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Ten Years Ago: Galileo's Long Journey Begins
By Andrew Chaikin
Executive Editor, Space and Science
posted: 06:59 am ET
18 October 1999
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galileoanniv_991015By all accounts, it has been one of NASA's most rewarding missions, but it started out as one of the most unlucky. On October 18, 1989 the Galileo spacecraft was launched aboard the Space Shuttle Atlantis. Originally slated to be carried aloft by a shuttle in 1986, the mission was postponed because of the Challenger tragedy, which occurred in January of that year. During the delay, Galileo's Centaur booster was replaced with a less powerful rocket that was deemed safer for the shuttle to carry. The change meant mission planners had to design a new, circuitous path to Jupiter, using a technique called gravity assist. After leaving Earth orbit, the spacecraft passed close to Venus, and then swung past Earth two times. Each close passage acted like a slingshot to speed Galileo toward its destination. Even so, it took Galileo six years to reach Jupiter, far longer than originally planned. In April 1991 Galileo was already well on its way to Jupiter when mission controllers discovered that its main communications antenna had failed to open completely. Attempts to free the antenna by remote commands from Earth failed. In the end, controllers were forced to send all of Galileo's data using the spacecraft's low-gain antenna, at a tiny fraction of the speed originally planned. To salvage as many of the planned scientific observations as possible, engineers devised advanced data-compression techniques to fit more information into Galileo's trickle of data. By the time Galileo reached Jupiter in December 1995, controllers were prepared for the handicaps of their mission. Even before then, Galileo had racked up a significant achievement, as the first spacecraft to take close-up pictures of an asteroid. And as it neared Jupiter, there was another first: the craft released an instrument-laden probe that plunged into the jovian atmosphere, providing the first in-situ data on a gas-giant planet's composition and structure. Meanwhile, the Galileo orbiter began its scheduled 23-month tour of Jupiter and its moons, armed with a sophisticated array of cameras and other sensors that have given scientists their most detailed look at the jovian system. Among the most exciting results are images suggesting the moon Europa harbors an ocean of liquid water beneath its icy crust. Europa has since gained interest as a possible abode of life beyond Earth. When the primary mission ended in 1997, the probe was given a two-year extension to perform additional close inspections of Europa and neighboring moon Io.
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