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jfribrg
2006-Aug-09, 03:43 PM
I am having some trouble understanding the mechanics of a gravity assist. I've been trying to make sense of the chapter on this topic in Prussing and Conway's "Celestial Mechanics", especially Fig 7.9 on Page 133 (http://dretechnologies.com/images/page133.jpg). Taking the New Horizons spacecraft as a concrete example, From a jovain-centric frame of reference, it will be on a hyperbolic trajectory, with the approach and departure at the same velocity (ignoring for this discussion the thrust from any near-perijove rocket firings). At the same time the heliocentric speed will increase. This sort of makes sense to me, but then I see the above mentioned diagram and discussion. They mention that the heliocentric velocity increases if the spacecraft passes behind Jupiter and decreases if the spacecraft passes in front of jupiter. It seems to me that they have it backwards. If anyone can clear this up or point me to another reference, I would appreciate it.

NEOWatcher
2006-Aug-09, 04:33 PM
...They mention that the heliocentric velocity increases if the spacecraft passes behind Jupiter and decreases if the spacecraft passes in front of jupiter. It seems to me that they have it backwards. If anyone can clear this up or point me to another reference, I would appreciate it.
Because from behind, the heliocentric velocity is "pulling" the spacecraft along. In other words, the overall hyperbolic system is moving the same as the direction of travel.
From the front, the hyperbolic system is moving toward the spacecraft, thus the entire system is slowing it down.

mugaliens
2006-Aug-15, 12:00 AM
True. Moving behind slows Jupiter and accelerates the craft.