NASA is replacing one of the astronauts assigned to fly on the first crewed test launch of the CST-100 Starliner, a capsule being developed by Boeing to take passengers to the International Space Station. NASA announced today that astronaut Eric Boe will no longer be flying on the mission due to unspecified “medical reasons.” Instead, NASA astronaut E. Michael “Mike” Fincke will take Boe’s place.

Boe was assigned to this flight in August of 2018, along with eight other astronauts taking part in the inaugural missions for NASA’s Commercial Crew Program. (In fact, Boe’s been involved with the program for the last three years.) Through this initiative, two private companies — SpaceX and Boeing — have been developing capsules to ferry astronauts back and forth from low Earth orbit, where the ISS resides. SpaceX has been upgrading its Dragon cargo capsule to carry crew, while Boeing has been working on the CST-100 Starliner, an entirely new vehicle.

After five years of development, both capsules are finally supposed to launch to space for the first time this year. The plan is for each vehicle to do an uncrewed test flight first, where they will launch empty and dock at the ISS for a few days. If those trips are deemed successful by NASA, then the companies will fly the capsules again, this time carrying the very first test passengers. Boe was supposed to be on that critical flight for Boeing, along with NASA astronaut Nicole Mann and Chris Ferguson, a former NASA astronaut who now works at Boeing. The launch is tentatively set for August, a month after SpaceX is slated to fly its first crew.

Fincke comes with plenty of spaceflight experience, as this will be his fourth trip to space. A retired Air Force colonel, he’s flown on two Russian Soyuz rockets, as well as the Space Shuttle Endeavor. NASA says Fincke will start training right away with both Mann and Ferguson. Meanwhile, Boe will still stay on with the program, by reporting to the chief of Commercial Crew in NASA’s astronaut office, according to the space agency.