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From: carroll@uiucdcsb.CS.UIUC.EDU
Newsgroups: net.space
Subject: Re: slingshot effect
Message-ID: <15700028@uiucdcsb>
Date: Wed, 13-Nov-85 12:49:00 EST
Article-I.D.: uiucdcsb.15700028
Posted: Wed Nov 13 12:49:00 1985
Date-Received: Sat, 16-Nov-85 08:37:55 EST
References: <2360@watale.UUCP>
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Nf-ID: #R:watale.UUCP:2360:uiucdcsb:15700028:000:1156
Nf-From: uiucdcsb.CS.UIUC.EDU!carroll    Nov 13 11:49:00 1985


Very simple explanation: Suppose the space craft is traveling radially out
from the sun. Now, it approaches a planet, say toward the back side (i.e.
on the side away from the direction the planet is traveling). It will be
attracted to the planet, and therefore gain speed in the direction the
planet is moving. Because the planet is in a nearly circular orbit, it's
radial velocity is about zero, so the space craft picks up radial velocity
coming in and loses it going out in equal amounts. It does, however, gain
some velocity perpendicular to that by being attracted to the planet as the
planet moves away. So, the total velocity vector is now larger than it
was originally. What the space craft gains is lost by the planet, but due
to the large size difference no one notices. Actually slingshot effects are
a lot worse than this, since the space craft starts with orbital motion, but
that's the general idea.
	Alternatively, consider two objects approaching each other, just
offset a little. If they attract, you could set it up so that they just go
around each other a half orbit, so that each ends up moving in the opposite
direction it started in.