Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!boulder!johnsonr
From: johnsonr@boulder.Colorado.EDU (JOHNSON RICHARD J)
Newsgroups: sci.space.shuttle
Subject: Re: Galileo Gravity Boost
Message-ID: <13020@boulder.Colorado.EDU>
Date: 20 Oct 89 17:57:22 GMT
References: <12027@eerie.acsu.Buffalo.EDU> <34700003@uxh.cso.uiuc.edu>
Sender: news@boulder.Colorado.EDU
Reply-To: johnsonr@boulder.Colorado.EDU (JOHNSON RICHARD J)
Organization: University of Colorado, Boulder
Lines: 53

stein-c@acsu.Buffalo.EDU writes:

> > Can anyone explain how Galileo will be gaining energy by flying near
> > planets? [...]
> > The only way I can see it is if Galileo picks up
> > some of the energy from the velocity of the planet. 

johnsone@uxh.cso.uiuc.edu (Erik A. Johnson) writes in reply:

>The spacecraft does not get a boost in its speed (it speeds up approaching a
>planet, e.g. Venus, but slows back down leaving), but rather a change in its
>direction of motion as it sling-shots around Venus, and thus a change in the
>orbital trajectory about the sun.

The spacecraft does indeed its direction of motion about the sun when it
encounters a planet.  However, as others have already pointed out, it also
gains some (or loses some) speed from the encounter.  The Pioneers and
Voyagers currently leaving the solar system are a case in point.  They didn't
reach solar escape velocity on the strength of human-made boosters alone.

The description of how a gravity assist works in terms of roller-skaters on
intersecting sidewalks was a great explanation.  You can also think of it in
terms of a slingshot.

Imagine Big John tied to the sole big yellow, glowing tree on an island in
the middle of an icy lake.  Big John is keeping the rope taught by moving
in a circle around the island.  Little Joey the prankster tosses a rock so
it passes just behind Big John, who catches it momentarily in his sling. 
Big John's lazy, so he doesn't try and add any velocity to the rock - he
just lets it swing around him until its heading more along his path.  When
he releases it, perhaps it's heading out so as to pass just behind Big
Bertha... Swinging it around toward his front did slow him down a little bit,
but because he's so big and heavy, no-one really notices.  The rock, however,
is small enough that being swung around passively like that causes a very
noticeable increase in speed.

In case you haven't figured it out :-), Big John is a planet, the rock is a
spacecraft, Big Bertha is another planet, the Tree is the sun, Little Joey
is JPL, and Big John's sling is a poor analog for the force of gravity.

Now, if Little Joey tosses the rock *in front* of Big John, the rock will be
slowed down, and Big John will pick up a miniscule amount of speed.

On a lighter note, if such is possible after that analogy, think of some
possible future names of environmental groups...  How about "The Momentum
Conservation Society", dedicated to keeping angular momentum, a very
valuable and non-renewable resource, where it belongs.  =:-O 


| Richard Johnson                           johnsonr@spot.colorado.edu |
|    CSC doesn't necessarily share my opinions, but is welcome to.     |
|  Power Tower...Dual Keel...Phase One...Allison/bertha/Colleen...?... |
|   Space Station Freedom is Dead.  Long Live Space Station Freedom!   |