10

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.

Mark
  • 525
  • 4
  • 11
  • 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 02 '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? – jamesqf Oct 02 '17 at 16:45
  • @ jamesqf : https://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 04 '17 at 12:11

5 Answers5

16

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.

Mark Foskey
  • 10,971
  • 34
  • 47
Mark Adler
  • 58,160
  • 3
  • 171
  • 251
  • 5
    could you elaborate why (in principle) this wouldn't be possible without the Moon? – mb21 Oct 02 '17 at 14:01
  • But the probability that all parameters would fit for an orbit is low, I think? – Uwe Oct 02 '17 at 14:17
  • 6
    @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. – Ajedi32 Oct 02 '17 at 14:21
  • @Ajedi32 What about aerobraking? –  Oct 02 '17 at 14:30
  • 1
    Orbits are closed paths, so if you go through the atmosphere once, you will return to it. The asteroid may be in orbit for one or maybe two orbits, but then it enters. Unless you have a really light aerobrake, and your resulting apoapsis goes out to the Moon, and the Moon is there when you get there and you fly by the correct side of the Moon, then your periapsis will be raised to prevent a reencounter with the atmosphere. Now however you are in a Moon-crossing orbit, which is unstable and will likely result in either being ejected or entering Earth's atmosphere anyway. – Mark Adler Oct 02 '17 at 14:38
  • 2
    @tubes aerobraking won't put it into a stable orbit either because the perigee has to be in atmosphere and there is no way to raise it even if the capture worked. Again the Moon might help I theory. – jkavalik Oct 02 '17 at 14:38
  • @Ajedi32 But an object moving towards the Earth-and-Moon will also gain momentum due to gravity as it approaches. So why doesn't the same conservation of energy argument apply there, too? – David Richerby Oct 02 '17 at 14:41
  • As for not needing the Moon, if you happen to have a really low $V_\infty$ relative to Earth, then it is possible for a very slight tug from Jupiter at the same time as an encounter with Earth to put you in Earth orbit. But such an orbit is so incredibly loose that it is not stable and you will soon depart again. – Mark Adler Oct 02 '17 at 14:42
  • 16
    @MarkAdler It's better to put the full explanation in the answer; comments in SE are considered ephemeral. – uhoh Oct 02 '17 at 14:47
  • 1
    @DavidRicherby Since two celestial bodies are involved there, it's possible for the object to lose momentum (relative to Earth) by essentially giving that energy to the Moon using a maneuver known as a Gravitational Assist. The physics of that are a bit harder to explain with text. Try looking up a few visual simulations of gravity assists on YouTube. – Ajedi32 Oct 02 '17 at 14:47
  • 5
    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). – Mark Adler Oct 02 '17 at 14:47
  • 3
    @uhoh Feel free to edit my answer. – Mark Adler Oct 02 '17 at 15:01
  • 1
    @MarkAdler the propulsion can be pure light sail, in which case the entire initial object remains together. Not sure if you can still call it a two-body system though? – Ross Presser Oct 02 '17 at 15:08
  • @RossPresser Good point! Yes, not a two-body system. The other bodies here however are not the Sun, but the photons! Also on really long time scales, the Yarkovsky effect could get something into a really loose orbit. Again photons absorbed and re-emitted for the momentum transfer. – Mark Adler Oct 02 '17 at 15:14
  • 1
    What about tidal forces? The moon Phobos of Mars is lowering its orbit by tidal forces and the other moon Deimos is rising. But could tidal forces explain a capture of an asteroid? – Uwe Oct 04 '17 at 12:06
  • Great question! Yes, tidal forces could, though the effect of a tidal force is vanishingly small for a single pass. The approaching thing would be need to be at a $V_\infty$ of very nearly zero for one pass to pull it over the edge. – Mark Adler Oct 04 '17 at 15:02
8

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.

RobJ
  • 81
  • 2
  • 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. – PearsonArtPhoto Oct 02 '17 at 15:48
  • 1
    @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 02 '17 at 16:22
  • 2
    Periapsis can become lower through aerobraking, but that doesn't lead to a stable orbit. :) – Ross Presser Oct 02 '17 at 16:23
  • 2
    Yeah, that's what I meant, you can't raise the periapsis by aerobreaking... – PearsonArtPhoto Oct 02 '17 at 19:02
3

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.

Loren Pechtel
  • 11,529
  • 26
  • 47
2

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.

Nuclear Hoagie
  • 3,702
  • 1
  • 19
  • 16
1

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.

Onyz
  • 111
  • 2