Energy

The main power sources available in space are solar PV and various forms of nuclear power: radioisotope decay, fission, and perhaps eventually fusion.

Power Sources

Current and near future satellites and probes get their power as follows.

Power sources in space. Sources: European Space Agency ¹ for solar, Gibson et al. ² for fission, and World Nuclear Association ³ for radioisotopes.

Active areas of research include higher efficiency, 4- to 6- junction solar cells ¹, new small fission reactors such as NASA's Kilopower project ², and reverse field configuration fusion ⁴.

Launch Safety

The use of radioisotope thermal generators (RTG) or nuclear fission in space entails launching radioactive material from Earth, which raises safety concerns. Results have been found as follows.

Under international regulations, a fission reactor such as NASA's Kilopower could not be activated until it safely escapes Earth. However, there are scenarios under which a reactor could become critical in a launch failure ⁸, and therefore rules are needed to govern the launch of reactors that go beyond what has been learned with RTGs ⁹, ¹⁰.

KiloPower uses highly-enriched uranium (HEU) as its fuel. A reactor that uses low-enriched uranium (LEU) instead would have greater mass, increasing launch costs, but might save money overall because LEU would have lower security costs than HEU ¹¹.

Nuclear thermal rockets (NTR) have greater exhaust velocity than chemical rockets ¹² but low impulse ¹³. This, together with the safety and regulatory issues around operating fission reactors in the atmosphere, might make them suitable for propulsuion but not for launch. In that case we imagine an NTR would have safety issues comparable to launching a fission reactor. Nuclear pulse propulsion, such as by Project Orion, entails the use of nuclear explosions for propulsion and is not permitted under the Partial Test Ban Treaty, even in space ¹⁴.