[X] My Assistant

[Chmee]

A question that has bugged me for a long time is how planets capture asteroids, etc into an orbit. My understanding of orbital dynamics is that a body approaching a planet would need to be "braked" in order to be captured into orbit. In the same manner that our space probes use their rockets to slow them down enough or they would shoot past.

So for moons like Triton, Deimos, and Phobos (as well as the small, distant moons of Jupiter/Saturn) how were they captured? What provided the braking?

[Guest_BruceMoomaw_*]

If the relative velocity of the planet and the potential moon is low enough, third-body gravitational effects can jockey the moon into orbit around the planet -- that is, the fact that both objects are orbiting the Sun at about the same distance out can by itself allow the moon to approach the planet slowly enough to be captured into orbit by it when it reaches the planet's vicinity. But for this to happen, their orbits and thus their velocities must be very close to begin with -- which was always one problem with the "capture" hypothesis for the origin of Earth's own Moon.

Instead, most captured moons are captured by one of two mechanisms: either they brushed through the cloud of gas that was still around a forming planet and were slowed down by friction, or they collided with a hunk of debris from the large cloud of solid debris that was still orbiting (or being pulled into) the planet while it was forming and got braked into orbit that way. There is a debate as to which of these mechanisms was more important in the early Solar System, and whether we'll ever be able to estimate whether a particular captured moon was captured by one or the other. (This is also one reason for the continuing dispute over the origin of Phobos and Deimos: it's hard to conceive of either mechanism being strong enough around the forming Mars for their capture to be likely.)

[Bob Shaw]

If the relative velocity of the planet and the potential moon is low enough, third-body gravitational effects can jockey the moon into orbit around the planet -- that is, the fact that both objects are orbiting the Sun at about the same distance out can by itself allow the moon to approach the planet slowly enough to be captured into orbit by it when it reaches the planet's vicinity.  But for this to happen, their orbits and thus their velocities must be very close to begin with -- which was always one problem with the "capture" hypothesis for the origin of Earth's own Moon.

Bruce:

The recent classic instance of a more-or-less (Solar) co-orbital object being captured by a planet must be the Apollo 12 S-IVB, but without some actual change in velocity such captures will be quite 'loose' - like the outermost semi-moons of the giant planets, which may not know quite whether they are asteroids, Trojans or moons. Phobos and Deimos, however, seem to me to defy all logic, though the very fact there's two of them surely suggests some common factors...

Do you have any good links regarding capture mechanisms?

Bob Shaw