NASA won’t put humans on the first flight of its big rocket after all

When NASA’s next big rocket launches for the first time, chances are good it won’t have people on board.
For the last two months, the space agency has been studying what it would take to fly a crew of two on the maiden flight of the Space Launch System, or SLS — the monster rocket that NASA has been developing to take people into deep space and on to Mars. Specifically, NASA wanted to know if such a crewed flight could be done safely by 2019. But after figuring out the costs an