With NASA astronauts docked at the International Space Station far longer than planned, the agency’s leadership on Wednesday acknowledged potential alternatives to Boeing’s Starliner for returning the crew to Earth.

Still, the Boeing’s spacecraft remains the primary option for returning crew, officials said.

Officials say Starliner capsule “Calypso” may return as soon as the end of this month from its extended ISS stay, pending results of testing a faulty propulsion system. Starliner has now been in space 36 days and counting as the agency and Boeing perform additional testing in New Mexico before clearing the spacecraft to return.

The mission is the first time Starliner is carrying people, flying NASA astronauts Butch Wilmore and Suni Williams.

NASA’s Commercial Crew manager Steve Stich emphasized during a press conference that the first “option today is to return Butch and Suni on Starliner,” adding, “we don’t see any reason” currently to turn to the agency’s other transportation option, which would be SpaceX’s Crew Dragon, to bring back the astronauts.

Stich — while acknowledging that a SpaceX capsule could be part of contingency plans in case Starliner were to return from the ISS empty — noted that NASA does not yet need to “make a decision as to whether we need to do anything different.”

“Certainly we’ve dusted off a few of those things to look at relative to Starliner, just to be prepared in the event that we would have to use some of those kinds of things,” Stich said.

“[But] there’s really been no discussion with sending another Dragon to rescue the Starliner crew,” Stich added later.

Boeing and NASA on July 3 began testing the spacecraft’s thruster technology back on the ground in White Sands, New Mexico, aiming to replicate an issue that caused as many as five of Calypso’s thrusters to shut down when the spacecraft was maneuvering to dock with the ISS. The ground testing is being done to “make sure that with all these pulses and all the heat we’re putting into it, that it doesn’t cause any damage to the thruster,” Stich said.

Stich noted that an “end of July” return for Starliner is “optimistically” based on completing the testing. Boeing and NASA teams in White Sands are conducting inspections of the test thruster over this next week.

But “so far we’ve not been able to replicate the temperatures that we saw in flight,” Boeing’s Mark Nappi, vice president of the Starliner program, said during the press conference.

“What we’re trying to do with this testing is fill in some gaps, because … what we’re trying to do is understand if the thrusters are performing [as expected], then we will be able to undock and just return. If the thrusters were somehow damaged, then what would we do differently?” Nappi said.

“We don’t believe that we have damaged thrusters, but again, we want to fill in the blanks and run this test to assure ourselves,” Nappi added.

Wilmore and Williams, speaking to press from the ISS, both expressed confidence about returning on Starliner.

“We trust that the tests that we’re doing are the ones that we need to do to get the right answers to give us the data that we need to come back,” Wilmore said.

Starliner was once seen as a competitor to SpaceX’s Dragon, which has made 12 crewed trips to the ISS over the past four years. However, various setbacks and delays have steadily slipped Starliner into a secondary position for NASA, with the agency planning to have SpaceX and Boeing fly astronauts on alternating flights.

The Starliner crew flight test represents a final major step before NASA certifies Boeing to fly crew on operational, six-month missions beginning as soon as February.