----- Original Message -----
From: "Robert J. Bradbury" <[EMAIL PROTECTED]>
To: "Europa Icepick" <[EMAIL PROTECTED]>
Sent: Thursday, October 11, 2001 7:55 AM
Subject: Re: Europa Orbiter's nuclear power source
>
>
> 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?
Nope -- Europa Orbiter itself is a considerably bigger spacecraft than the
Pluto probe, and needs two RTGs by itself.
> > 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.
Some kind of gravity assist by an inner-planet flyby is looking increasingly
necessary for any kind of Europa mission -- Europa orbiters and landers both
have very big delta-V requirements and thus must carry very large amounts of
fuel. NASA's original approach was to develop super-miniaturized new
electronics for Europa Orbiter and then launch it on a direct
Earth-to-Jupiter trajectory -- but these new advanced electronics are
proving very hard and expensive to develop. So the obvious alternative
approach -- without enlarging the size and cost of the launch vehicle -- is
to use inner-planet flyby assists, just as Galileo and Cassini did.
But NASA has also made it entirely clear that Cassini will be the last Earth
flyby by a plutonium-fueled spacecraft for the foreseeable future.
Fortuantely, Venus and (to a less extent) Mars are available as
alternatives. A trajectory involving 3 successive Venus flybys would allow
a Europa mission's mass to be about double, for the relatively low cost of a
3.5-year increase in flight time to Jupiter.
> > 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).
No doubt -- but my point remains true: the more we can minimize the amount
of this stuff that we use, the better.
> > 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.
I wasn't talking about LIVING cells, but SOLAR cells for spacecraft.
> > 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.
I agree -- in 1999, the Europa Icepick group had a tremendous wrangle over
whether there was any alternative to nuclear power for the Europa Cryobot,
and we simply couldn't find anything even remotely workable. An inflatable,
tiltable solar reflector to power a simple Europa lander -- which of course
doesn't have to melt its way through kilometers of ice -- is a very
different matter; but it still presents a serious weight problem -- and the
problem of solar-cell vulnerability to the high radiation levels at Europa
remains prohibitive.
> > -- 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.
Again, I agree. The best Web source for data on the design of such an
inflatable collector is www.lgarde.com/programs/powant.html , which
describes the "Power Antenna" that the L'Garde Company has designed for
JPL -- and which includes data on the relative size and mass of reflectors
capable of providing a modest amount of electrical power to spacecraft at
Jupiter, Saturn, Uranus and Neptune. (This would double as a very efficient
high-gain antenna dish -- thus its name.) For Jupiter, a reflector capable
of providing 75 watts would be only 4 meters wide and weigh 25 kg, so mass
alone is not prohibitive for a Jupiter orbiter (which requires much less
total delta-V than an orbiter of lander of a Jovian moon) -- but the
radiation problem remains.
Europa Orbiter was, in fact, originally proposed as a Discovery-class
mission in the second round of competitive Discovery mission selections --
but it got the knife both because of cost, and because the review panel
concluded that the solar panel that the original design used for power would
be knocked out after only a week in Europa orbit. A modified version with
the solar panel replaced by an RTG, and the steerable high-gain antenna
replaced by a fixed one, was the basis for JPL's Europa Orbiter design --
but its cost continued to soar higher and higher as the true technological
dificulty of the mission became apparent.
> 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.
What kind of radiation does Gd-148 give off (as compared to the alpha
particles that Pu-238 emits)?
> (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).
Don't worry -- ALL Europa missions (as well as Europa itself) are completely
fair subjects for this list.
==
You are subscribed to the Europa Icepick mailing list: [EMAIL PROTECTED]
Project information and list (un)subscribe info: http://klx.com/europa/
