----- Original Message -----
From: "Robert J. Bradbury" <[EMAIL PROTECTED]>
To: "Europa Icepick" <[EMAIL PROTECTED]>
Sent: Monday, October 08, 2001 11:41 AM
Subject: Re: More jolly Space Station news
>
>
> On Mon, 8 Oct 2001, Bruce Moomaw wrote:
>
> > It's what it has been from the start: some kind of thermionic nuclear
> > generator converting the heat from Pu-238 into electrical energy.
However,
> > NASA would like to develop a new system more efficient at this
conversion
> > than the current RTGs, thereby reducing the amount of plutonium
necessary by
> > a factor of 2 or 3.
>
> A conversation with Chris Chyba over a year ago, seemed to suggest to
> me that there was only one remaining RTG in existence (perhaps a spare
> from the Cassini mission). In order to do Europa & Pluto the development
> of a new source was a *necessity*.
The situation is as follows: There is one fueled "F-5" RTG (a spare from
Galileo), one unfueled "E-8" RTG (a Cassini spare, which if fueled would
produce about 1/3 again more power than the "F-5"), and enough parts to
build a second unfueled "E" RTG. The US currently has enough domestic
Pu-238 to fuel any one of these RTGs (enough for the Pluto probe) -- but not
two of them (and Europa Orbiter by itself would require two). There is
considerable discussion about either trying to get more Pu-238 from the
Defense Department, or buying it at low cost from Russia.
______________________________________
> > [snip] but, as I say, it would use far less plutonium,
> > which is good both from the safety point of view and from the viewpoint
of
> > how extremely costly it is to manufacture the stuff.
>
> Why is it costly to manufacture? Or is it just the handling costs that
> drive up the expense?
I believe it's mostly the manufacturing cost.
_______________________________________
> > Any generator that uses Pu-238 at all, of course, runs a safety risk
from a
> > launch accident. NASA insists that the risk, even in the event of a
launch
> > explosion or accidental reentry, is miniscule; but then NASA lies a lot.
>
> Ah, making claims without citing "factual" data (shame shame).
> It would appear that the RTGs were designed by the DOE, so you would
> need to make an argument that the DOE is lying about their safety, *not*
NASA.
>
> Here is a page about the RTG architecture -- judge the safety for
yourself.
> http://spacepwr.jpl.nasa.gov/rtgs.htm
The DOE does manufacture the RTGs -- but their safety in a space mission was
estimated by NASA. I will agree that the odds are very high that they would
be safe even in a major launch explosion or reentry accident -- after all, a
Delta carrying a Nimbus weather satellite with two smaller RTGs blew up in
1968, and the plutonium cores were retrieved from the ocean floor off
Vandenberg in such good shape that they were used to fuel the replacement
Nimbus the following year.
But the fact remains that NASA has routinely and grotesquely underestimated
the odds of accidents before -- for instance, the official NASA estimate was
that there was only 1 chance in 100,000 of a Shuttle blowing up shortly
before Challenger did just that. (The Defense Department pegged it at more
like one chance in 50, which is why they started pressuring Congress for
some Titans as emergency backups before the accident -- fortunately.) And
NASA assured us that there was no chance whatsoever of a serious
navigational error during Cassini's Earth flyby, just a few weeks before
they flew Mars Climate Orbiter into the planet. I still think it possible
that an RTG launch accident could release enough Pu-238 to kill several
hundred people over the following years (as does Prof. Michio Kaku, who is
no hysterical environmentalist crank). So, both from the viewpoint of
safety and of manufacturing cost, the less of this stuff we have to use on
space missions, the better.
_______________________________________
> > Solar power now looks perfectly practical for Jupiter flybys, since
solar
> > arrays have gotten a lot lighter than they used to be -- but solar-cell
> > damage from Jupiter's intense radiation belts is still a very serious
> > problem for a Jupiter orbiter, and the weight problem is also very
serious
> > for a Europa orbiter.
>
> They can get *much* lighter still, but they probably have to be assembled
> in space due to the high g forces during Earth launches. The limits would
> appear to be at least 3 orders of magnitude lower than current areal
densities.
Again, the main problem for solar cells on any Jupiter orbiter is the high
radiation level, and no solution for that seems to be on the horizon. As
for the mass problem for solar arrays on missions to planets further away,
the probable solution for that is an inflatable concentrating reflector,
which would be far cheaper and simpler than attaching super-flimsy solar
panels to an already-orbiting spacecraft -- but the inflatable reflector
technology has not yet been developed (and, in the case of a Neptune
orbiter, we're still talking about 200 kg or so more weight than an RTG of
comparable output).
==
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