On Aug. 28, 1993, NASA's Galileo spacecraft flew by the asteroid Ida while on its way to Jupiter.
Galileo was the first spacecraft to fly by an asteroid in the asteroid belt. Ida was actually the second one it visited after another flyby of the asteroid Gaspra in 1991.
Galileo whizzed by asteroid Ida going almost 28,000 mph and came within 1,500 miles of the asteroid's surface. During the encounter, Galileo actually discovered a tiny moon orbiting Ida! This was the first confirmed satellite of an asteroid. The International Astronomical Union named the moon Dactyl after a group of mythological beings who lived on a sacred mountain called Mount Ida.
Why it mattered
Asteroid 243 Ida was actually the second asteroid ever to be visited by a spacecraft (Galileo previously flew by the asteroid Gaspra in October 1991), but this flyby clinched the theory that asteroids could have moons.
Until the Galileo spacecraft photographed Ida and Dactyl, astronomers suspected some asteroids could have their own moons, but Galileo proved it once and for all. During the flyby, Galileo approached within 1,500 miles (2,400 kilometers) of Ida as it made its way to Jupiter.
"It was previously thought that natural satellites of asteroids could form, but they probably weren't common," Torrence Johnson, Galileo's project scientist, once said of the flyby in a NASA statement. "Having found one fairly quickly, we can say that they're probably more common than previously thought."
Ida is an S-type asteroid made mostly of silicate rock and sits in the main asteroid belt. Astronomers suspect it is what's known as a Koronis asteroid, leftover debris from an ancient crash of two much larger objects, according to NASA. It was discovered on Sept. 29, 1884 by Austrian astronomer Johann Palisa at the Vienna Observatory.
Galileo imaging team member Ann Harch actually discovered Ida's moon Dactyl on Feb. 17, 1994, months after the flyby, while reviewing data, NASA said.
Galileo did not stop to study Ida and Dactyl. As with its Gaspra flyby, the probe zoomed by to continue its trip to Jupiter. NASA launched the Galileo mission on Oct. 18, 1989 and it arrived at Jupiter on Dec. 7, 1995, orbiting the giant planet for nearly 8 years. The mission ended on Sept. 21, 2003, when Galileo intentionally crashed into Jupiter.
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