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Considering that NASA have been running short of plutonium for their RTGs, it makes sense to consider alternatives. For the inner solar system up to Jupiter, solar cells have been demonstrated on missions e.g. Juno. However, for missions beyond such as Pluto, solar cells reportedly are inadequate.

I made some quick calculations with Wolfram Alpha reaching on the order of 50 metric tons for a 1kW power budget and a 20 year mission. These figures are not precise but hopefully on the right magnitude.

Question - Will dropping launch costs (from reuse or otherwise) ever make these types of power systems reasonable for deep space missions? Or will some other technology make efficient use of the extra weight possible to launch when costs drop?

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    $\begingroup$ Plutonium isn't the only source of nuclear power - we can also build nuclear reactors. $\endgroup$ – ikrase Jul 16 at 7:28
  • $\begingroup$ I did the sums for hydrogen (which is best in terms of energy/mass) and I got about 43 tonnes, most of it oxygen of course, so I think you're in the right ballpark for mass. $\endgroup$ – tfb Jul 16 at 8:49
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    $\begingroup$ Relevant to the comment by @ikrase: the TOPAZ-I reactor was designed to produce $5\,\mathrm{kW}$ for 3-5 years. It weighed about $320\,\mathrm{kg}$: you could lift more than 100 of these things for the effort of lifting the chemical fuel. $\endgroup$ – tfb Jul 16 at 9:01
  • $\begingroup$ @tdb do you have any info on the expense of those? That could be relevant. $\endgroup$ – lijat Jul 16 at 14:59
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We have the capability to launch this already, so it is feasible, just very expensive when there are better alternatives. A spacecraft as you describe isn't just 50 tons of solar panels, you need a huge structure to hold them on, huge amounts of maneuvering fuel to align the spacecraft, extremely powerful gyros, and other elements that would have to be super-sized, meaning a lot more weight. We'd also have to have the capability of building the spacecraft in orbit, meaning bringing up many tons of equipment to support the build and the astronauts carrying it out.

That is a massive, expensive effort which also carries a great deal of risk from its sheer size. It would be far easier, cheaper and less risky to simply make more plutonium, which is exactly what NASA and the US Department of Energy plan to do at the Oak Ridge National Laboratory.

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    $\begingroup$ Update on the plutonium production program for NASA was presented at the Small Bodies Assessment Group meeting in 2018 $\endgroup$ – astrosnapper Jul 15 at 22:04
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    $\begingroup$ Interesting link, thanks @astrosnapper! $\endgroup$ – GdD Jul 15 at 22:17
  • $\begingroup$ Given the potential to just dock a bunch of stuff together I don't see why assembly in orbit requires astronauts. $\endgroup$ – ikrase Jul 16 at 2:33
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    $\begingroup$ You're talking a spacecraft 1/5 the size of the ISS @ikrase. We haven't built anything that size automatically yet. $\endgroup$ – GdD Jul 16 at 7:24

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