Xref: utzoo sci.space:4525 sci.space.shuttle:533
Newsgroups: sci.space,sci.space.shuttle
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Payload of shuttle flight directly after last Challenger.
Message-ID: <1988Feb8.192335.27728@utzoo.uucp>
Organization: U of Toronto Zoology
References: <347@flatline.UUCP> <1988Feb3.133415.12432@utzoo.uucp>, <572@kksys.UUCP>
Date: Mon, 8-Feb-88 19:23:29 EST

> I was under the impression that the experiment packages left up on the
> moon by the manned Apollo shots used a small Pu-powered nuclear
> reactor as their power source.  I seem to remember a small, black,
> cylindrical device with radiating fins as the center imstrument
> deployments.  Can anyone else confirm or deny this?

Sigh.  I guess I had better explain all this from scratch; there are
probably others with similar misunderstandings.  Everyone who isn't sure
what's going on, please listen -- I'm only gonna say this once! :-)

There are two ways of getting power from nuclear energy.  One is with a
nuclear reactor:  a complex piece of machinery that maintains a controlled
chain reaction in a suitable fissionable isotope, usually Uranium 235 but
sometimes Plutonium 239.  Non-fissionable isotopes like Uranium 238 may
be present but do not participate.  Reactors can yield very high power
outputs, but are relatively big, heavy, and complicated, and once in
operation they emit lots of radiation and have a stew of nasty isotopes
inside them.

The other way is to use the natural decay of a radioactive isotope.  To
get useful amounts of power without troublesome radiation, one picks an
isotope that decays rapidly by emitting non-penetrating radiation like
alpha and beta particles.  Plutonium 238 (note, 238, *not* 239) is one
fairly good choice.  Such isotopes do not have to be fissionable and in
fact usually aren't.  Small isotope generators can use thermoelectric
devices for converting heat to electricity, which means no moving parts.
It is difficult to get high power out of isotope generators, but they are
simple and reliable and (unlike reactors) can be built in small sizes.
It is relatively easy to package the isotope in such generators in an
armored capsule that can survive a re-entry or a launch accident; this
is routinely done, and such capsules *have* survived launch accidents.
An isotope generator is *not* a nuclear reactor.

The US has launched only one reactor, the SNAP-10A experimental reactor
of the late 60s.  The Soviets use reactors to power their military
radarsats, which need lots of power.  That is basically it for reactors
in space.

Isotope generators are used in some military satellites where the
vulnerability of solar panels is undesirable, but see their main use in
planetary missions.  The Apollo surface experiments and the Viking landers
used isotope generators to continue operations despite long periods of
darkness.  Outer-planet probes like the Voyagers and Galileo use isotope
generators because the sunlight is pretty faint out there.  The waste
heat from isotope generators can also be useful to keep equipment warm
in cold environments.
-- 
Those who do not understand Unix are |  Henry Spencer @ U of Toronto Zoology
condemned to reinvent it, poorly.    | {allegra,ihnp4,decvax,utai}!utzoo!henry