WASHINGTON — As the two companies developing commercial crew systems reiterated that they were on schedule to carry out test flights later this year, a government analysis of schedules concluded those vehicles may not be certified to carry NASA astronauts until late 2019 or early 2020.

That assessment, delivered by the Government Accountability Office at a Jan. 17 hearing by the House space subcommittee on the commercial crew program, raises the potential of a gap in U.S. access to the International Space Station when the agency’s current agreements for Soyuz seats run out next year.

Christina Chaplain, director of acquisition and sourcing management at the GAO, said in her testimony that despite current schedules, which call for certifying both Boeing’s CST-100 Starliner and SpaceX’s Crew Dragon in the first quarter of 2019 after the completion of planned uncrewed and crewed test flights late this year, NASA’s own estimates project that certification to be significantly delayed.

“We found that the program’s own analysis indicates that certification is likely to slip into December 2019 for SpaceX and February 2020 for Boeing,” she said. Those certifications are required before the vehicles can begin regular crew rotation flights to and from the International Space Station.

Chaplain said the companies assumed aggressive schedules, in part to motivate their teams working on the vehicles, assumptions NASA does not necessarily accept. “According to NASA, both contractors assumed an efficiency factor in getting to the crewed flight test that the program office does not assume in its schedule,” she said.

Such delays, she noted, are not uncommon in major NASA programs. “But in this case, the delays and uncertain final certification dates raise questions about whether the U.S. will have uninterrupted access to the space station beyond 2019,” she said.

NASA currently has access to the station through the first half of 2019, thanks in part to the purchase of five Soyuz seats from Boeing, which acquired them from RSC Energia as settlement for litigation involving the two companies’ partnership in Sea Launch. Three of those seats are on flights launching in the spring of 2019, returning in the fall.

Bill Gerstenmaier, NASA associate administrator for human exploration and operations, said NASA was studying potential ways to deal with any certification delays beyond the fall of 2019. That will not involve the production of additional Soyuz spacecraft, he said, since each Soyuz has a production time of about three years.

“We are brainstorming ideas to provide additional schedule time, if needed,” he said, without elaborating. “The ISS program is looking at ways to maximize ISS operations while allowing for some delays in launch dates.”

He emphasized that NASA would not succumb to schedule pressures to make changes that could adversely affect safety. “NASA is aware of the schedule, but not driven by the schedule,” he said.

Patricia Sanders, chair of the agency’s independent Aerospace Safety Advisory Panel, concurred. “Despite the volume of remaining work, the technical challenges and the upcoming end of the Soyuz transportation for U.S. crews, the Panel sees no evidence that the program leadership is making decisions that prioritize schedule over crew safety,” she said.

Despite the assessment from the GAO, representatives of Boeing and SpaceX expressed confidence at the hearing about their latest schedules. Updated versions of the commercial crew flight test schedule, released by NASA Jan. 11, call for an uncrewed CST-100 Starliner flight in August, followed by a crewed mission in November, a timeline unchanged from prior assessments.

“We have high confidence in our plan,” said John Mulholland, vice president and program manager for commercial programs at Boeing Space Exploration. He said he believed the vehicle would be certified by the spring of 2019.

SpaceX is planning an uncrewed flight of its Crew Dragon in August and a crewed flight in December. SpaceX’s schedule represents a slip of four months from its prior schedule for the two test flights.

Hans Koenigsmann, vice president of build and flight reliability at SpaceX, said the company had completed most of the technical development of the Crew Dragon vehicle, and was confident the company could meet the latest schedule. However, he, like Mulholland, emphasized safety over schedule.

“Safely and reliably flying commercial crew missions remains the highest priority for SpaceX, and we will launch NASA astronauts only when both we and NASA are ready,” he said.

SpaceX in particular faced tough questions from some committee members, who questioned the company’s ability to safely transport astronauts given the failures of two Falcon 9 launches and uncertainty about the outcome of the company’s latest launch, of a classified payload known as Zuma, Jan. 7.

Koenigsmann reiterated previous company statements that the Falcon 9 performed as planned on the Zuma launch. He also noted that, in the case of the June 2015 launch failure on a Dragon cargo mission to the ISS as well as the pad explosion during preparations for a static-fire test of a Falcon 9 in September 2016, astronauts could have safely escaped using the abort system on a Crew Dragon spacecraft.

Some members expressed concerns about both the safety of commercial crew vehicles and schedule delays, as well as worries that they could result in increased costs for NASA even though the agency’s commercial crew contracts with the two companies are fixed-price.

“With the end of the ISS on the horizon, the clock is ticking on maximizing the return on the taxpayers’ investment,” said Rep. Lamar Smith (R-Texas), chairman of the House Science Committee. He cited delays in other NASA programs, such as the Space Launch System and Orion, as cause for broader concerns. “NASA and its contractors must restore American confidence in their ability to deliver safe, cost-effective leadership in space.”

However, another senior committee member, Rep. Dana Rohrabacher (R-Calif.), praised the commercial crew program and called Boeing and SpaceX “two terrific companies” for demonstrating an alternative to traditional government-led approaches to human spaceflight. “This program has saved money. We’ve got different approaches that are now being proven. So it looks like the program is going along as we thought it would, even though there have been glitches.”