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Why are we still using solar power for spacecraft? Would it be possible to replace the giant solar arrays and batteries on spacecraft with Kilopower generators?

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NASA's Kilopower project is still in its development phase.

After successful completion of the Kilopower Reactor Using Stirling Technology (KRUSTY) experiment in March 2018, the Kilopower project team is developing mission concepts and performing additional risk reduction activities to prepare for a possible future flight demonstration. Such a demonstration could pave the way for future Kilopower systems that power human outposts on the Moon and Mars, enabling mission operations in harsh environments and missions that rely on In-situ Resource Utilization to produce local propellants and other materials.

NASA is targeting 2020 for an initial in-flight demonstration.

Even when Kilopower becomes operational, it won't replace solar panels on most spacecraft soon.

  1. Because Kilopower is a nuclear power system, it will be expensive to build and launch, which limits its applications.
  2. NASA is targeting a system that produces 1 kW at a weight of 400 kg. In Earth orbit, solar panels can produce that power at a much lower weight: 50 W/kg, so 20 kg for 1 kW. (this ignores batteries, which you need for orbiters and ground-based applications)

Kilopower is targeted at planetary exploration: missions to Mars and the outer solar system. On Mars' surface, solar panels can be used but have drawbacks (dust accumulation, 30% lower performance than on Earth). And the further out from the Sun you get, the lower solar panel performance will be. The ground-based Kilopower system also has a higher power/weight ratio (1500 kg/10 kW) because it can use the ground as a heat sink. This makes a comparison with solar panels more favorable.

NASA's current nuclear power source, the GPHS RTG, costs about \$65-90 million apiece (cost estimate for the New Horizons mission) and provides ~250 W at launch. The goal is to supplement this with a system that provides more power at a lower cost. So far (including the KRUSTY demo reactor which didn't have Stirling generators) NASA has spent \$20M on Kilopower development.

The RTG may still be used for some missions: it has a proven lifetime of closing in on 50 years, while NASA targets "at least 10 years" for Kilopower. Because Kilopower contains moving parts (and an RTG doesn't), it's more difficult to keep a Kilopower reactor running for very long missions (e.g. Voyager).

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  • $\begingroup$ The problems of a lifetime of about 50 years are not only the moving parts of Kilopower. The reactor core should remain critical but also stable and controlable over the long time despite the consumption of fissile material and the accumulation of long-lived neutron poisons. But due to the very different reactor core design many experiences from more than 70 years of reactor operation are not applicable. $\endgroup$ – Uwe Feb 11 at 22:38

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