Are Nuclear Powered Mars Rovers a Good Idea?

meatybeans writes "NASA officials are meeting today, with concerned residents around Cape Canaveral, regarding the power system for the upcoming Mars Science Lab mission. MSL is going to be like our current rovers on steroids. The plans call for a larger, heavier rover with a lot more juice for gadgets. This meeting however brings to light the issue of the power system for the MSL. The Mars Science Lab originally called for a nuclear power source, much like the Cassini and New Horizon missions use. Some vocal opposition to this has been voiced in the past. As a result, NASA has backup plans to employ solar power and small amounts of RTG's ^? if arguments against straight nuclear for MSL win out. As with most, things 'NIMBY' ^? seems to be in full effect when it comes RTG's. Does the recent success of the rovers show us that RTG's are not needed for Mars exploration? Are 1:420 odds of an accident that bad? Finally, are the hearings that are taking place between NASA and the public really just a formality in the name of public relations?"

Re:They'll be perfectly fine

[Detritus]

Pu-238 is not fissile, and an RTG (radioisotope thermoelectric generator) is not a nuclear reactor, it uses the decay heat of the radioisotope to produce electricity.

Re:wrong question

[Ironsides]

'what's wrong with continuing to build solar rovers that we need a nuclear one?

Solar powered rovers can't
1) Operate in shadow for long
2) Supply enough power if you want more insturments
3) Work through the martian winter

Re:Result of accident?

[Ironsides]

Would it result in more radiation than an "open air" nuclear explosion test? What does it compare to?

A Radioisotope Thermal Generator (RTG) basically using nonfisile radioactive material as a heat source to create electricity. This is what has powered the two Voyager probes for the past 30 years. The amount of readiation released is effectively zero. An open air nuclear explosion releases several kilograms worth of fisile material into the atmosphere.

Oh, and as to the dangers of RTGs in case of a launch accident. We've actually launched radioactive material on a rocket where the rocket exploded partway into the flight. The nuclear material was recovered inside it's intact casing and reused on a later mission.

There is zero danger involved here.

RTGs are proven safe.

[AWeishaupt]

Scores, if not hundreds, of RTGs have been used in space exploration, going back to the '60s. There have only ever been three - iirc - incidents where the RTG's have been breached, resulting in detectable radioactive release.

Despite always having been controversial, RTGs have been proven safe.

Even if you run the space probe from solar cells, you cannot have analytical instruments such as Alpha particle X-ray spectrometers and Mossbauer spectrometers without radioactive sources.

Re:Result of accident?

[Ironsides]

Great. What are the arguments against the use of a RTG then? If there isn't any "real" damage aven locally why does it seem to such a big issue?

As the other guy said, an irrational fear of nuclear. I remember hearing that during one of the nuclear launches in the 70's, there were people protesting saying that NASA was going to kill them all by launching a nuclear powered Satelite/Probe (I can't remeber which). They protested at the launch holding up there babies holding signs "You're going to kill me." Launch went off without a hitch.

Nuclear power and weapons detonation has released far less radiation than Coal and Fossil fuels in the past 60 years (Coal contains small ammounts of Uranium). If anything, they should be protesting Coal, not nuclear.

Re:New meme?

[Goaway]

Except of course that they are, in fact, really very safe.

So it's a good idea, right?

Re:RTGs are not dangerous

[AKAImBatman]

But the real answer to your quest is that RTGs aren't dangerous, so the entire premise of the question is flawed. A launch failure isn't going to make Florida a radioactive wasteland. We've launched dozens of RTGs in past missions.

Don't forget that we've blown up a few of them, too. The original RTGs were designed to be burned up in the atmosphere. (Russia even burned one up over Canada.) So far, there are no nuclear wastelands because of it. NASA quickly figured out, however, that burning up expensive nuclear fuel in the atmosphere was probably not the best idea. So they started cladding the fuel in tough containers designed to withstand a launch failure.

Those containers have been proven twice. Once on the Nimbus launch vehicle (which was destroyed by the range officer) and the other was the emergency landing of Apollo 13. The Nimbus RTG was recovered from the sea bed, washed off and resused. The Apollo 13 unit fell in the Troga Trench and has been sitting there unpenetrated.

