NASA is going back to an old idea as it tries to get humans to Mars. It is teaming up with the Defense Advanced Research Projects Agency (DARPA) to test a nuclear thermal rocket engine in space with the aim of using the technology for crewed missions to the red planet. The agencies hope to "demonstrate advanced nuclear thermal propulsion technology as soon as 2027," NASA administrator Bill Nelson said. "With the help of this new technology, astronauts could journey to and from deep space faster than ever — a major capability to prepare for crewed missions to Mars."