Nasa has decided that the two astronauts currently stuck on the International Space Station will return next February on a SpaceX-crewed Dragon flight where two seats have been made available for Sunita “Suni” Williams and Barry “Butch” Wilmore. Space agency officials said there was “too much uncertainty” for the astronauts to return on the craft that brought them to the space station, Boeing’s Starliner, which has had problems after the capsule sprang small leaks and some of its thrusters failed. The announcement comes after an “agency-level review” on Saturday that included Bill Nelson, the agency administrator. “Nasa has decided that Butch and Suni will return with [SpaceX’s] Crew-9 next February, and that Starliner will return uncrewed,” Nelson said in a press conference on Saturday. “I want you to know that Boeing has worked very hard with Nasa to get the necessary data to make this decision. We want to further understand the root causes and understand the design improvements so that the Boeing Starliner will serve as an important part of our assured crew access to the ISS,” he added. “Space flight is risky, even at its safest and even at its most routine, and a test flight by nature is neither safe nor routine, and so the decision to keep Butch and Suni aboard the International Space Station and bring the Boeing Starliner home uncrewed is a result of a commitment to safety,” Nelson continued. Williams and Wilmore, two veteran astronauts, arrived at the ISS on 6 June as part of a crucial test by Starliner before it can receive Nasa approval for routine flights. However, the planned eight-day mission turned into a months-long stay for Williams and Wilmore after technical issues emerged, including reaction control thrusters that failed during Starliner’s first docking attempt. Four of the spacecraft’s five failed thrusters have since reactivated in orbit, the Associated Press reports, adding that the thrusters are crucial for the spacecraft to back away from the ISS after undocking and for maintaining the capsule in proper position for the deorbit. On 2 August, Boeing said in a blog post that it had conducted “extensive testing of its propulsion system in space and on the ground”. The embattled manufacturer, which has struggled to compete with SpaceX and has taken in $1.6bn in losses on the Starliner program, added: “The testing has confirmed 27 of 28 RCS [reaction control system] thrusters are healthy and back to full operational capability. Starliner’s propulsion system also maintains redundancy and the helium levels remain stable.” Describing the decision to bring the Starliner back uncrewed in September, Steve Stich, program manager for Nasa’s commercial crew program, said on Saturday: “The bottom line relative to bringing Starliner back is … there was just too much uncertainty in the prediction of the thrusters. “If we had a model, if we had a way to accurately predict what the thrusters would do for the undock and all the way through the de-orbit burn and through the separation sequence, I think we would have taken a different course of action, but when we looked at the data and looked at the potential for thruster failures with a crew on board … it was just too much risk with the crew,” Stich added. As they wait on the ISS to join the SpaceX crew next February, Williams and Wilmore will do science station maintenance, among other research duties, according to Dana Weigel, manager of Nasa’s ISS program. “They’ll execute the SpaceX 31 research and cargo mission,” Weigel said on Saturday, referring to the commercial resupply service mission to the ISS. “We may have a couple space walks for them towards the end of their expedition. Since they’ve been up there, they’ve been a welcome set of helping hands. They’ve already done about 100 hours of work on 42 different experiments, and they’ve helped us with some of the critical station maintenance that we’ve had on board,” she added. In response to a question on how Nasa plans to rebuild trust with Boeing, Nasa associate administrator Jim Free replied: “I don’t think it’s a trust issue at all. I don’t think we’re rebuilding trust. I think we’re looking at the data, and we view the data and the uncertainty that’s there differently than Boeing does. It’s not a matter of trust. It’s our technical expertise and our experience that we have to balance.” Ken Bowersox, associate administrator of Nasa’s space operations mission directorate, acknowledged the “tense discussions” that Nasa has had with Boeing before going on to say: “People have emotional investment in either option and that gives you a healthy discourse. But after that, you have to do some work to to keep your team together, to keep your team restored and ready for the next issue and I’ll acknowledge that we have some work to do there.” According to Nasa, SpaceX’s Crew-9 mission will launch no earlier than 24 September. Currently, Nasa and SpaceX are working on several items prior to launch including seat reconfiguration on the Crew-9 Dragon, in addition to adjusting the manifest to carry additional cargo and personal effects, as well as Dragon-specific spacesuits for Wilmore and Williams.