How to be captured by a planet ?

[Tatonf]

Ok I played KSP for two years and I know how to capture a rocket with retroburn or aerocapture.

I also know what a gravity assist is.

But I was wondering : in real life, planets can "capture" asteroids. How does it works ? I mean, the asteroid can't do a retroburn, and if they manage to get "aerocaptured" it means that their periapsis is inside the planet's atmosphere and they will eventually fall on the planet.

So, how does it work ? How Eve have been able to capture Gilly for exemple ? Can we do something similar with spacecraft ?

Edited August 4, 2015 by Tatonf

[Mastikator]

A moon can gravity assist an asteroid into orbit.

It actually happened in a game of mine once. I discovered a E class asteroid in orbit around Kerbin, I didn't put it there.

[Tatonf]

Oh ok that's interesting.

But that mean that Gilly shouldn't be where it is.

[Senormeens]

its happens when two bodies similar enough orbital velocities interact with each other, so for eve and gilly, imagine that eve is orbiting around the sun at (i forget what it is in game and for simplification reasons) 1000m/s and an asteroid (gilly) is orbiting at 1050m/s (again simplifying for the "capturing" part, i don't know the maths to say what is necessary for an orbit, and not lithobraking)

If gilly were to enter eve's SOI its velocity now compared to eve is around 50m/s, so if eve's gravity is strong enough, it can prevent gilly from exiting its SOI, so it now has gilly captured.

Now that is way over simplified and that scenario would probably end in gilly crashing into eve with such a low orbital velocity, eve's gravity would just pull it into eve. what the exact numbers are for a stable orbit? that requires maths and physics knowledge i don't have.

Edit: my computer decided to change "gilly" to "billy"

[Snark]

Oh ok that's interesting.

But that mean that Gilly shouldn't be where it is.

Well, Venus has no moons IRL...

A couple of plausible ways that Gilly could be orbiting Eve:

1. It could have formed there in the first place

2. There could have been two bodies passing by Eve at around the same time (Gilly plus one other), which interacted in a way that left Gilly in orbit while ejecting the other.

its happens when two bodies similar enough orbital velocities interact with each other, so for eve and gilly, imagine that eve is orbiting around the sun at (i forget what it is in game and for simplification reasons) 1000m/s and an asteroid (gilly) is orbiting at 1050m/s (again simplifying for the "capturing" part, i don't know the maths to say what is necessary for an orbit, and not lithobraking)

If gilly were to enter eve's SOI its velocity now compared to eve is around 50m/s, so if eve's gravity is strong enough, it can prevent gilly from exiting its SOI, so it now has gilly captured.

Actually, it doesn't work that way-- the original poster had it right, that's a scenario where capture doesn't really work. Setting aside for the moment that planets in real life don't have an actual SOI (gravity goes on forever): if the asteroid entered the planet's field at some relatively slow velocity (say, 50 m/s) and managed not to collide with the planet itself, then it would just swoop around in a parabola and exit again with the same 50 m/s that it entered with.

The real answer to the question of "how do things get captured" is that there has to be an interaction with a 3rd body involved. Two bodies in isolation cannot result in capture.

Edited August 4, 2015 by Snark

[Tatonf]

Ok thank you !

[KerikBalm]

supposedly I've head that non-uniformity in the planets gravity (ie, its not a perfect sphere with its mass evenly distributed with no asymetries in density) can result in capture of bodies in very similar orbits.

[cantab]

In real life there aren't clear SOIs with sharp boundaries where nothing but the primary affects your orbit. Objects can be weakly bound walking the edge between escaping and not escaping. Other planets and moons can perturb the orbit subtly. Tidal forces can play a role, in particular in making circular an orbit that starts out eccentric. Binary objects can have one member captured by a planet while the other one is sent flying off.

[Kaos]

Not to forget that the sun also disturbs the orbits. Some time in the past, the laws of physics in KSP must have changed to patched conics physics, hence this sun-supported capture is not possible anymore

[Yasmy]

Likely scenarios:

1) The moon formed from the same cloud of material as the parent, in which case it was already captured.

2) The moon formed from ejecta from the parent as a result of a collision with the parent.

Much less likely:

3) The moon formed from a portion of an asteroid which broke up due to tidal forces from the planet. The remaining portion was not gravitationally bound and left the orbit of the planet. Many asteroids are fairly weakly bound material, so breakup due to tidal forces is not unusual, but you have to have just the right conditions to leave some material behind.

(I just noticed that cantab said essentially the same thing: binary objects can have one member captured.)

4) The moon formed from debris of asteroids which collided. Space is big, and asteroids are tiny. This is incredibly unlikely, but not impossible.