The space shuttle program has just two launches remaining on the calendar, one April 29 and one in June. After that, no one knows what the next U.S.-based rocket to take astronauts to orbit will look like, when it will launch, or who will have built it. But all indications are that the rocket won't be NASA's—the space agency is hoping private firms will soon be capable of safely ferrying U.S. astronauts to and from orbit.

A few frontrunners to take that role emerged April 18, with the announcement that NASA is awarding four companies a combined $270 million to develop manned spaceflight systems for use after the space shuttle is retired.

Blue Origin will receive $22 million to develop vehicle designs and test engines and an escape system; Boeing will get $92.3 million to develop its CST-100 crew capsule; Sierra Nevada Corp. will receive $80 million to pursue its Dream Chaser spacecraft; and SpaceX will get $75 million to continue work on its rockets and capsules and to develop technologies such as a launch-abort system. The companies will be expected to provide their own funds as well, albeit on a smaller scale than the NASA contributions, to develop their technologies.

NASA received 22 proposals for funds, said Philip McAlister, NASA's acting director for Commercial Spaceflight Development, in an April 18 teleconference with reporters. He said the goal was to select "a portfolio of approaches" that might return NASA astronauts to flight soon, without foreign assistance. (After the space shuttle retires, NASA will rely on Russia to deliver U.S. astronauts to orbit until a successor is ready.)

"We are trying to make an American-made system that will get our astronauts to low Earth orbit," said Edward Mango, program manager for the Commercial Crew Program at Kennedy Space Center in Florida. He said that the NASA-fueled development of commercial space companies should cut short the interlude during which Russian rockets will be the only route to space.

McAlister implied that the U.S. spaceflight gap may be as short as a few years. "We are targeting the middle part of this decade to have services available," he said.

Artist's conception of the Sierra Nevada Dream Chaser: Sierra Nevada Corp.