WASHINGTON (AP) — President Barack Obama sought Tuesday to reinvigorate his six-year-old call for the U.S. to send humans to Mars by the 2030s, a mission NASA has been slowly and quietly trudging away at.

The White House was calling attention to government contracts awarded to six companies to build prototypes for "habitats" that could sustain human life in deep space. One such privately developed habitat — an inflatable room —is already attached to the International Space Station. Obama also...