Starship Asterisk*

Captured Object Question

Often I read about a moon or planet that may be a captured object.

What I question is how such a capture can happen?

It would seem that any object would have to undergo some kind of orbital evolution to end up in a circular orbit?

Norval

exactly - most objects that are suspected captures are in elliptical orbits - hence the reason they are suspected.

For example, certain moons of Saturn (can't remember which off of the top of my head) have far flung out, elliptical orbits, and they also have retrograde orbit, which makes them prime candidates for capture objects.

Comets and such have highly elliptical orbits that often pass thorugh planetary orbits - gas giants such as Jupiter and Saturn have such a large gravitational influence that they could easily capture such objects...

Empeda
True, one of Saturn's moons is in a retrograde orbit, Phoebe.
It is also the farthest out orbit, but not very elliptical.

Quoted from this page, http://nineplanets.org/phobos.html
"Open Issues
It seems likely that Phobos and Deimos formed elsewhere and were subsequently captured by Mars. But how did the capture occur? Was it made possible by a thicker Martian atmosphere long ago?"
Both Phobos and Deimos are not elliptical in their orbits. And I think they are grasping at straws about a thicker atmosphere, but then again maybe not. For a look at Deimos go here, http://nineplanets.org/deimos.html

While doing some research on these "captured objects" Gale (FieryIce) and I came across this page describing Trojans of Jupiter.
http://www.ifa.hawaii.edu/~sheppard/sat ... rojan.html
We had never heard of them till today.

Astronomy is indeed a fascinating learning experience.

Norval

Some of it had to do with the speeds of an elliptical orbit and the micro collision that take place during aphelion (slower orbital speeds) and perihelion (faster orbital speeds). Over all, micro impacts (mostly from the solar winds) have a negative effect on the speed of an orbiting body; the collisions have a greater negative effect during perihelion (E=MV^2) assisting in the decrease the elliptical properties of the orbit.
Other factors are gravitational and magnetic resonances (the reason the Moon keeps the same face to the Earth). It’s rare to see moons that don’t have some type of resonance because their orbits remain unstable and more likely to escape or crash into the host body.

craterchains wrote: Both Phobos and Deimos are not elliptical in their orbits.

Of course, all orbits are slightly elliptical but I see what you're saying...

Aren't we just looking a statistical resonances here? What I mean it that for every Pheobe that makes it in an orbital resonance, there must have been many hundreds, maybe thousands more that didn't? (I'm thinking Shoemaker-Levy 9 in 1994 for example).

There are many aspects to consider, that is for sure, in trajectories and forces acting on such an object to allow for it's capture, AND eventual semi-circular orbit. The same "solar wind" that could act to slow it could also increase it's momentum / speed if in the correct position. Asteroids / comets have allot of mass to get slowed down by this method I would think.

Just to get Cassini parked around Saturn took allot of force to slow it down on several elliptical orbits.

But, as it is stated, we are all scratching our heads over some of these objects orbits in our solar system.

Norval