Asteroid Capture?

[GluttonyReaper]

Not entirely sure if this is the right place to ask this...

Anyway, I've learnt from my time in KSP that to get something captured around a planet/moon, you need to apply a force to slow it while inside its SOI. But, IRL, things like asteroids obviously don't have engines... so how do they are able to be captured by a planet's gravity? I did quick Google search, but didn't get any clear answers.

[Spacial_Anomaly]

Probably by a gravity assist (desist?) (or more generally some N-body-interaction)

[Skyler4856]

Gravity assists have the greatest effect, but out gassing could also play a major role

[GluttonyReaper]

Ok...so, just to check I'm understanding this right, the asteroid would be slowed by the gravity of a moon? I assume out gassing would be a bigger factor when considering asteroids that orbit far from their planet, like a lot of Jupiter's moons...

[lajoswinkler]

They are captured if their speed relative to the planet is less than the second cosmic speed. Sometimes, that is really the case and the object gets captured. In most cases it is not. That's why planets have way less captured objects than the real number of such objects in the Solar system.

[drakesdoom]

Just like hitchhiking if the planet swings by slow enough they can hitch a ride.

[PakledHostage]

Ok...so, just to check I'm understanding this right, the asteroid would be slowed by the gravity of a moon?

A gravitational assist from a moon is one method. Another is passing through a Lagrange point at low energy. This is how J002E3, suspected to be the S-IVB stage from Apollo 12, was recaptured from solar orbit. Yet another mechanism is two objects orbiting each other passing close to a third much more massive object. If the trajectories are just right, one of the pair of objects may be captured while the second of the pair is flung away at higher speed. This is the mechanism that is believed to give hypervelocity planets and stars their incredible speeds.

[Spacial_Anomaly]

After a quick search it seems a configuration where a binary asteroid is interrupted (tidal force) and one of them is captured is favored by physists (or at least a lot looked at). Also a circumplanetary disk could slow down the asteroid/them moon and lower their orbit (i.e. here: ADS)

[hugix]

Real life doesn't work like KSP does. In the Real world everything is attracted to everything. Not like in KSP where a object is only being affected by its parent object. For instance Jupiter also has a small (But not negligeble) effect on our spacecrafts in orbit.

Our moon got formed because we collided with another proto planet that left a lot of debris in orbit around the Earth, that debris was also attracted to the other pieces of debris that eventually formed our moon as we know it. Even funnier, our moon doesn't even orbit our planet but it orbits the sun, and the Earth gently nudges it aropund giving us the impressing it revolves around us (Because the Earth also spins). If you look at our system from the Suns point of view the Moon is being nudged from left to right on our orbital path.

perhaps the best illustration is J002E3, it's an discarded Apollo booster that was trown into an heliocentric orbit. In 2001 it was close enough to Earth to be pulled in. Because of a gentle dance between the object, Lagrange 1, the Moon and Earth it orbited the Earth for a while. See the path it made here : http://neo.jpl.nasa.gov/j002e3/j002e3c.gif

[GluttonyReaper]

Yet another mechanism is two objects orbiting each other passing close to a third much more massive object. If the trajectories are just right, one of the pair of objects may be captured while the second of the pair is flung away at higher speed. This is the mechanism that is believed to give hypervelocity planets and stars their incredible speeds.

Interesting. Perhaps this could be related to comets and their very elliptic orbits, maybe?

perhaps the best illustration is J002E3, it's an discarded Apollo booster that was trown into an heliocentric orbit. In 2001 it was close enough to Earth to be pulled in. Because of a gentle dance between the object, Lagrange 1, the Moon and Earth it orbited the Earth for a while. See the path it made here : http://neo.jpl.nasa.gov/j002e3/j002e3c.gif

In any case, I suppose using KSP physics as a basis was ... not the best idea. Also, that animation of J002E3 shows fairly clearly what's going on exactly. Real life physics makes very strange things happen...

Edited November 27, 2013 by GluttonyReaper

[K^2]

They are captured if their speed relative to the planet is less than the second cosmic speed.

Which is impossible, because from far enough away, that speed is zero. Good luck finding an asteroid going with negative speed.

Probably by a gravity assist (desist?) (or more generally some N-body-interaction)

Yeah, there have to be more than two bodies involved, because in two-body interaction energy of each body is conserved.

Well, there can be some perturbations due to masses not being perfectly spherical, which can be just enough to take an object from just above escape velocity to just below, but this is extremely unlikely to happen. In practice, any real capture is going to be an n-body interaction.

[lajoswinkler]

Which is impossible, because from far enough away, that speed is zero. Good luck finding an asteroid going with negative speed.

I don't understand the word you just said. If an asteroid is in a similar orbit with a planet, their relative speeds are low. Why would a capture be impossible if they come near each other?

Billions of years ago, such scenarios were quite common.

[andrewas]

Because the asteroid will accelerate as it approaches Earth, and wind up in a hyperbolic orbit rather than a closed one.

[K^2]

Precisely. It's either already in an orbit around the planet, in which case there is nothing to capture, or it's on an escape trajectory, and it's not going to be captured.