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From: jmc@wuphys.UUCP (Jimmy Chen)
Newsgroups: net.physics
Subject: slingshot effect
Message-ID: <395@wuphys.UUCP>
Date: Wed, 13-Nov-85 23:35:20 EST
Article-I.D.: wuphys.395
Posted: Wed Nov 13 23:35:20 1985
Date-Received: Fri, 15-Nov-85 04:54:20 EST
Organization: Physics Dept., Washington Univ. in St. Louis
Lines: 40
Keywords: gravity

>Can anyone explain to me (in 250 words or less) the famed
>'slingshot' effect that is used to accelerate space probes?
>
>I understand this effect is used not only to change the
>trajectory of the craft, but also its *speed*.
>
>I know it has to do with angular momentum, but *how* is the
>planet's momentum transfered to the space craft?
>
>PS. keep it simple if possible..
>
>thanx
>
Newton's Law of Gravity says that, in principle, gravity is a
universal force and exerts its influence everywhere.
In practice, this is not always relevant.  For example,
to calculate the trajectory of a falling pencil it is
not necessary to include the gravitational effects of
the sun, the moon, etc.  Only the earth's gravity is of
any importance.  So, we can imagine the sun, planets,
moons, and other heavenly bodies of the universe being
surrounded by a domain of influence where its gravity
dominants all other heavenly bodies.  Consider the
trajectory of a satellite, say the Voyager.  During most of its
trajectory it orbits the sun everything else being
irrelevant.  When it enters Jupiter's
domain of influence we can imagine the sun's gravity
being turned off and Jupiter's on.  Since Jupiter is
itself orbiting the sun, Voyager is carried along as a
rock is carried in a slingshot which we whirl around
our head.  Just as the rock picks up the momentum of
the slingshot, Voyager picks up the momentum of
Jupiter.  When Voyager leaves the influence of Jupiter
for that of the sun its velocity is increased if it
leaves in the direction of Jupiter's motion and
decreased if it leaves in the opposite direction.
That's roughly it.


                          Jimmy Chen (wuphys!jmc)