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.

"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.