US Space Force, the space service branch of the US armed forces, has awarded California nuclear startup Antares a $161m (€139m) contract to develop a fission reactor for spacecraft, an award that the company says is the largest current Department of War award of its kind.

The award came from the Office of the Assistant Secretary of the Air Force for Space Acquisition and Integration, which runs Space Force acquisition. Antares announced the selection on 11 September.

Under the award Antares will run a ground demonstration of the R1-S, a space variant of its terrestrial heat-pipe microreactor, then work to integrate that reactor with a spacecraft for flight certification ahead of an eventual launch.