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1995-05-24
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The Galileo spacecraft made the first and second close observations of asteroids in 1991
and 1993, as it passed through the main asteroid belt on its way to Jupiter. The first
encounter occurred nine months into the Galileo mission, while the probe was still
gathering up speed through gravity-assists with Earth and Venus.
On October 29, 1991, Galileo made the first asteroid encounter in history. The probe
passed just 1,000 miles from the asteroid Gaspra at a speed of 18,000 miles per hour.
Scientists collected pictures of Gaspra and other data on its composition and physical
properties.
Observations of Gaspra revealed a cratered, complex and irregular body about 22 by 14 by
12 miles. The asteroid also had a thin covering of dirt-like "regolith," a top-blanket of
rock. Instruments on Galileo detected a possible magnetic field, indicating that the
asteroid may have a large percentage of metallic components in its structure.
On August 28, 1993, Galileo had a second asteroid encounter, with a larger more distant
body called Ida. This asteroid is about 34 by 12 by 15 miles in size. It rotates every 4.6
hours on its short axis. Like Gaspra, Ida seems to have a magnetic field, a sign that metals
may be present.
The closest approach that Galileo made to Ida was about 1,500 miles, at a relative speed
of nearly 28,000 miles per hour. The images and other data revealed a surface covered
with craters, suggesting a much greater age than previously thought. The surface of Ida is
much older than that of Gaspra.
Almost immediately, Galileo discovered the first moon of an asteroid. Ida's satellite was
found in both a camera frame and an infrared scan. The 1-mile diameter satellite, later
named Dactyl, was estimated to be about 60 miles from the center of the asteroid, in a
roughly circular orbit.
In the future, robotic and human missions to the asteroid belt may be sent to prospect and
mine useful minerals, metals, and fuels for industry.