not the sun. And a new mystery: the atmosphere holds much more ammonia and methane than expected. Theories of planet formation cannot explain this, and may have to be rewritten.

Looping through moons

Over the years since then, the craft has looped in and out of Jupiter’s system of moons, observing a solar system in miniature. Io, the innermost large moon, is the most volcanic world we know. Its volcanoes were thought to be spewing sulphur, at a relatively cool 100 to 500 °C. But Galileo saw lava at 1500 °C, even hotter than on Earth. The best bet now is that Io’s volcanoes resemble those that existed on Earth more than 2 billion years ago, disgorging molten rocks rich in magnesium.

Alien havens

Back in the 1970s, the two Voyager spacecraft saw hints that Europa, Jupiter’s second moon, might have an ocean under its icy surface. Galileo made that almost certain: it sent back images of ancient ice floes and detected a magnetic anomaly around the moon that is best explained by a salty, conducting ocean. Similar magnetic traces suggest under-ice oceans on frigid Callisto and the giant moon Ganymede. All these worlds, especially Europa, are now considered prime sites for extraterrestrial life.