NASA tested a nuclear rocket engine dozens of times in the 1960s, proved it could work, then shelved it before it ever flew — and now the same basic idea is being revived because it could make the trip to Mars far shorter than chemical rockets allow.

NASA tested a nuclear rocket engine dozens of times in the 1960s, proved it could work, then shelved it before it ever flew — and now the same basic idea is being revived because it could make the trip to Mars far shorter than chemical rockets allow.

In the 1960s, NASA and the Atomic Energy Commission built nuclear rocket engines and test-fired them dozens of times in the Nevada desert. The engines worked. Then the program was shelved before a single one ever flew. In recent years the same basic idea came back, driven by the promise that a nuclear engine could shorten the journey to Mars in a way chemical rockets cannot. The revival’s own story, it turns out, has been almost as stop-start as…