re: For your amusement - Galileo.

pchien@ids.net
Mon, 17 Jul 1995 21:58:35 -0400

=46rom: "Neil T. Clifford" <neil@eeyore.demon.co.uk>

>Did anyone observe Galileo
>during either of its two passes as part of the VEEGA gravitational
>assist maneuvers?

The first pass was during the STS-35 shuttle mission (December 1990), and=
 ground controllers calculated where the astronauts should look.  Jeff=
 Hoffman and Mike Lounge tried but didn't see anything.

> I guess though, that the [earth gravitational assist] passes
>would be targeted over poorly populated areas due to the RTG's onboard.

Actually there's an important falacy in your logic.  When a spacecraft uses=
 a planet for a gravitational assist it is *NEVER* pointed at the planet. =
 To do so would cause the mission to change from a gravitational assist=
 flyby into a crash landing. ;).  It's an incorrect impression many people=
 have - and an important one to correct.  There is a 'target' for each probe=
 making a gravitational assist (whether it's using the Earth or another=
 body) and in the weeks before the encounter the target is refined closer=
 and closer through midcourse corrections.  The spacecraft is rarely more=
 than a couple of miles off the optimum location, and well within the error =
boxes.

It's also important to note that the Earth's gravitational model is much=
 better understood than any other planet (we have yet to put a LAGEOS or=
 POSEIDON over another planet!) so the accuracy of the pointing can be incre=
ased.

Gravitational passes over the Earth are targeted based on when the=
 spacecraft was originally launched, errors which have built up during the=
 journey, and the accuracy of spacecraft maneuvers, including previous=
 gravitational assists.  In other words Issac Newton determines which area=
 of Earth a gravitational assist occurs over - not politics or population.


Philip Chien, Earth News - space writer and consultant  PCHIEN@IDS.NET
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