On Thu, 11 Oct 2001, Bruce Moomaw wrote:
> 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).
I assume this is because you are assuming a "lander" and not just an orbiter,
correct?
> There is considerable discussion about either trying to get more Pu-238 from the
> Defense Department, or buying it at low cost from Russia.
This report:
http://www.lanl.gov/orgs/nmt/nmtdo/AQarchive/01spring/01spring.pdf
seems to suggest Los Alamos should have the production capacity (5 kg/yr)
to meet the requirements for space missions. Cost is certainly an issue.
The RTGs require 11-33 kg of plutonium and the price from Russia (from
at least one web source) appears to be $1.2m/kg. This seems to suggest
it is a significant fraction of mission cost.
> 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.
Yes, but there is a big difference in navigating something as far away as
Mars and something approaching the Earth. What are the distance limits
of our radars detecting probe sized objects approaching the Earth such
that we could detect and correct any orbital errors? Are there any known
routings to Europa that would require an Earth flyby? If not, then this
is probably off topic.
> 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).
I've seen Kaku speak and was unimpressed. I've also read Visions and he clearly
doesn't understand molecular nanotechnology (as is the case of the chemists
Smalley & Whitesides as the recent Scientific American articles clearly
demonstrate). These all fall under the category of people speaking outside
their areas of expertise. I'd believe a mission controller or a JPL expert
on computing gravity assist flybys before I'd believe Kaku knows what he
is talking about. Similarly, I'd trust DoD experts over the NASA "experts"
on the safety of the RTGs, if they designed them to survive launch accidents
or reentry.
I don't know what Bruce or Kaku are basing their estimates on but I'd bet
you make the excessively conservative assumption that there will be no
improvement in technologies for treating cancer. It is *highly* unlikely
that that will be the case (I'm speaking as a biotechnology expert here).
> 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.
If you discover "organic" molecules in the Europa ocean but nothing you
consider to be definitively alive (not dissimilar from the current situation
with Mars) -- you *are* going to want to go back with bigger and better
probes. I doubt the problem is going to go away.
> 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.
Not true -- bacteria are perfectly capable of recycling their components that
have been subjected to radiation damage. I'm not sure of the radiation delivery
rates around Jupiter, but Deinococcus radiodurans can be hit with a
megarad and keep on ticking.
What one needs is a completely closed system that allows the bacteria to
produce electricity. That isn't so "implausible" when you consider they
create a proton gradient for the production of ATP.
> 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
I don't think there is that much difference between the minimum thickness
of a mirror and the minimum thickness of a solar cell array so it probably
becomes a cost issue of which is easier to manufacture. In that situation
the mirrors probably win. I agree that this would be useful for the space-based
orbiters/surveyors but I don't think this will work well on or below the surface
of Europa.
> -- 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).
This suggests that engineering better RTG's isn't a problem that will go
away anytime soon.
Just as a FYI for mission planners, Robert Freitas has estimated in
Nanomedicine (Section 6.3.7.1) that a reasonably safe power source for
nanorobots in the human body is Gd148, supplying ~100W/0.2 kg.
So if one wanted a safer, reasonably dense power source for space missions
one should setup a breeder program for Gd148.
(I'm trying to keep this about Europa mission(s). If the purpose of the
list is to focus on *the* Europa mission, then someone should let me know
offlist).
Robert
==
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