The return of the Boeing Starliner has been delayed once again with astronauts now slated to return to Earth from the International Space Station no earlier than June 25, Boeing said.

NASA astronauts Suni Williams and Butch Wilmore became the first crew to fly aboard the Starliner on June 5 for its maiden mission. They were initially going to spend just a week at the space station, but have now remained there since they docked the Starliner spacecraft there on June 6.

The astronauts have been "testing spacecraft systems and performing tasks aboard the orbiting laboratory" ever since, according to the release.

"Mission teams supporting NASA’s Boeing Crew Flight Test continue to review Starliner’s data from the completed test objectives," Boeing said.

When will the Boeing Starliner return?

The primary undocking opportunity is now targeting "no earlier than 10:10 p.m. EDT" on June 25, Boeing said in the update. The astronauts will tentatively land around 4:51 a.m. on June 26 at the White Sands Space Harbor in New Mexico.

Before the new return date, the Starliner was previously scheduled to be back on Earth by June 22. Before the June 22 return date, the spacecraft was anticipated to land no earlier than June 18.

The changes in return dates are partially due to the crew staying longer to better prepare the Starliner for the "long-term goal of having it perform a six-month docked mission at the space station," Steve Stich, manager of NASA’s commercial crew program, said in a news release announcing the June 22 return date.

Some ways the crew is preparing the Starliner include performing "additional hatch operations," repeating some "safe haven" testing and assessing piloting and thrusting capabilities, according to the release.

“We have an incredible opportunity to spend more time at (the) station and perform more tests which provides invaluable data unique to our position,” Mark Nappi, vice president and program manager for Boeing, said in the release. “As the integrated NASA and Boeing teams have said each step of the way, we have plenty of margin and time on station to maximize the opportunity for all partners to learn – including our crew.”

Boeing Starliner problems, including helium leaks

Amid the Starliner's launch from Cape Canaveral, Florida on June 5, the crew dealt with helium leaks in the spacecraft and a malfunctioning valve on the rocket.

During a media teleconference on Tuesday, Stich spoke more about a series of helium leaks in the Starliner's propulsion system and the next steps forward for the spacecraft. So far, five leaks have been discovered since Starliner has been on the space station, he added.

"The path going forward is to continue to look at the helium system to try to understand what's happening, again look at simulations for the remainder of the flight... and then the operational techniques to manage the helium system," Stich said.

The valve that went faulty during launch also did not come back online when the crew test-fired the spacecraft's thrusters on Saturday. The crew did not try to test-fire the defective valve and they don't plan to use it during the return flight "out of an abundance of caution," according to Stich.

NASA and Boeing believe the thrusters went offline due to the high heat generated while the system was firing "in rapid succession" to keep the spacecraft on course with the space station, Stich said.

“We’re taking our extra time given that this is a crewed vehicle, and we want to make sure that we haven’t left any stone unturned,” Stich said. “We also want to look at the systems and potential interaction between the systems, and make sure we haven’t missed something before we return. And we’re getting a lot of great data while we’re at the space station for not only this flight but for the next flight.”

How will the Boeing Starliner land?

Once undocked and deorbited, Starliner will "descend under parachutes to land in the desert grounds of White Sands," according to the aerospace company. Airbags attached to the bottom of the spacecraft will soften its landing, Boeing added.

"The landing will mark the first time an American capsule has touched down on land with astronauts aboard," the company said in the release.

NASA and Boeing specialists will pick up the crew after they land successfully, according to Boeing.