CAPE CANAVERAL, Fla. — On the eve of NASA’s next-to-last space shuttle launch, the U.S. space agency trotted out four private companies it is counting on to build commercial spaceships to carry astronauts to low Earth orbit and the international space station after the space shuttles retire.

NASA recently awarded the second wave of contracts in its Commercial Crew Development program, which aims to spur creation of a private replacement for the space shuttles. NASA hopes outsourcing travel to low Earth orbit will free it to focus on building a rocket to carry astronauts to an asteroid and Mars.

The four contract winners —  Blue Origin, Sierra Nevada, Space Exploration Technologies (SpaceX) and Boeing — were on hand April 28 at NASA’s Kennedy Space Center here to brief reporters covering the Space Shuttle Endeavour’s final mission.

While the timeline is murky, officials say the earliest a commercial spacecraft would be ready to carry crews to orbit is around 2014-2015.

The firms are at various stages of development; Hawthorne, Calif.-based SpaceX is the only one that has flown its version of an orbital spacecraft on a test flight. In December 2010, SpaceX launched its Dragon capsule atop its Falcon 9 rocket to orbit, and then safely retrieved the capsule from the ocean after landing. It was the first time a commercial company had ever achieved such a feat.

“We’re very proud of that and basically we’re taking it to the next level,” former astronaut Garrett Reisman, SpaceX Commercial Crew Development program manager, said during the NASA-organized press conference here.

Boeing is developing a capsule called the Crew Space Transportation-100 (CST-100), which would be compatible to fly atop various rockets, including SpaceX’s Falcon 9 and Denver-based United Launch Alliance’s Atlas 5 and Delta 4 boosters.

“We’re focusing on keeping it simple,” said John Elbon, vice president and program manager of Commercial Crew Development at Boeing in Houston. “Having a simple system makes it safe and reliable and helps to keep operational costs low.”

Sierra Nevada Corp. of Sparks, Nev., is building a space plane called Dream Chaser that would launch atop an Atlas 5 rocket like a capsule but glide down for landing like a plane or the space shuttle.

“Our vehicles have been in design and testing now for over 12 years,” said Mark Sirangelo, executive vice president and chairman of Sierra Nevada Space Systems, the Louisville, Colo.-based group leading the Dream Chaser project.

Rounding out the group is Blue Origin, a company founded in 2000 by founder Jeff Bezos.

“We’re committed to developing safe and affordable commercial human spaceflight,” said Rob Meyerson, Blue Origin project manager.

Blue Origin is the most secretive of the space companies, keeping specifics of its spacecraft and timeline under wraps. When asked when the company’s first orbital spaceflights may occur, Meyerson said, “I’m not prepared to release those dates.”

He said the company will become more open once it makes progress toward flying spacecraft. “As Blue Origin has more accomplishments and meets our milestones, we’re going to be sharing more information with the public,” Meyerson said.

The four companies are hoping NASA will not be the only customer looking to buy passage aboard their orbital spacecraft. They intend to sell seats to space tourists as well as to astronauts from countries without their own space vehicles.

Elbon said he absolutely believes the market for orbital space travel is there.

“Customers will be picking based on price, and I believe there’s a market for multiple suppliers on orbit, as long as the price is competitive,” he said.