This is the best summary I could come up with:

Traditional chemical propulsion is great for blasting rockets off the surface of the Earth, but such machines are terribly inefficient for moving around the Solar System.

The basic idea is straightforward: A nuclear reactor rapidly heats up a propellant, probably liquid hydrogen, and then this gas expands and is passed out a nozzle, creating thrust.

Finally, in 2020, the curious folks at the US Defense Advanced Research Projects Agency said they wanted to test a flyable nuclear thermal propulsion system.

On Wednesday, NASA and DARPA announced they had selected Lockheed Martin to serve as the primary contractor to assemble the experimental nuclear thermal reactor vehicle (X-NTRV) and its engine.

This final orbit has yet to be determined, but it is likely to be 700 to 2,000 km above the surface of the Earth, such that the vehicle’s reentry into the planet’s atmosphere will take place hundreds of years after any nuclear reactions occur.

Dodson said this mission would attempt to store liquid hydrogen in its ultra-cold state for a couple of months, allowing enough time for multiple tests of the nuclear thermal engine.

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