[X] My Assistant

[Rakhir]

Death of a Spacecraft: The Unknown Fate of Cassini
http://space.com/businesstechnology/061108_cassini_fate.html

I like the Cassini Mercury crash option, even if unlikely.

Rakhir

[dvandorn]

I will point out that the Apollo 13 situation isn't entirely applicable to Cassini. The RTG fuel element for Apollo 13's ALSEP was not stored in its RTG. It (as all Apollo ALSEP fuel elements) was stored in a graphite fuel cask which was specifically designed and located on the outside of the LM to *survive*, intact, an entry into Earth's atmosphere at circumlunar speeds. (The astronauts removed the fuel element from this cask and installed it into the ALSEP RTG during the ALSEP deployment.) That's not exactly the same thing as a fueled RTG, which is likely to break apart under stresses such as entering any significant atmosphere.

However, the Apollo RTG fuel elements were quite different from the elements that fuel modern probes such as Cassini and New Horizons. The Apollo ALSEP RTGs used a simple rod of plutonium which generated an awful lot of heat -- it was strictly forbidden for the astronauts to touch the rod directly, mostly because of heat effects. Modern RTG fuel is, as far I understand it, encapsulated in many small spheres of reinforced graphite. The entire mass of the fuel generates less heat than the ALSEP fuel rods, but modern thermocouples are able to efficiently turn this somewhat smaller heat flux into ample amounts of power.

This makes the modern fuel elements safe to launch within their RTGs -- in case of a launch vehicle explosion, etc., each small sphere within the RTG fuel will survive without releasing any of the toxic plutonium inside. In Apollo, that fuel rod's cask had to survive intact, with *all* of the fuel in one container.

So, as I said, the situations aren't exactly comparable.

-the other Doug

[Greg Hullender]

This makes the modern fuel elements safe to launch within their RTGs -- in case of a launch vehicle explosion, etc., each small sphere within the RTG fuel will survive without releasing any of the toxic plutonium inside. In Apollo, that fuel rod's cask had to survive intact, with *all* of the fuel in one container.

Since it's so important to outer planet missions, I just wanted to point out that in real life Plutonium is only about as toxic as lead -- short of critical mass. In fact, I don't think you can HAVE a critical mass of Pu238, which is what space probes use.

http://en.wikipedia.org/wiki/Plutonium#Precautions

The current reputation it has as the world's most toxic substance is originally due to exaggeration in a Ralph Nader fundraiser.

http://www.phyast.pitt.edu/~blc/book/chapter13.html

(There are probably better references for this point.)

Anyway, at least among ourselves, it'd be nice if we didn't repeat the other side's propaganda.

--Greg

[Rakhir]

Since it's so important to outer planet missions, I just wanted to point out that in real life Plutonium is only about as toxic as lead -- short of critical mass. In fact, I don't think you can HAVE a critical mass of Pu238, which is what space probes use.
...
Anyway, at least among ourselves, it'd be nice if we didn't repeat the other side's propaganda.
--Greg

Just to come back to the topic, again, the problem is not with the Plutonium itself but with the by-product : the heat.
Actually, the issue is not the completely unlikely risk of killing a hypothetical local life but to allow earthly microbes to survive on the surface (I guess also very unlikely).
So in this case, any radiation leaking from a damaged RTG could even be a *good* thing as it may help to finalize the sterilization (I agree, this one is a bit far-fetched )