Gaspra, Ida, Dactyl - MBA Galileo Flybys
NASA's Galileo spacecraft, whose mission was to study Jupiter, was the first to do a close flyby of asteroids. (However, the NASA Viking Mars mission did photograph Mars' two moons which many people think are captured asteroids.)
The first asteroid roaming freely on its own around the Sun which a manmade probe viewed close up was asteroid 951 Gaspra. The Galileo probe en route to Jupiter flew within 1,600 km (1,000 miles) of this asteroid in 1991.
Gaspra is a large asteroid of shape and size 20 km x 12 km x 11 km. It has relatively few craters, and thus the consensus between scientists is that it may be the remnant of a relatively recent breakup of a larger asteroid, e.g., around 400 million years ago.
Gaspra is classified as an S-type asteroid, composed of metal rich silicates and free metal granules.
Gaspra is not an economically attractive asteroid, especially compared to many near Earth asteroids, but the data from the probe is still interesting.
There is a webpage at LANL which gives pictures of Gaspra and additional information.
Ida and Dactyl
Galileo passed within 3,000 km of the main belt asteroid 243 Ida in August, 1993. When it took its photos of Ida, it discovered that Ida has a moon, which was subsequently named Dactyl. (Actually, the moon was discovered in the data transmitted back to Earth long after Galileo had passed them.)
Ida is a large asteroid, at 56 x 24 x 21 km, and is an S-type, believed to be descended from a large body which broke up due to an impact. Dactyl is also an S-type, albeit a bit different. It may have been formed in the same collision, though it's possible that Dactyl was chipped off of Ida by a subsequent collision. We don't know the shape of Dactyl's orbit around Ida. It's unlikely that Dactyl was formed elsewhere and captured by Ida due to their weak gravities.
Neither is economically attractive due to their distance and the transportation energy required.
However, the fact that this second asteroid flyby revealed a moon supports a school of thought that asteroids are likely to come in swarms, as collisions are likely to produce groups of asteroids orbiting each other. In fact, the moon showed up only because it happened to be along the same line of sight from the camera as Ida, which was an unlikely and lucky moment if Dactyl is its only moon. Dactyl was in the foreground, i.e., a little closer to the Galileo probe than Ida. This was the first discovery of an asteroid having a moon, or a "binary asteroid". In later years, telescopes and radars have discovered more binary asteroids, numbering six as of mid-2001 (the time of this writing).
Recall in the section on radar imaging of three asteroids that passed close to Earth: two of those appear to be so-called "contact binary" asteroids, which means that they are two bodies either very close or in contact with each other, and the third asteroid imaged by a radar may also be a contact binary, as it is the most elongated, cigar-shaped asteroid known.
We can't tell if an asteroid is really one asteroid or a cluster until it gets very close to us ... or better yet, when we get close to it.
Final note here on Ida: Ida's composition varies over its surface, and Dactyl shows a small variation in composition overall. We saw that only close-up from the probe.
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