Since reaching Jupiter in December 1995, the Galileo spacecraft has carried out its planned mission by returning a wealth of images and data on Jupiter and its moons. But the intrepid spacecraft also made some important discoveries during its long journey to Jupiter, which began when it was released from the cargo bay of the space shuttle Atlantis on October 18, 1989. While making the orbital "slingshot" maneuvers that would eventually propel it to Jupiter, Galileo made planned observations of two asteroids beyond the orbit of Mars.
Until the NEAR mission, this was humankind's first look at these mysterious objects. Here are the best images constructed from the Galileo transmissions:
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Galileo's approach to Gaspra on October 29, 1991, coming within 1,600 kilometers (1,000 miles), marked the first time that a spacecraft made a fly-by of an asteroid. This image is a composite of the highest resolution data captured during the encounter.
Gaspra was discovered by Grigoriy N. Neujamin in 1916 but received little attention until it was discovered that Galileo's trajectory would take it close to the asteroid. Gaspra measures approximately 19 by 12 by 11 kilometers (12 by 7.5 by 7 miles). The area shown here is about 18 kilometers (11 miles) from lower left to upper right. Unlike Mathilde which contains carbon-based material, Gaspra is an S-type --or stony--asteroid.
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On August 28, 1993, Galileo captured the data used to make this image of Ida just 14 minutes before its closest approach but still about 10,500 kilometers (6,500 miles) away. Like Gaspra, Ida is classified as an S class (stony or stony iron) asteroid. But Ida is more than twice the size of Gaspra, measuring 56 by 24 by 21 kilometers (35 by 15 by 13 miles). Ida's heavily cratered surface suggests that it has existed in its present form for at least a billion years
The most impressive discovery of the Galileo encounter was that Ida has a moon (right). This tiny satellite, the only known object orbiting an asteroid, was not identified until images stored in Galileo's tape recorder were transmitted to Earth in February 1994. It has since been named Dactyl.
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