Re: SPACE: Cassini Mission Consequences

Michael Lorrey (
Sat, 20 Sep 1997 17:14:20 -0400

Natasha V. More (fka Nancie Clark) wrote:
> Although I have been in the periphery of the adversarial panoply of Cassini,
> I'd like to address some data and if anyone has a difference of opinion,
> please do let me know. If this information has already passed through the
> list and either as a solo post or thread of comments, would someone kindly
> advise me and I'll go to the archives and look it up.

There is 73 lb of a P-238 based ceramic compound on the RTG units. How
much of that is actual plutonium isotope I don't know. The protesters
disingenuously claim that the P-238 on board is "280 more radioactive
than P-239, which is used in atomic weapons" (notice they don't say
nuclear, apparently they've given up on using the word since you can
never find one of them that can pronounce it correctly. ;)). What they
fail to inform people is that P-238 emits alpha radiation while P-239
emits gamma radiation, horses of entirely different colors. Alpha
radiation is so weak that a sheet of paper is sufficient sheilding from
it. So while P-238 emits 280 times more radiation "particles" than
P-239, the difference in terms of safety, the environment and health
risk is overwhelmingly the reverse. This compound has a melting point of
1000 deg F, while a reentry caused by straying too close to the earth's
atmosphere at almost 40,000 mph would raise surface temperatures above
10,000 deg, which would obviously boil and burn the RTG into vapor. The
casing, a high temperature compound that I forget the name of at the
moment, has a melting point of 4500 degrees.

I have yet to hear about what altitude the probe will be at at closest
approach. Since they protesters aren't saying, I assume its a couple
thousand miles at the very least. Since this flyby maneuver is to
accelerate the probe, not decellerate it, it obviously should be nowhere
near the atmosphere.

Considering that the average navigational accuracy for US interplanetary
space probes is in the order of 1 mile per billion miles travel, the
chance of the sort of disaster people are fearmongering about is
ludicrously small. For this to happen, the probe must perform both Venus
flybys flawlessly, then have its navigational systems fail as it reaches
the halfway point between earth and venus. If this were to happen, we
would obviously have several tens of millions of miles and several
months to either overcome the problem, or shunt the probe off course so
that it completely misses the earth-moon system.

As an engineer, and looking at history, it is far more likely that the
failure will be when it is launched on the Titan IV launcher. The reason
protesters are focusing on the flyby for their propaganda is because
such a disaster threatens potentially millions of people, while a launch
failure would only threaten some fish in the Atlantic Ocean, and because
they know that the RTGs have been specifically designed to survive
launch failures (and have sucessfully done so in the past, to be
refurbished and reused on a later mission).

			Michael Lorrey
------------------------------------------------------------	Inventor of the Lorrey Drive
MikeySoft: Graphic Design/Animation/Publishing/Engineering
How many fnords did you see before breakfast today?