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From: KFL@MIT-MC.ARPA ("Keith F. Lynch")
Newsgroups: net.space
Subject: Slingshot effect
Message-ID: <[MIT-MC.ARPA].718657.851114.KFL>
Date: Thu, 14-Nov-85 22:12:51 EST
Article-I.D.: <[MIT-MC.ARPA].718657.851114.KFL>
Posted: Thu Nov 14 22:12:51 1985
Date-Received: Sat, 16-Nov-85 01:37:46 EST
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    Date: 12 Nov 85 23:53:24 GMT
    From: tektronix!uw-beaver!ssc-vax!gml@ucbvax.berkeley.edu
    (Gregory M Lobdell)

    > 
    > Can anyone explain to me (in 250 words or less) the famed
    > 'slingshot' effect that is used to accelerate space probes?
    > 
    I am told, though I haven't actually written a simulation for it
    that the slingshot effect is due to the fact that you are falling
    into a moving object, so as you fall, it pulls you along.

  (Why didn't I see the message that this is in reply to?)
  When a spacecraft falls towards a planet but misses it, its
trajectory is a hyperbola.  It leaves with the same velocity as it
arrived with.  BUT note that this is relative to the planet!  Relative
to the Sun, it looks very different, and it is possible for the
spacecraft to have accelerated from zero to twice the oribital speed
of the planet *relative to the Sun*.  This is what allows the Pioneer
and Voyager probes to reach solar escape velocity (which, in the
vicinity of Jupiter is 1.414 times the orbital speed of Jupiter) with
negligible fuel consumption.
  It is possible to imagine a set of neutron stars or other dense
massive objects set up to fly by the solar system and another star at
a large fraction of the speed of light.  Using these, one could travel
between the stars at a large fraction of the speed of light with very
little energy consumption.
  A second slingshot effect is that it is more effective to burn fuel
when passing close to a massive object.  It has been pointed out that
with today's technology, we can send a probe past Jupiter in such a
way that Jupiter's slingshot effect will cancel out most of the
probe's solar orbital velocity, and will cause the probe to drop
towards the Sun (this is much more energy efficient than trying to get
to the Sun directly, since you have to cancel out earth's orbital
velocity somehow).  By the time it gets near the Sun it will be moving
quite rapidly, mostly because of its long fall from Jupiter.  Just
when it's closest to the Sun is when you burn all your fuel as rapidly
as possible.  This will cause it to fly away from the Sun at about
1000 miles per second!  At that velocity, about 100 times that of the
Voyager probes, it can reach Earth in a day, Jupiter in a week, Pluto
in 6 weeks, and Alpha Centauri in about 8 centuries.  It could be
used for exploration or perhaps for energy by catching it in an
electromagnetic gun (you actually get more energy out than you put
in.  The extra comes from dropping part of the Earth's mass (the fuel)
into the Sun, and from dropping Jupiter's orbit a tiny fraction of an
inch closer to the Sun.)
								...Keith