# Can an asteroid enter Earth orbit?

Can an asteroid pass near Earth at a right velocity and angle to bend its trajectory and make an orbit?

Note: I know about objects such as 2016 HO3, which are quasi-satellites of Earth due to Lagrange points. I'm asking whether an asteroid can become an actual satellite of Earth.

• You can read more about 2016 HO3 in this question. The phrase "quasi-satellite" is only a "quasi-scientific" term. HO3 is in orbit around the Sun, but will spend a while in a 1:1 resonance with the Earth due to perturbations. If the Earth suddenly disappeared, it would still be in a very similar orbit around the Sun. An answer to your question may involve temporary "capture" in a chaotic "mini-moon"-like orbit. See this question for more on that.
– uhoh
Oct 2 '17 at 12:21
• I think it's safe to say yes. Earth is a planet. Other planets from Mars on out have satellites which appear to be captured asteroids (or KBOs), so why shouldn't Earth be able to capture one? Oct 2 '17 at 16:45
• @ jamesqf : en.wikipedia.org/wiki/Moons_of_Mars "The origin of the Martian moons is still controversial" We don't know for sure that the moons of Mars are captured asteroids (or KBOs).
– Uwe
Oct 4 '17 at 12:11

Our Moon makes this possible. An asteroid with a low $$V_\infty$$ with respect to Earth making a close flyby of the Moon in the right direction could get into a distant orbit around the Earth. That object would likely continue to encounter the Moon, and could be ejected again.

It is important to realize that things cannot just drift into orbit without something to slow them down. This results from conservation of energy. An object moving towards Earth will gain momentum due to gravity as it approaches. Unless something else interferes to slow the object down (like a gravitational assist by the Moon), that kinetic energy will also necessarily be enough to allow the object to escape Earth's gravitational pull.

• could you elaborate why (in principle) this wouldn't be possible without the Moon?
– mb21
Oct 2 '17 at 14:01
• @mb21 Conservation of energy. An object moving towards Earth will gain momentum due to gravity as it approaches. Unless something else interferes to slow the object down (like a gravitational assist by the Moon), that kinetic energy will also necessarily be enough to allow the object to escape Earth's gravitational pull. Oct 2 '17 at 14:21
• @MarkAdler It's better to put the full explanation in the answer; comments in SE are considered ephemeral.
– uhoh
Oct 2 '17 at 14:47
• In a two-body system, an object approaching on a hyperbolic trajectory leaves on the same hyperbolic trajectory with the same energy it came in with. The only way to change it from hyperbolic to elliptical is either with a third body (the Moon, Jupiter, etc.), or with drag (atmospheric pass), or with propulsion (in which case some of the initial object ends up in orbit, not all of it). Oct 2 '17 at 14:47
• @uhoh Feel free to edit my answer. Oct 2 '17 at 15:01

Aerobraking would not work. If the asteroid shaved off enough speed to put it into orbit, the orbit would be highly eccentric resulting in repeated encounters with the atmosphere and eventual impact.

• You might add in a piece like the periapsis can't be changed by aerobreaking, but otherwise, this is a decent answer. Also, you might factor that the Moon might allow for a gravitational slingshot type effect that could allow it to orbit. Oct 2 '17 at 15:48
• @PearsonArtPhoto Except that isn't true, the periapsis will change with aerobreaking, just not by nearly as much as the apoapsis. In order for the periapsis to not change during aerobreaking, all of the aerobreaking has to occur at the periapsis, and none before or after, no?
– Matt
Oct 2 '17 at 16:22
• Periapsis can become lower through aerobraking, but that doesn't lead to a stable orbit. :) Oct 2 '17 at 16:23
• Yeah, that's what I meant, you can't raise the periapsis by aerobreaking... Oct 2 '17 at 19:02

Capture inherently requires three bodies. By far the easiest way is with the moon being the third body. There is another capture scenario, though--take two asteroids that are orbiting each other. They approach very slowly, when they pass Earth they are moving just over escape velocity. They pass very close, the one moving forward at that point hits the atmosphere and goes splat. This leaves the other one with a periapsis that is dangerously low and an apoapsis that is high enough to be unstable.

• Brilliant, bravo!
– uhoh
Aug 17 at 4:26

Very unlikely.

One interesting thing about orbital trajectories is that they are reversible in time. You can watch a rocket take off from earth and enter orbit, and if you want to know how to land it, you can imagine doing the exact same maneuvers but in reverse. This is handy to get an intuitive grasp on what's possible. If an asteroid could come from deep space and get captured in earth's orbit spontaneously, that would mean that it's possible to do the reverse - have an object in orbit around earth that gets spontaneously launched into deep space completely for free without spending any fuel at all. The only way anything like this would be possible is with a very precise gravitational interaction with a third body such as the moon, but that would have to occur just right to capture an object or launch it out of orbit. Any object approaching earth from deep space is moving at escape velocity, and will need a way to actively slow down to enter earth's orbit.

It really depends whether you're willing to consider collisions, and how large an asteroid.

An object of sufficient size impacting the Earth would shear off a good amount of material and disturb the orbit of the Earth. This could potentially slow down the impacting object enough while still keeping it distinct from the Earth, causing it to orbit around the Earth.

I believe the currently predominant theory is that this is how the Moon was formed.

• I refer to that as lithobraking. Oct 2 '17 at 17:30
• Is crashing into the Earth considered "becom(ing) an actual satellite of Earth"? Because that's what the question asks. Oct 3 '17 at 3:28
• It is an extreme example of that, though one could argue that the thing being orbited is no longer Earth. Oct 3 '17 at 4:52