SpaceX, based in Hawthorne, Calif., is working on a variant of its Dragon capsule. The capsule has made two cargo trips to the International Space Station so far, and is slated for two more this year under a cargo contract with NASA through at least 2015. The crewed version of Dragon would use rocket motors during the final moments of descent for a soft landing on Earth at mission's end. It would also include an escape system that could be used if a launch was aborted shortly after liftoff. The company has said it plans to test the escape system on the launch pad as early as next December.
The other entrant, Boeing, has teamed up with Bigelow Aerospace on another capsule-based approach to crew transportation that would also be expected to use the Atlas V as the launch vehicle.
Under NASA's contracts with the three companies, they receive incremental payments that are issued as they successfully meet agreed-upon milestones. If all three meet their milestones for the current contracts, NASA will have invested $1.5 billion in developing a commercial-crew capability for the US since 2009.