It was the spacecraft that wouldn't quit. Delayed, rerouted, nearly silenced, and shot through with radiation, the Galileo spacecraft followed its final orders as directed on 21 September. It was an act of self-destruction, intended to avoid contaminating the jovian moon Europa, a possible habitat for life. "Galileo is gone," intoned the flight controller once it dove into Jupiter at 48 kilometers per second, transmitting data until the end.
One of the last of the great planetary explorers, Galileo opened new frontiers, exploring everything from the solar system's most massive planet to motes of dust from other stars. Along the way, it overcame a raft of obstacles.
The craft almost didn't get off the ground. Scheduled for launch in 1986, from the cargo bay of the space shuttle, the mission was nearly scrapped after the Challenger disaster. Galileo got under way in 1989, but took a roundabout route to get the gravity boosts to make up for the change in plans. Various irksome problems cropped up, but "the problem with the main antenna really threatened the mission," says Torrence Johnson of the Jet Propulsion Laboratory, project scientist from start to finish. The umbrella-like antenna designed to return data at 130,400 bits per second just wouldn't unfurl, no matter how hard engineers beat on it from afar.
The fallback antennas' data rate of 10 bits per second "would have been virtually useless" for doing science, notes Johnson. So engineers and scientists teamed up to increase the backup antennas' data rate through hardware upgrades at ground receiving stations and tricks like data compression on the spacecraft. In the end, says Johnson, "we accomplished far more than 100% of what we anticipated at the start" of mission planning.
Those achievements ran the gamut from the nanoscale on up, since Galileo carried just about every instrument planetary scientists could wish for. The dust detector discovered streams of smoke-sized particles shot out from the planet by its powerful magnetic field, as well as dust from other stars. The magnetometer detected an ocean beneath the icy surface of the moon Europa (ScienceNOW, 9 April 1997), which along with Mars has become astrobiologists' prime focus in the solar system. And on the way to Jupiter, the camera found the first satellite of an asteroid, tiny Dactyl circling asteroid Ida.
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