# Small Reactor For Rover

Is a very low powered fission reactor (~25kWe) feasible for powering a large (car sized) Lunar or Martian rover? Or is it impractical (today) due to weight, size, radiation, or another technical reason?

I understand that radioisotope thermoelectric generators are planned for future Mars rovers. Is NASA even considering a fission rover option? It seems the 100x additional power that fission provides could be extremely useful for better mobility and experiments.

There was a series of reactors designed for space, only one SNAP10 was flown http://en.wikipedia.org/wiki/Systems_Nuclear_Auxiliary_Power_Program
According to the spec it made 30KW and weighed 1/4 ton.

I've run across similar info. It seems that if such reactors were possible back then, current tech should allow a somewhat smaller package that would fit in a rover.

Am I wrong about the upside of such an increase in power for experimentation?

I fairly certain a 'fission rover' could cover a lot of territory in its lifetime. Heck, give it 3-5 years and a fission lunar rover could probably go from lunar north pole to lunar south pole. Such a journey would captivate school kids and science geeks worldwide.

As you probably know, one of the issues with nuclear power for space is shielding. Shielding is dead weight that contributes nothing to the mission. It is something like $5000/lb to put something in space... so just for the reactor at 500 lbs, that is$2.5M. In a very cash strapped explorations budget, that is a lot of money. I am assuming here that the 500 lbs includes the proper shielding of the reactor.

So, you say lets skimp on the shielding since this rover will be unmanned. Well, it turns out that neutrons can be very nasty to electronics and would be a major problem for any scientific payload.

There may in fact be a need for a large power source on a rover (I cannot imagine one right now, but that doesn't mean some project wouldn't need one) but I think it would be much more likely for a plan to attempt to use a smaller power source and attempt to be more energy efficient.

MMRTG is much more efficient than a full blown reactor. In fact the MSL will have it.

MMRTG is much more efficient than a full blown reactor. In fact the MSL will have it.

More efficient @ 1/100th the output...

More efficient @ 1/100th the output...

We aren't building the hoover dam here.

MMRTG is much more efficient than a full blown reactor. In fact the MSL will have it.

What definition of efficient are you using? The MMRTG uses 2000w of heat to produce 120w of electricity. The Advanced Stirling Radioisotope Generator (ASRG) will produce 143w electricity from only 500w of electricity.

The ASRG only needs 2 General Purpose Heat Sources (GPHS) versus 8 for the MMRTG. and only weighs 20kg versus 45kg for the MMRTG.

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We aren't building the hoover dam here.

20kWe is the power produced by a non-sport motorcycle engine.

Such power would allow travel over 100's (if not thousands) of kilometers over a rover's lifetime. Such power would allow deeper excavation and more complex experiments. I don't doubt it's technically difficult in many ways - but there are potentially significant benefits.

my first thoughts are

if its stupid but it works...its not stupid

imagine combing these two designs and you have a fearless go anywhere rover

Interesting ideas. My thoughts run toward a Brayton Cycle micro turbine w/ helium gas. Temp management may be a headache in a rapidly changing lunar sun / shade environment.

My guess is that weight could be a serious issue. I doubt one of these animals could be built on Earth at a mass < 2000kg... Getting it to the moon or mars would require some sort of orbital assembly in stages and I doubt that's gonna happen soon. (Although maybe the Japanese / Chinese may be adventurous to try).

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there is also a process that converts nuclear radiation directly to electricity...have to Google it, i do know craploads of stuff but im thin in this area

It's not the nuclear radiation that's converted into electricity. It is the HEAT caused by nuclear decay that is directed towards thermopiles to generate electricity.
At least with RTG's that is. Of course there are ways to generate electricity using the nuclear radiation itself, but RTG's are very stable and relatively simple.

I think RTG's were used in the still traveling Pioneer spacecraft. If I got that right, my understanding is that the RTG's(not sure how many) are boom extended beyond the spacecraft by many feet(Limited shielding, needs to keep it away from sensitve electronics)

Anyway, I heard that the plutonium isotope mass would generate heat on the order of 700-900 degree's F for 70 years! which is directed to the thermopiles. My specifics might be incorrect but should be really close. I also do not know the net usable electrical generation from this type of set-up. Apparently sufficeint for that spacecraft.

mgb_phys
Homework Helper
It's not the nuclear radiation that's converted into electricity. It is the HEAT caused by nuclear decay that is directed towards thermopiles to generate electricity.
In some systems it's thebeta particles (electrons) used directly, in others the gamma are used with effectively a solar panel.

I think RTG's were used in the still traveling Pioneer spacecraft. If I got that right, my understanding is that the RTG's(not sure how many) are boom extended beyond the spacecraft by many feet(Limited shielding, needs to keep it away from sensitve electronics)
Yes, they aren't as powerfull as solar panels but are good when you are heading 1/2Bn miles from the sun.

Anyway, I heard that the plutonium isotope mass would generate heat on the order of 700-900 degree's F for 70 years!
It's more about power than temperature - a small enough piece of anything can reach a high temperature - think of a spark from a sparkler, is't at 3-4000F but doesn't have much power.
Pu238 is nice because it's an alpha emitter so not much sheilding is required and it's half life is 90years so your don't notice much power loss over a reasonable mission. 2g of Pu238 gives about a Watt.

Using the heat in a Stirling engine is probably more efficent than a thermo-electric generator, especially if you have the cold of space to dump the radiator into. But it's more of a maintenance and reliability issue if you need it to run for 20years on a space probe.

Remember that a lump of radioisotope just decaying on it's own gives much less energy than the same material fissioning in a reactor.
You could use the same Stirling engine/RTG system with a reactor heat source - it's just more heavy and complex engineering.
There is also a minimum size for a reactor since you need to maintain criticality

Great details, mgb!!