Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!shadooby!ginosko!xanth!mcnc!thorin!unc!state
From: state@unc.cs.unc.edu (Andrei State)
Newsgroups: sci.space.shuttle
Subject: Re: Galileo Gravity Boost
Summary: How it the gravity boost works
Message-ID: <10150@thorin.cs.unc.edu>
Date: 23 Oct 89 15:53:08 GMT
References: <12027@eerie.acsu.Buffalo.EDU> <34700003@uxh.cso.uiuc.edu> <18630@pasteur.Berkeley.EDU>
Sender: news@thorin.cs.unc.edu
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The "gravity boost", more correctly referred to as "swing-by" or
"gravity-assist" maneuver, has been used by NASA at least since the
days of Pioneer and Voyager, possibly earlier.  It involves approaching
a planet with a spacecraft under a certain angle on a hyperbolic
trajectory.  Roughly the spacecraft intersects the planet's orbit around
the sun passing from "inside" to "outside", with the spacecraft's
velocity having a component parallel to the planet's tangential
(orbital) velocity (i. e., we approach the planet from "behind" and from
"inside" its orbit -- I wish I could draw a sketch, I'm no good at
this).  During the approach, spacecraft and planet attract each other,
but since the spacecraft's mass is much smaller than the planet's, it
gets a higher delta v; nevertheless, the planets gets a delta v as
well (you guessed it: it's NEGATIVE), i. e. it slows down a little bit
(tanstaafl).  Assuming the planet's orbit was (ideally) circular in the
beginning, then the swing-by point on the planet's orbit becomes the
apohelion of an elliptical orbit, so the (poor) planet will be nearer
to the sun exactly half a year later, at the perihelion of the (new)
elliptical orbit.  Theoretically, if we do this often enough, we can
bring a planet down into the sun; but then again, we don't have enough
mass on Earth to do it.

________________________________________________________________________
Andrei State                       alias                state@cs.unc.edu
more movie buff than grad student at the    University of North Carolina