NASA says that its latest tests are a milestone towards the next stage of space exploration — and Mars. It successfully fired an RS-25 rocket engine for 500 seconds on Thursday March 10th. This is the same engine that will eventually send astronauts on the first deep-space mission in more than 45 years. It's a key part of NASA's new Space Launch System (SLS), aimed at taking humans to nearby asteroids and, after that, the red planet. Four of these engines power the SLS — making up a combined thrust of two million pounds.