Newsgroups: sci.space.shuttle
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Galileo Gravity Boost
Message-ID: <1989Oct20.162803.17916@utzoo.uucp>
Organization: U of Toronto Zoology
References: <12027@eerie.acsu.Buffalo.EDU>
Date: Fri, 20 Oct 89 16:28:03 GMT

In article <12027@eerie.acsu.Buffalo.EDU> stein-c@autarch.acsu.buffalo.edu (Craig Steinberger) writes:
>Can anyone explain how Galileo will be gaining energy by flying near
>planets? I understand that the spacecraft will be gaining velocity due
>to gravity forces as it approaches Venus, etc., but wouldn't all of that
>energy be lost as Galileo goes away from the planet and has to go
>against gravity forces? The only way I can see it is if Galileo picks up
>some of the energy from the velocity of the planet. 

That's exactly what happens.  Galileo hasn't gained anything as seen from
the planet -- it leaves at the same velocity, *with respect to the planet*,
as it approached at.  But the planet is moving, so to really understand
the situation you have to add (using vector addition, in which directions
matter) the planet's velocity to Galileo's inbound and outbound velocities.
Then you see a difference.
-- 
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry henry@zoo.toronto.edu