Re:wrong question

[Anonymous Coward]

Solar powered rovers can't
1) Operate in shadow for long
2) Supply enough power if you want more insturments
3) Work through the martian winter

All true - but you missed the most important one:

4) Work at northerly or southerly latitudes.

Current technology can only operate close to the Martian equator. It's like being able to search for life in the Sahara desert. I'm sure we'd want to look closer to the poles if we could.

Re:RTGs are not dangerous

[Anonymous Coward]

If you are referring to Cosmos 954 crashing in the Northwest Territories, that wasn't a RTG. It was a nuclear reactor on a satellite. From Wikipedia, it is a BES-5 reactor fueled with U-235. According to one source (http://www.svengrahn.pp.se/trackind/RORSAT/RORSAT .html [svengrahn.pp.se]), there was 30 kg of 90+% enriched U-235 (as U-Mo alloy) in the core. Power output was 3kW, obtained by thermoelectric generators. The heat source is fission, not decay heat.

Re:Check the RTG packaging.

[Ash Vince]

Actually no, if the launch fails very late on after the container has left the atmosphere but before it escapes earths magnetic field then the container would have to withstand re-entry.

Anyone who has studied re-entry will tell you this is bad. The container could quite easily (no atmosphere to slow it down in space) attain speeds of the order of 1000+mph (http://www.physlink.com/Education/AskExperts/ae15 8.cfm). Something travveling at this speed hitting the atmosphere will get hot, very hot. I am sorry I cant be bothered to go into all the details now as I am on my lunch break at work at have no access to my old Physics with Space Technology Uni notes.

Basically, you would be hard pressed to design a container that can with stand anything more than a 5 degree angle re-entry. Something tumbling out of control will almost certainly come in much steeper, and it will burn up. This will disperse its contents over a wide area (half the globe).

Nuclear fissile material will not become magically inert in this process though. This is because even if it gets hot enough become a gas it is still too heavy to be captured by the earths magnetic field (normal radiation from the sun is trapped by earths field and enters at the poles producing the funny lights in the sky). Think Chernobyl, this caused radioactive material to rain (litterally in rain) down over a wide swaith of Europe and this was a near meltdown at ground level.

A nuclear meltdown in low earth orbit would be very bad. Not planet destroying, but still bad.

That said, I am still in favour of using nuclear power, it just has to be done very carefully in all circumstances.

Sorry this article is light on technical details, but I could write 30 page essays on this and still leave bits out. If anyone is that interesting try this wikipedia link for a start:

http://en.wikipedia.org/wiki/Atmospheric_reentry [wikipedia.org]

Re:Yes, of course

[ozmanjusri]

People should get over their irrational fear of decaying nuclei.

They already use some nuclear power. Each rover has eight Radioisotope Heater Units, powered by Plutonium 238, so it's not fear that's preventing the use of RTGs.

My guess is that weight is the problem. NASA's standard General Purpose Heat Source RTG generates about 290W and weighs about 60Kg, while the rover's existing power system weighs about a third of that.

Re:Yes, of course

[Rei]

RTGs can be nice, but they're not without problems. Anything that involves working with plutonium-238 is expensive (it's made from Np-237, and you can only recover that very slowly from nuclear fuel; then you have to irradiate it. You can also make it from Americanium, but that's also very costly). If you don't use something with a reasonably long half-life like Pu-238, you don't have the ability to have "unexpectedly long lifespans" like the MERs have had. While you have "power" for a long time, you don't have peak power for that long. For RTGs in general, ionizing radiation from daughter products can interfere with systems. You need oversized radiators to deal with the peak power production and to get any sort of efficiency out of your system. Etc.

RTGs have their role to play. So does solar. For the forseable future, both will continue to be used in space exploration, and that's a good thing. The big question is, "Which is better for Mars exploration?" The assumption used to be "RTGs" because of the dust problem. Now we're seing that the "dust problem" isn't as much of problem as we once thought.

As to the "irrational opposition" to RTGs, yes, there is some. However, it's not very widespread. Most people don't even pay attention to what's inside most spacecraft. Heck, *most* people would have trouble naming just one interplanetary probe that's been launched in the past decade.