We’re building thermonuclear spaceships again—this time for real 

Enlarge / Artist concept of the Demonstration for Rocket to Agile Cislunar Operations (DRACO) spacecraft. (credit: DARPA)

Phoebus 2A, the most powerful space nuclear reactor ever made, was fired up at Nevada Test Site on June 26, 1968. The test lasted 750 seconds and confirmed it could carry first humans to Mars. But Phoebus 2A did not take anyone to Mars. It was too large, it cost too much, and it didn’t mesh with Nixon’s idea that we had no business going anywhere further than low-Earth orbit.