WASHINGTON — NASA is continuing to study using commercial crew test flights as space station crew rotation missions, but won’t make a final decision regarding that until next summer.

At a meeting of the human exploration and operations committee of the NASA Advisory Council Aug. 27 at NASA’s Ames Research Center, agency officials said they were keeping the option open of using the crewed test flights of Boeing’s CST-100 Starliner and SpaceX’s Crew Dragon to maintain a U.S. presence on the International Space Station, while expressing confidence either or both vehicles will be certified for crew rotation missions by the end of next year.

“If we can bring commercial crew online this year and next year, then we have sufficient margin to overlap with the Soyuz capability,” said Bill Gerstenmaier, NASA associate administrator for human exploration and operations.

NASA’s access to Soyuz seats runs out in January 2020, a few months later than previously planned after NASA and the other ISS partners agreed to stretch out the schedule of Soyuz flights. SpaceX is currently scheduled to perform a crewed flight test in April 2019, followed by Boeing in mid-2019, according to updated schedules announced by NASA in early August.

Those dates have slipped significantly from original plans, raising concerns that the vehicles may not be certified — a milestone that takes place after a successful crewed fight test — until after access to Soyuz seats runs out. NASA announced earlier this year a modification of its contract with Boeing to study turning that flight test into a long-duration mission that could stay at the ISS for up to six months, carrying three astronauts rather than the previously planned two.

When NASA announced assignments for the initial commercial crew flights Aug. 3, it placed three people on the Boeing crewed demonstration flight: NASA astronauts Eric Boe and Nicole Mann and Boeing test pilot Chris Ferguson, a former NASA astronaut. That assignment of three people, versus two for the SpaceX crewed flight test, appeared to protect the option of using the Boeing flight as a longer duration mission.

Kathy Lueders, NASA commercial crew program manager, said later at the committee meeting that the crew for that mission, including Ferguson, had started training on ISS systems to prepare for the possibility his mission there would be extended. However, she said no decision would be made on it until next summer.

“We’re working with station and we’ll see the progress on where the crewed flight tests are, and then we have the flexibility to be able to make that a longer duration mission if we need to,” she said.

She added that NASA will continue to fly three people on that mission even if it remains a short-duration test flight. That prompted discussion among committee members, who questioned whether the benefit of a third crewmember was worth the risk. “What’s the justification for adding the human risk of a third person on that flight?” one committee member asked. “You don’t need three crewmembers for a short flight.”

Lueders noted that the vehicle would carry four people on later post-certification missions, and that both Boeing and SpaceX worked to address the risks of a crewed flight test by performing uncrewed flight tests first. “The government’s original concept, or minimal requirements, were for our first mission to be a crewed flight test,” she said. “Both providers are flying uncrewed flight tests to mitigate that activity.”

Gerstenmaier said that NASA was also considering a similar contract modification with SpaceX for using its crewed test flight as a long-duration ISS mission. “There’s potentially a contract change also with SpaceX,” he said, but didn’t state if that would involve adding a third person to that mission, after crews had already been assigned.