> >Deep space satellites (at least Pioneer) use simple solid
> >state nuclear heated peltier devices for power.
> 
> 
> any satellites that go beyond earths orbit from the sun use some form of
> nuclear powered power source, as the output from solar cells is just too
> low. About the only items that do not have nuclear power sources are the
> Mars Rover vehicles, which do have solar cells, but are also in a position
> where they can be left in a very low power mode for lengthy periods of time
> to recharge the batteries. Any orbiting spacecraft tends to be powered on
> all the time, so does not get the recharge periods, as the scientists back
> on earth don't want to miss any data scans of planet surfaces, as it may be
> quite some time before the orbit goes over that spot again, and then it may
> be "on the dark side" away from the sun.

Bit of an oversimplification, that.  For example, Mars Climate Orbiter and
Global Surveyor both had solar panels, as did the Viking orbiter.  I'm not
sure if they used backup radioisotope thermoelectric generators or not, but
I'm thinking not because there wasn't the big hullabaloo one usually hears
with the launch of an RTG equipped spacecraft.

NASA had some superb documentation on the usage of RTGs in deep
space probes, with specific emphasis on the Cassini-Huygens mission.
One of the more illuminating diagrams features a to-scale representation
of the size of solar panels required to do anything useful at Earth orbit,
Mars orbit, Jupiter orbit and Saturn orbit.

Why Cassini couldn't use solar power...
http://saturn.jpl.nasa.gov/spacecraft/safety/solar.pdf

Cassini's RTG systems...
http://saturn.jpl.nasa.gov/spacecraft/safety/power.pdf

General info on RTG systems...
http://saturn.jpl.nasa.gov/spacecraft/safety/rhu.pdf

All in all, a pretty useful exploration of nuclear decay harnessed as
a fairly safe energy source.

Mike H.
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