CAPE CANAVERAL, Fla. -- Space shuttle Atlantis is slated to return to Earth next week and close out NASA’s 30-year-old human spaceflight program. When it does, no U.S. spacecraft will be ready to replace its ability to rocket people into orbit.

As a stopgap measure, NASA recently agreed to a $763 million contract for 12 Russian rocket rides from 2014 through 2016. By that time, the space agency hopes at least one of four private companies it's seeding with cash will demonstrate a crew-ready spaceship.

SpaceX (Space Exploration Technologies), a start-up firm started by PayPal founder Elon Musk, is widely considered to be leading the pack of firms that includes Blue Origin, the Boeing Company and the Sierra Nevada Corporation.