NASA launched Galileo on October 18, 1989, seven years after its original launch date, and its mission ended in a blaze of success on September 21, 2003. Galileo explored the Jupiter system for seven years, having taken nine years to reach the planet. The mission was redesigned five times, as specifications for the trip changed, and different launch dates required different flight paths through the solar system. The spacecraft was designed to be launched from the space shuttle, but because of delays in the launch date that created problems with the route to the outer solar system, when it was finally launched, Galileo did not have enough propellant to take a direct route. An ingenious system of gravitational boosts were used to get the probe all the way to the outer solar system, starting with a launch actually toward the inner solar system, where it swung around Venus, then came back to swing twice around the Earth, before catapulting into the outer solar system.When Galileo came back by Earth the second time, a special communication had to come from NASA to the North American Aerospace Defense Command that the fast-moving object seeming to come from the Middle East was not an attack but the return of their own spacecraft on its way to Jupiter.

Two years after launch, Galileo had reached the asteroid belt, and was scheduled to open its large antenna and start streaming data back to Earth. When the command was sent, the antenna stuck: It would not open! With only the secondary, smaller antenna, Galileo would only be able to send data at a rate that would transmit one picture per month, a completely inadequate rate.What followed was one of the most daring engineering feats of the space age: The original designers of the aged computer processors were brought out of retirement for consultation, and over the course of a year or so, all the software in Galileo was replaced. An error could have meant a complete loss of communication with the spacecraft, but the radio link worked, and the software replacement worked, and in the end Galileo was able to send 200 images per month. There was still a team assigned to make the large antenna open, and they worked at it for five years, without success.

Galileo took pictures of the four largest Jovian satellites, and studied Jupiter's atmosphere, magnetic field, rings, and small satellites, even dropping a probe into Jupiter's clouds. The probe entered Jupiter's atmosphere 280 miles (450 km) above the one-bar pressure level and began measuring composition, temperature, and pressure, and sending the data back to Galileo. Its first parachute opened at 14 miles (23 km) above the one-bar level, and soon after, the heat shield dropped off. The probe survived to an atmospheric level of 22 bars, when its transmission ended.

Galileo conducted 40 flybys of planets and moons, far more than any other space mission. It passed close by two asteroids, Gaspra and Ida, and took the first high-resolution images of asteroids. In these images it was discovered that Ida has its own orbiting moon! Some scientists refer to the tiny moons of asteroids as "moonlets." Galileo was the first to document active volcanism on Io, and it photographed the pieces of the Shoemaker-Levy comet falling into Jupiter. It detected nine new moons of Jupiter.

On September 21, 2003, Galileo was directed to fly into Jupiter and destroy itself.This directive was made necessary by discoveries Galileo itself had made about the moon Europa. Data from Galileo indicates that, with its probable water oceans, Europa is a highly possible site for life in the solar system. Rather than risk contaminating Europa with any microscopic life still on Galileo, should Galileo crash onto Europa as it aged and its systems failed, NASA made the determination that it should be definitively destroyed in Jupiter's upper atmosphere, where all possible life on it will be completely vaporized, while Galileo was still responsive enough to allow guidance from Earth.

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