Today’s Factismal: The first sighting of a moon orbiting an asteroid was made in 1994.
If you have ever gone on a long journey, you know that it is often a good idea to stop every once in awhile to look at the local scenery. You can rest your legs and get more out of the trip than you would if you simply raced on to your destination. The folks at NASA know this, too.
That’s why they planned the Galileo mission so that it could take an up-close look at an asteroid while the probe headed out to Jupiter. Galileo was one of only three space probes launched by the Space Shuttle (other probe launches were cancelled after the Challenger disaster due to the risk). Using a combination of gravity-assist and chemical rockets, Galileo’s primary mission was to study Jupiter. But on the way, it would pass through the asteroid belt, which provided an excellent opportunity to study these rocks in detail.
The asteroid that NASA chose to study was 243 Ida. From spectroscopy, we knew that it was a S-type (rocky) asteroid, but didn’t have many details about the composition. Was it uniform or chunky? Was it one big rock or lots of little ones (a “rubble pile”)? This was a rare chance to find out; 243 Ida would be only the second asteroid ever visited by a space probe.
As Galileo raced past the asteroid, it took a series of pictures and sent them back to Earth for analysis. And, while looking through those images, we discovered a lot of interesting things. 243 Ida was shaped like a croissant. It was made up of chunks of rock, blanketed over the surface in a loose regolith (“rock blanket”) and covered with impact craters. But, most astonishing of all was the fact that 243 Ida had a tiny moon orbiting it.
Since then, moons have been discovered around other asteroids. Now that we know what to look for, we have seen it elsewhere. But this discovery changed how we think about asteroids and has led to more research into how they form and interact.
If you’d like to be part of that work, then why not join the Lowell Amateur Research Initiative?