United Launch Alliance (ULA) announced Feb. 2 that it has passed two key milestones — a NASA-led systems requirement review and safety analysis — meant to lead toward the eventual certification of the Atlas 5 rocket for human spaceflight.

Three of the four companies receiving funding under NASA’s Commercial Crew Development (CCDev) program have chosen Atlas 5 as the rocket they would use to launch the spacecraft the hope to build to transport astronauts to and from the international space station.

ULA is no longer receiving CCDev funding — the $6.7 million the Denver-based company won in 2010 during the program’s first round ran out last year — but the launch service provider continues to collaborate with NASA on human-rating the Atlas 5 under an unfunded agreement awarded last April.

In a statement, ULA said it conducted a series of reviews with NASA in December to show how Atlas 5 already meets the intent of NASA’s human spaceflight certification requirements.

“This was the first time we were able to share detailed Atlas 5 design and flight data with NASA Human Spaceflight experts,” George Sowers, ULA’s vice president of business development and advanced programs, said in a written statement.

In addition to the Tailored Systems Requirements Review, ULA also completed for NASA’s benefit a Probabilistic Safety Analysis to show, in Sowers words, how the demonstrated reliability of the Atlas 5 “offers significant benefits towards meeting NASA’s stringent crew safety requirements.”

Since making its debut in 2002, Atlas 5 has flown 28 missions without a failure.

ULA said it will offer human-certified Atlas launch services to NASA’s would-be crew transportation services providers as the agency moves forward with its Commercial Crew program.

A solicitation for a third round of Commercial Crew funding, aimed at getting at least two competing teams ready to start building and testing their system, is due out Feb. 7.