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I was thinking about asteroids which travel through solar system and I have a question. Is it possible to inject small object into an orbit around one of them and use momentum of this asteroid to increase range of our object?

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closed as unclear what you're asking by TildalWave, Hohmannfan, Brian Tompsett - 汤莱恩, Rikki-Tikki-Tavi, GdD Feb 12 '16 at 13:22

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    $\begingroup$ What do you mean by "increase range"? If you mean a gravity assist, 1) asteroids are too light, 2) gravity assists don't work like that. $\endgroup$ – SF. Feb 12 '16 at 9:12
  • $\begingroup$ Well, there is Dawn. It's not that small as far as robotic spacecraft go, but it has orbited Vesta and is now orbiting Ceres, both in the asteroid belt. And there were other spacecraft that were in the neighborhood. What do you mean with increasing range? To use these objects for gravity assist? $\endgroup$ – TildalWave Feb 12 '16 at 9:14
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    $\begingroup$ @Zgr3doo Sure, but if you match its orbit to inject into orbit around it, you don't really require it to "extend range", you'd be on that trajectory anyway. Also, Rosetta is currently doing that. And there were others before, and there's a couple upcoming ones, like Hayabusa 2 on its way to 162173 Ryugu, OSIRIS-REx that is scheduled to launch to 101955 Bennu this September,... Here's e.g. NEO delta-v chart. $\endgroup$ – TildalWave Feb 12 '16 at 9:39
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    $\begingroup$ @Zgr3doo: Hohmann transfer is the most basic technique of matching orbits. $\endgroup$ – SF. Feb 12 '16 at 10:52
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    $\begingroup$ You'll have to very nearly exactly match its orbit first before going into orbit around it. There's no other way, otherwise you have hyperbolic excess velocity with respect to it and orbit insertion will fail. Most asteroids have only a tiny escape velocity at reasonably stable orbital altitudes. Go that fast or faster in any vector with respect to it, and your inertia will cause you to drift away from it. Unless, of course, you hit it. That's also one way of matching its orbit (assuming you don't bounce off or the asteroid fragments on impact). $\endgroup$ – TildalWave Feb 12 '16 at 11:03
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This is possible, but entirely pointless. Spacecraft are not limited by range, only by how much they can change their velocity, usually called delta-v. If you can change your velocity enough to catch up with the asteroid, you are already in the same orbit as it. That means, You can do that even if the asteroid was not there.

You are perhaps also thinking about a gravity assist, by which a spacecraft gain momentum from a fly by of a planet. But that only works with massive objects, like planets, and the gain from an asteroid fly-by is incredibly small. Also, if this manoeuvre is performed by first braking into orbit, as you describe, more delta-v is already wasted than you can possibly gain from the manoeuvre.

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Orbiting does not work that way.

Getting into orbit around an object means to get close to it and then change your speed and direction to be almost the same as the object. That means you can't "hitchhike" with a fast asteroid, because the energy you would need to rendezvous with it would be the same energy you need to get onto the same trajectory it already has.

What might give you a speed boost is going onto a collision course with the asteroid and having it ram you (like a billard ball). But building a spacecraft which can survive such a collision would be quite challenging.

What might give you a much safer speed boost is doing a fly-by with the asteroid to give you a gravity assist. But the speed-gain by a gravity assist depends on the mass of the object, so you won't get much from an asteroid. A gravity assist is usually only worth the trouble when it's with a planet or at least a large moon.

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  • $\begingroup$ quite challenging, what an understatement. $\endgroup$ – gerrit Feb 12 '16 at 11:13

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