Asteroids can have moons

This afternoon my postman delivered a review copy of The Space Book by Jim Bell. This is the latest book in a series that includes Cliff Pickover’s math, physics, and medical books. Like the other books in the series, The Space Book alternates one-page articles and full-page color images.

Here’s something I learned while skimming through the book: Asteroids can have moons. (That’s the title of the article on page 414.) This has been known since the early 1990′s, but it’s news to me.

The first example discovered was a satellite now named Dactyl orbiting the asteroid 243 Ida. The Space Book says Dactyl was discovered in 1992. Wikipedia says Dactyl was photographed by the Galileo spacecraft in 1993 and discovered by examining the photos in February of 1994. Since that time, “more than 220 minor planet moons have been found.”

Posted in Science
7 comments on “Asteroids can have moons
  1. mpleger says:

    My naive thought was that the meteor that landed in Russia earlier this year was a moon of the larger asteroid that just missed earth. However, I never got around to investigating it further. That being said, the MSM were reporting that they weren’t meant to be connected.

  2. In something as chaotic and numerous as the minor chunks of rock in the solar system, if it can happen it will happen, and of course any two things can get gravitationally bound together. So this doesn’t surprise me.

    The sci-fi author Stanislaw Lem has some amusing stories, one of which I think involved a space explorer with some humorous trash item orbiting his spaceship. I don’t remember the particulars, just a vague memory.

  3. The suffocated says:

    I too ain’t surprised by the fact that an asteroid has a moon (although this is new to me). What I find surprising, however, is that when Dactyl was spotted, it was only 90km away from Ida.

  4. John says:

    The surface gravity of 243 Ida is about 1/100 that of earth. Escape velocity would be around 1/600 that of earth, around 20 m/s. So it’s surprising that something so small could capture and retain a satellite.

  5. Ed Davies says:

    mpleger, no, the two were on completely different trajectories.

    http://en.wikipedia.org/wiki/Chelyabinsk_meteor#Coincidental_asteroid_approach

    Also, they were so far apart that any gravitational interaction between them would have been minute – even if they’d been on parallel trajectories they couldn’t have been called moons by any stretch of the imagination.

  6. Ed Davies says:

    I don’t know for sure but my guess would be that asteroid moons are not typically captured objects but rather lumps which have been knocked off the main rubble pile but collision with a much smaller object – but not quite fast enough to get away.

  7. There used to be an xscreensaver that simulated gravitationally attracting masses in 2D, it was interesting to watch all the chaotic dynamics. One way they could get bound would be direct physical collisions, another way is a multi-body gravitational “collision” where several things all get attracted to a bigger mass and in the process they could get spit out with a large velocity relative to the larger object but a small velocity relative to each other, so that they bind together.