Without Plutonium, Deep-Space Probe Missions May Sputter Out

cold fjord writes with this excerpt from Wired: "Most of what humanity knows about the outer planets came back to Earth on plutonium power. ... The characteristics of this metal's radioactive decay make it a super-fuel. ... there is no other viable option. Solar power is too weak, chemical batteries don't last, nuclear fission systems are too heavy. So, we depend on plutonium-238, a fuel largely acquired as by-product of making nuclear weapons. But there's a problem: We've almost run out. 'We've got enough to last to the end of this decade. That's it,' said Steve Johnson, a nuclear chemist at Idaho National Laboratory. And it's not just the U.S. reserves that are in jeopardy. The entire planet's stores are nearly depleted. ... what's left has already been spoken for and then some. ... Political ignorance and shortsighted squabbling, along with false promises from Russia, and penny-wise management of NASA's ever-thinning budget still stand in the way of a robust plutonium-238 production system." The plutonium shortage has been deepening for a long time, leading to some creative solutions. The Wired article alludes to the NASA project underway to create more, but leans toward gloom.

Comments

Re:Why are nuclear fission systems too heavy?

[mark-t]

Mass doesn't disappear just because something is in outer space. That mass carries with it a certain amount of inertia, and the heavier something is on earth, the more energy will be required to manipulate it with any kind of acceleration, even in space.

Re:Why are nuclear fission systems too heavy?

[fuzzyfuzzyfungus]

Launch price.

Shoving something out of ye olde gravity well is always expensive, if you go over the weight/size limit of one of the reasonably-commodified launch systems, things go from 'expensive' to 'heroically expensive'.

Depending on exactly what trajectory you have in mind, a more massive craft may also require more fuel/more powerful thrusters if you are making any course corrections along the way.

rather sensationalist

[rubycodez]

there are alternative isotopes, with much longer half lives even if battery weight is three or five times what a pu-238 one would be. not the heaviest thing in a spacecraft...anyway, the equipment to make the pu-238 exists, just a matter of getting serious about making the stuff

Re:rather sensationalist

[mirix]

The Russians always used strontium 90. Slightly lower heat output and shorter (~30 vs ~90 yr) half life. Much cheaper.

Of course the reduced half life means power will drop off sooner, but I'd think thermocouple aging factors weigh more heavy anyway (for the first decade or two, at least). Maybe not?

So for long missions You'd want something else, I guess.

Re:rather sensationalist

[Xyrus]

There are other isotopes that have longer half-lives, but they are not alternatives.

In order to be a decent RTG power source, and isotope needs to have:

1. Good power density
2. Good half-life
3. Require little to no shielding

Plutonium 238 is the ideal fuel because it is the best (or close to it) in all three categories. Strontium-90 has a much shorter half-life and lower decay energy. Polonium-210 has a high power density but comes at the cost of an extremely short half-life (138 days). Curium-242/244 is a gamma and neutron emitter so requires heavy shielding.

The only reasonable alternative at this time is the same material they put into smoke detectors: Americium-241. It has a much longer half-life than plutonium, however due to that half-life it only has about 1/4 of the power density. It does emit more penetrating radiation but doesn't require a lot of shielding.

Re:112 tonnes enough?

[compro01]

That's the wrong kind of Plutonium. RTGs need Plutonium-238. That stockpile is Plutonium-239, 240, 241, and a bit of 242.

Re:Why are nuclear fission systems too heavy?

[CrimsonAvenger]

I don't know anything about them, but I have to ask why anything is too heavy in space? Is it too heavy when assembled on earth?

A very long time ago I was in the Navy, sailing about in a nuclear submarine.

The power plant of that submarine outmassed the ISS.

Re:Voyage To The Bottom Of The Sea.

[RevDisk]

Tapping undersea cables.

Re:Why are nuclear fission systems too heavy?

[ShanghaiBill]

But I should think the minimum safe distance from an unshielded reactor would preclude anybody actually getting near enough the spacecraft to prep it for launch.

A fission reactor that has been assembled, but never operated, does not produce much radiation. Enriched uranium and/or pure plutonium are not particularly dangerous (unless inhaled or ingested). It is the fission byproducts from actually operating the reactor that are dangerous. Even this minimal radiation could be avoided by using temporary shielding that is removed (possibly by a robot) immediately before the launch.

Re:Upside

[Baloroth]

Pu-238 isn't usable for nuclear weapons. The only use to which it is put is power generation. The only connection between Pu-238 and nuclear weapons, in fact, is that weapons production facilities naturally make good production facilities for Pu-238.

I thought they restarted production back in March

[charnov]

I thought NASA struck a deal with DOE back in March to do 2 kilos per year of Pu-238 back in March. Did it get de-funded or something? http://www.universetoday.com/100875/u-s-to-restart-plutonium-production-for-deep-space-exploration/

Re:Why are nuclear fission systems too heavy?

[ShanghaiBill]

WHAT WOULD HAPPEN?

The reactor would mostly likely fall into the ocean, where it would be retrieved intact. RTGs are designed to survive a launch failure, and several accidents have
already happened, without any significant release of radiation.