R1-S
In Development
Antares Nuclear
Overview
Space variant of the R1 heat-pipe microreactor. Under a $161M Space Force Strategic Breakthrough award, Antares will run a nuclear ground demonstration of the R1-S, integrate it with a spacecraft for flight certification, and use Mark-1 to power an operational ground-to-space asset during 2027 testing.
Award announced September 11, 2026. Ground demonstration and spacecraft integration are funded; no flight date has been published. The award is described by Antares as the largest current Department of War award in space nuclear power.