A mismatch between funds and ambitions likely will force NASA to revise its commercial crew strategy and schedule in the future, according by an external safety advisory group.

NASA wants at least one of three commercial crew systems currently under development to begin flying missions to the international space station this decade. But Congress has not funded the program at the levels sought by the White House.

“For the last two years, the [Commercial Crew Program] appropriation has been approximately one half of the budget request,” the Aerospace Safety Advisory Panel (ASAP) said in its latest annual report. “We predict this planning-funding disconnect will again drive a change to acquisition strategy, schedule, and/or safety risk.”

The White House in February said it needed $850 million a year for the Commercial Crew Program, but Congress has since signaled a willingness to provide as much as $500 million a year. Under the continuing resolution funding federal activities for the first six months of 2013, the Commercial Crew Program is funded at a rate equivalent to $400 million per year.

In August NASA awarded a combined $1.1 billion in Space Act Agreements to three commercial crew hopefuls — Boeing, Sierra Nevada Corp. and Space Exploration Technologies Corp. — that covers design and development work for a two-year period. NASA says it cannot certify designs as safe under Space Act Agreements, and in December awarded the three companies so-called Certification Products Contracts worth $10 million each that allow NASA to enforce safety standards.

However, NASA has left the door open for performing certification-related activities via optional milestones under the Space Act Agreements. All three companies have said they could perform orbital tests with non-NASA crews under these optional milestones, and that data gathered during these flights could be made available to the agency for certification purposes.

The ASAP was particularly concerned about using optional test-flight milestones in the Commercial Crew Program. Doing so “could yield two standards of safety — one reflecting NASA requirements, and one with a higher risk set of commercial requirements,” the panel said.

Asked via email if NASA planned to trigger the test-flight milestones under the Space Act Agreements, agency spokesman Trent Perrotto said only that “NASA is committed to launching American astronauts from U.S. soil as soon as possible and is working aggressively to accomplish this goal.”