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Please help me understand: As far as I understand it, when an object gets thrown off the surface of a planet by an impact and doesn't reach escape velocity it is on a suborbital trajectory and therefore will impact on the planet eventually. If that is true, how can debris from an impact reach a stable orbit? How can the periapsis be higher than the point of the impact?

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  • $\begingroup$ Both of these are good questions, I would separate it out into two different questions, however. $\endgroup$ – PearsonArtPhoto Jan 19 '18 at 19:54
  • $\begingroup$ Simply delete the second part of your question, and ask it as a new question. $\endgroup$ – peterh says reinstate Monica Jan 19 '18 at 20:15
  • $\begingroup$ How can the periapsis be higher than the point of inpact? Why not? The point of impact is just the "launching site". While the "rocket" is the impactor. The velocity and trajectory of the ejected debris will determine the sub- ot stable orbit. $\endgroup$ – Alchimista Jan 24 '18 at 13:41
  • $\begingroup$ Obviously if something (friction during ascent or in principle other debris) slow down an otherwise escaping debris $\endgroup$ – Alchimista Jan 24 '18 at 13:53
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You are correct that if a single object is thrown off of a planet that it will either escape, or else return to that object, that orbiting isn't a solution. The way that an object can orbit is by interacting with something after it has launched. For instance, it might pass close to another large clump that will use gravity to change it's path. Another solution is that it could impact another object, causing it to enter an orbit.

The third solution is that the impact could change the orbit of the host body itself. I believe this is what was actually done in the case of forming the Moon. Also of some note is that the velocity transfer isn't an instantaneous value, but rather happens over some period of time. The periapsis can be as late as the latest change. So for a side impacting asteroid, much of the speed will remain even after it has left the Earth's surface, and will continue to accelerate pieces that went ahead of the event.

To see how this could actually happen, take a look at this paper.

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  • $\begingroup$ Could some light debris benefit randomly from gravity assist of one doomed massive highly elliptic debris? $\endgroup$ – qq jkztd Jan 19 '18 at 21:08
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    $\begingroup$ @qqjkztd It could, in theory. $\endgroup$ – PearsonArtPhoto Jan 19 '18 at 21:37
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    $\begingroup$ It would make it easier. In KSP, the (Exaggerated) case often happens where you can send in a space probe towards a moon of a planet, and end up in orbit around the planet. Same thing goes for something in space. $\endgroup$ – PearsonArtPhoto Jan 20 '18 at 1:00
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    $\begingroup$ +1 OK, well KSP doesn't handle 3-body effects correctly, but I get the idea ;-) That's probably what you mean by "Exaggerated"? $\endgroup$ – uhoh Jan 20 '18 at 1:22
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    $\begingroup$ @uhoh That and the ratio of mass to planet and distance to planets in KSP are much less then in real life. $\endgroup$ – PearsonArtPhoto Jan 20 '18 at 3:06
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You can actually have this happen in freespace. (I.e. no other bodies.)

(Imagine the following in 2d, to make things easier.)

Imagine you kick off a rotating piece of debris "North" at somewhat under escape velocity. Then somewhere near apoapsis it breaks into two pieces - one flung "West" and the other "East". The two pieces each end up with enough tangential velocity to form an orbit around the parent body.

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  • $\begingroup$ There are also a bunch of other methods for this to happen in practice. Solar wind, light in general, thermal radiation, offgassing, other gravitational influences, other electrical or magnetic influences, etc, etc. $\endgroup$ – TLW Jan 20 '18 at 2:47
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    $\begingroup$ @uhoh - have you ever seen a flywheel explode? "Flung" absolutely follows from breaking in this case. The energy is already stored in the system as rotational kinetic energy. As I said, "Imagine you kick off a rotating piece of debris". $\endgroup$ – TLW Jan 21 '18 at 5:02

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