The Dream Chaser space plane continues to take steps toward flight, even though NASA did not select the private vehicle to ferry astronauts to and from the International Space Station.

Sierra Nevada Corporation, which is building Dream Chaser, recently checked off a milestone laid out in the company's last commercial-crew contract with NASA, which was signed in 2012. (The space agency did not award funding to Sierra Nevada in the final phase of the program, known as the Commercial Crew Transportation Capability, instead choosing SpaceX and Boeing to provide astronaut taxis.)

The optional work, called Milestone 15a, had Sierra Nevada show that Dream Chaser's reaction control system can operate in a vacuum chamber characterized by some of the conditions found in space. The system is supposed to help Dream Chaser maneuver in orbit, and also guide it to landings on runways.

A prototype thruster for Sierra Nevada's Dream Chaser space plane undergoes testing in a vacuum chamber to simulate the orbital environment. (Image credit: SNC/ORBITEC)

"In passing this milestone, we are able to validate our performance and safety, while decreasing the risk for this critical propulsion system," Mark Sirangelo, corporate vice president of Sierra Nevada's Space Systems Division, said in a statement. "Reaching this milestone propels us even closer toward the critical design review and orbital flight of our complete system."

Sierra Nevada and NASA agreed to the optional milestone in 2013 under the company's existing Commercial Crew Integrated Capability (CCiCap) contract. The company now has achieved all but one of the 13 miletones outlined in the contract. The work should be useful for future crewed and uncrewed missions using the spacecraft, Sierra Nevada representatives said.

NASA announced the Commercial Crew Transportation Capability awards on Sept. 16, giving $4.2 billion to Boeing to continue developing the CST-100 capsule and $2.6 billion to SpaceX to work on a crewed version of that company's Dragon spacecraft.

The goal of the Commercial Crew Program is to restart launches of astronauts from U.S. soil, a practice suspended in 2011 when the space shuttle retired. Current International Space Station crews launch with Russian Soyuz spacecraft, which leave Earth from Kazakhstan.

Sierra Nevada filed a protest with the Government Accountability Office (GAO) concerning the award of CCtCap on Sept. 26. The GAO rejected the protest on Monday (Jan. 5).

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