(Image credit: NASA/Bridget Caswell, Alcyon Technical Services)

There are no gas stations in deep space, so engines must be reliable for the long haul. NASA has successfully operated a free-piston Stirling engine for more than 110,000 hours of cumulative operation.

Engineers have virtually eliminated the mechanisms of wear and tear in the engines, which can operate on small spacecraft that need electrical power to run optics, sensors, recording devices, and communications systems to get data back to Earth.