Newsgroups: sci.space.shuttle
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Payload of shuttle flight directly after last Challenger.
Message-ID: <1988Feb4.130645.10670@utzoo.uucp>
Organization: U of Toronto Zoology
References: <347@flatline.UUCP>, <1484@boulder.Colorado.EDU>
Date: Thu, 4-Feb-88 13:06:40 EST

> As for Pu in space,  I've never understood how a satelite could use so much
> power that it would need a fision reactor insted of photovoltaic cells, and a
> battery?  Is there a weight problem?  Is it a political issue?

Solar arrays are excessively big, heavy, and cumbersome if you want a
whole lot of power [e.g. the Soviet radarsats] or are operating in the
outer solar system where sunlight is weak [e.g. Galileo].  They are also
less than ideal in environments which don't get more-or-less steady sunlight
[e.g. Viking landers on Mars -- 12+ hour nights and month-long dust storms].
There is also some reluctance to use them for crucial military satellites
because they are relatively large and vulnerable.

When one of these constraints applies, and the times involved are too long
to just use batteries or fuel cells, things that need a lot of power have
to use reactors [the Soviet radarsats] and the rest use isotope capsules
[planetary missions and some military satellites].  It is more or less an
accident that a good choice of isotope for the latter is plutonium 238;
this is not the bomb/reactor isotope (239) and in fact I don't think 238 is
fissonable at all.  The Soviet radarsat reactors use enriched uranium, as
I recall.
-- 
Those who do not understand Unix are |  Henry Spencer @ U of Toronto Zoology
condemned to reinvent it, poorly.    | {allegra,ihnp4,decvax,utai}!utzoo!henry