An illustration depicting Galileo in front of Jupiter.
Jupiter's volcanic moon Io, as seen by Galileo.
Scientists at NASA's Jet Propulsion Laboratory in Pasadena, Calif., will listen in Sunday as the Galileo spacecraft sends its final signals. After orbiting Jupiter for nearly eight years, mission managers decided it was best to destroy the spacecraft by crashing it into the giant planet. NPR's Joe Palca, who has been covering the Galileo mission for 17 years, reports on the mission and its accomplishments.
Galileo was a big success, Palca says. Its cameras returned thousands of pictures of Jupiter and its large, icy moons. Galileo's magnetometers found evidence that liquid oceans still exist under the ice of Europa. Other instruments analyzed Jupiter's turbulent atmosphere.
But Galileo was very nearly a flop, Palca says. It was constantly flirting with disaster, even before it left Earth.
"When the project started in 1977," says Bill O'Neil, one of Galileo's project managers, "the idea was that it would be launched in January 1982 and would arrive at Jupiter in the middle of July 1985. So it turns out that we actually arrived at Jupiter a little more than a decade later, in December of 1995, and launched seven-and-a-half years after we were originally scheduled to launch."
The problem was with NASA's brand new space shuttle that was to carry Galileo and its booster rocket into orbit.
A launch date was finally set for May 1986. But in January that year, the space shuttle Challenger blew up. After reviewing the safety of all future missions, O'Neil says NASA decided it was too dangerous for Galileo's powerful booster to fly on the shuttle even when flights resumed.
Without the booster, Galileo couldn't leave Earth's orbit and make it to Jupiter. The summer of 1986 was a bleak time for O'Neil and colleagues.
"We spent the month of July with no way to get Galileo to Jupiter, and for a time it looked like the only place we could get it to was the Smithsonian," he says. "And we weren't entirely kidding when we used to talk about that."
O'Neil says NASA would only let the shuttle carry a far less powerful booster, one that didn't have the power Galileo needed. System engineer Roger Diehl solved the problem in the nick of time:
"It was just on that last Friday in July that Roger had this brainwave to come by the Earth twice to get the gravity assist we needed to get to Jupiter," O'Neil explains.
So in what has to be one of the most counterintuitive flight plans of all time, the mission to the outer planet Jupiter actually started off flying to the inner planet Venus, then back for two loops around Earth. Finally, six years after it launched, Galileo arrived at Jupiter in December 1995 and began taking pictures and measurements of the Jovian system.
Galileo's main mission ended in 1997, but NASA has kept the project going — until now.
Claudia Alexander, Galileo's final project manager, says NASA officials worried that the spacecraft's orbit could send it crashing into Europa. They didn't want to risk contaminating a moon that might harbor life, so they decided a suicide plunge into Jupiter would be best.
Alexander says Galileo is scheduled to work right up to the end. "We are hoping, with fingers crossed, that we can go past the orbit of Amalthea and continue collecting science data," she says. "I actually don't think that's going to happen. I don't think the spacecraft is going to be able to handle that radiation environment."
But with Galileo, says Palca, you have to be prepared for pleasant surprises.