WASHINGTON — A NASA astronaut flying to the International Space Station next spring could be the only American on the station for an extended period because of uncertainty in the status of commercial crew vehicles.

NASA announced Oct. 30 that Chris Cassidy will fly to the station next April on a Soyuz spacecraft with Russian cosmonauts Nikolai Tikhonov and Andrei Babkin. Cassidy will be making his third spaceflight, and second long-duration mission, while Babkin and Tikhonov will each be making their first flights.

Cassidy said at a Nov. 7 briefing at the Johnson Space Center that there will be an overlap for about a week with the departing crew of Jessica Meir, Drew Morgan and Oleg Skripochka, then he and Babkin and Tikhonov will have the station to themselves, perhaps until the end of their mission in October 2020.

“We’re preparing for a six-month duration where it’s just the three of us,” he said. “We’re getting lots of extra training, at specialist levels, for Andrei and Nikolai on all the U.S.-side equipment.”

Cassidy said he’s hopeful that either a Boeing CST-100 Starliner or SpaceX Crew Dragon spacecraft will visit the station during their time on the station, each carrying two NASA astronauts and, in the case of Starliner, former NASA astronaut and current Boeing employee Chris Ferguson.

A Starliner mission would likely be an extended duration stay, as NASA has been working with Boeing on plans for that for some time. Agency officials said in October the SpaceX Demo-2 mission, the crewed flight test of the Crew Dragon spacecraft, could be extended as well.

However, neither crewed test flight has been scheduled yet, pending the outcome of upcoming tests and other work to certify the spacecraft to carry people. “With luck we’ll have commercial crew,” Cassidy said. “But we’re also ready operationally, mentally, all that, prepared to just be the three of us on the station.”

Delays in commercial crew has forced NASA to adjust flight assignments in order to maintain a presence on the station. The Japanese space agency JAXA had hoped to have one of its astronauts on the station next summer to coincide with the Summer Olympics in Tokyo, but the lack of available seats led NASA to fly Cassidy on the only available Soyuz seat.

Kirk Shireman, NASA ISS program manager, said last week that they assigned Cassidy to this flight because of his experience in spacewalking in particular. “We need a very experienced, exceptional spacewalk person,” particularly given the lack of experience of his Russian crewmates, he said after a Nov. 1 press conference about the NG-12 Cygnus cargo mission.

“There’s a lot of considerations,” he added of Cassidy’s selection. “Really, just being able to make sure we can maintain the ISS in all its dimensions.”

With a three-person crew, there will be reduced time for research. “There will be less available crew hours because you still have to devote your baseline number of hours per week to keeping the thing running,” Cassidy said of station operations with three people on board. “It will be slightly changed in philosophy in how we manage crew time, but the goal is still the same: to maximize science hours in research, and we’ll do our best to do that.”

While Cassidy has spacewalking experience, he and his crewmates don’t expect to perform any spacewalks during their stay unless one is required for urgent repairs. “I kind of doubt they’ll plan a normal, planned spacewalk” if there’s only three people on board, he said. “If it’s an emergency, a situation that requires a repair, then we’re prepared and ready to go.”

Cassidy will fly on the last Soyuz seat that NASA has currently reserved with the Russian state space corporation Roscosmos. NASA Administrator Jim Bridenstine said at an Oct. 24 press conference during the International Astronautical Congress that it was “highly likely we will enter into a negotiation, make the request from Roscosmos, for at least one additional Soyuz seat.”

Sergei Krikalev, the executive director for human spaceflight at Roscosmos, told the Russian news service Interfax Nov. 7 that NASA had so far made only a “preliminary request” to Roscosmos about additional Soyuz seats. “Roscosmos is thinking over it so far,” he said.

Cassidy, who became the 500th person in space when he flew on a shuttle mission a decade ago, said he’s not thinking much about the potential milestone of being the last American to fly on a Soyuz. “I care about that as much as I care about being the 500th person in space, which is pretty low,” he said.