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The pictures of Rosettas orbit show it doing some straight legs past 67P for the next while.-

Does this mean that at the current distance, the force of 67P's gravity is not really significant, and Rosetta is not really (yet) "in orbit" but rather just flying in the same general direction at the same time?

TildalWave
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GreenAsJade
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1 Answers1

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Rosetta will shift between different hyperbolic trajectories. This means that it won't move in an ellipse around the comet, but escape it if not repeatedly powered to another hyperbolic trajectory in a series of flybys. Trajectory changes are planned every Wednesday and Sunday. It is not what most ordinary people would think of as an "orbit", hence the confusion.

See from 16:45 in this video of the press conference August 6th.

Community
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LocalFluff
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  • So it is fair to say - almost by definition - that it is not "in orbit", because it has a hyperbolic trajectory; that in fact, it is just repeatedly passing by while it happens to be conveniently in the neighborhood? – GreenAsJade Aug 07 '14 at 12:43
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    Perhaps fair to say that the delta-V between spacecraft and comet will never drop below escape velocity (1.5 ft/s) during any of the flybys. Hence it will never be in a true orbit. – Anthony X Aug 07 '14 at 22:39
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    It will enter a more stable elliptical orbit once comet's mascons are mapped more precisely. IIRC it will reduce semi-major axis 10 fold from what's now roughly 100 km. – TildalWave Aug 08 '14 at 11:33
  • @TidalWave Yeah, what is all that "elliptical orbit" talk about? That tiny microgravity comet cannot gravitationally have Rosetta actually orbit it at 100 or 50 km distance. Or what? I suppose the spacecraft will use its engine continuously when it is in (a forced) elliptical orbit. – LocalFluff Aug 08 '14 at 12:53
  • @TildalWave Mascon? For the uninitiated please? – Everyone Aug 08 '14 at 13:05
  • @Everyone mass concentration http://en.wikipedia.org/wiki/Mass_concentration_(astronomy) – TildalWave Aug 08 '14 at 13:24
  • @LocalFluf https://www.youtube.com/watch?v=Mf1zsACcXc4 and I'm not sure I understand your comment. Your answer only addresses the hyperbolic quasi-orbit part, but surely it would be easier to complete your answer than bicker over semantics in the comments? BTW 67P isn't all that tiny. It's about the size of Mt. Blanc. – TildalWave Aug 08 '14 at 13:37
  • Mt Blanc is tiny! This question only addressed the hyperbolic phase of Rosetta's trip. I think a followup question (a separate question) would be interesting: "when Rosetta enters the next phase of its visit, where it is shown travelling in an eliptical path around P76, then is it truly in orbit, or is it thrusting the whole time to stay on that path?" But that would be a separate question to this one. – GreenAsJade Aug 11 '14 at 02:15
  • @TidalWave I don't mean to "bicker" about any semantics. If Rosetta actually orbits the comet by gravitational force, I'd love to learn about it. Climbing Mt Blanc is a hardship, but only because it has the Earth underneath it. And this tiny comet has a density lighter than cork. It is more of a cloud than a mountain. – LocalFluff Oct 01 '14 at 11:01