Subject: Re: Effect of Asteroids On Trajectories
From: Henry Spencer <henry@zoo.toronto.edu> 
Date: Mar 25 1996
Newsgroups: sci.space.science

In article <4iihbr$18g@lace.colorado.edu> fcrary@rintintin.Colorado.EDU (Frank Crary) writes:
>...It turns out that the best estimate is experimental:
>We've sent three spacecraft through the rings of Saturn
>without damage, and that statistic actually puts stronger
>limits on the risks to Cassini than using theoretical
>models of the rings...

Mind you, the record might not be so good if conservatism hadn't prevailed
in Pioneer 11's targeting...  There was a proposal to send P11 through
Cassini's Division (the apparent gap between the A and B rings), which we
now know to be just a thin spot in the rings.  There was a much more
serious proposal to send it inside the rings, between the C ring and the
planet -- more or less through the faint D ring -- and the P11 Principal
Investigators voted 11-1 in favor of doing this.  NASA management overruled
them, citing the risk of losing later data (including the Titan flyby) and
the desirability of examining the environment the Voyagers would pass
through, and dictated a flyby somewhat outside the visible rings.
-- 
Americans proved to be more bureaucratic           |       Henry Spencer
than I ever thought.  --Valery Ryumin, RKK Energia |   henry@zoo.toronto.edu

Newsgroups: sci.space.history,sci.space.policy
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Galileo and Cassini antennas (was Re: Smaller, Faster, cheaper)
Date: Sun, 26 Oct 1997 21:55:11 GMT

In article <62ud4l$rrf@ultranews.duc.auburn.edu>,
Andrew J Wohrley <wohrlaj@mail.auburn.edu> wrote:
>Here is my question.  The antenna for Gallileo had to remain furled in
>order to protect it from the sun as it followed a trajectory for Venus
>gravity assist.  Conversely, for Cassini, as I understand it, the (fixed)
>antenna is pointing directly at the sun in order to shade the rest of the
>spacecraft.

Correct.  In both cases, the Venus gravity assists put the spacecraft close
enough to the Sun for overheating to be a serious problem, and most of the
spacecraft has to hide behind protective shades.  In Galileo's case, the
shades were late add-ons due to the change of mission design; for Cassini,
the antenna is deliberately built to be heat-resistant so the rest of the
spacecraft can hide behind it.

>Doe this mean that the heating problem Gallileo tried to
>avoid by shading its furled antenna had been analyzed and found not to be
>a problem with Cassini?

The antenna designs are completely different.  More important, though,
Cassini's antenna was designed for the Venus-flyby environment, and
Galileo's wasn't.  (Galileo's mission changes came late and budgets were
tight, so wholesale redesign and requalification was out of the question.)

>More exactly, was Gallileo trying to protect the
>antenna's structure of spars, or its electronics?  I guess I had always
>vaguely understood that the reason Gallileo's antenna had to be furled
>related to protecting the antenna structure from thermal warping.

The issue on Galileo was that the antenna itself wasn't designed or
qualified for the warmer environment, nobody knew whether it could take it
or not, and there wasn't any money to find out.  So the antenna had to be
kept cool, which meant keeping it furled so it could hide behind a small
sunshade.
--
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        | henry@zoo.toronto.edu