CAPE CANAVERAL, Fla. (AP) — NASA is weighing the risk of adding astronauts to the first flight of its new megarocket, designed to eventually send crews to Mars. The space agency's human exploration chief said Friday that his boss and the Trump administration asked for the feasibility study. The objective is to see what it would take to speed up a manned mission; under the current plan, astronauts wouldn't climb aboard until 2021— at best. The Space Launch System, known as SLS, will be the most powerful rocket when it flies.