NASA’s plan to routinely ferry astronauts into orbit using private spacecraft—initially slated to start last year—has now slipped until at least the spring of 2019, and unresolved hazards threaten further delays.

New questions about the high-profile program, known as commercial crew transportation, emerged Thursday, with the National Aeronautics and Space Administration’s top outside safety panel flagging persistent dangers. Under the program, NASA would rely on capsules developed and operated on a commercial basis by Boeing Co. and Elon Musk’s SpaceX.

Potential problems identified in the group’s annual report range from unconventional rocket-fuel systems to the bombardment of aircraft in orbit by tiny meteor fragments and other space debris.

NASA has stipulated a statistical probability of no more than one fatal accident per 270 flights. SpaceX and Boeing are developing separate fleets of capsules but neither is likely to meet that longstanding safety standard, despite years of testing, re-engineering and high-level government concern about what are called micrometeoroids, according to the report.

Even with mandated on-orbit inspections to detect collision damage, the independent watchdogs concluded “the likelihood remains that the providers will not meet all” of the previously agreed-to requirements. NASA managers will then have to determine if the statistical risk—including significant statistical uncertainties—is acceptable, according to the panel.