Due to medical reasons, NASA has had to swap out one for another.
NASA astronaut E. Michael "Mike" Fincke will replace Eric Boe for the Boeing CST-100 Starliner's Crew Flight Test, scheduled to launch later in 2019.
"Boe is unable to fly due to medical reasons," NASA said in a statement Tuesday, but he will stay on the mission as assistant to the chief for commercial crew at NASA's Johnson Space Center -- the role Fincke previously held.
Fincke is a seasoned replacement, having completed three space trips since joining the astronaut corps in 1996. He has spent 382 days in space and performed nine spacewalks.
Fincke joins astronauts Nicole Mann from NASA and Chris Ferguson from Boeing for the mission.
The Starliner's Crew Flight Test will be the first manned launch of the spacecraft, which is part of NASA's Commercial Crew Program, a partnership with Boeing and SpaceX aiming to launch humans to the International Space Station (ISS).
If successful, Boeing's Starliner and SpaceX's Crew Dragon, reportedly due to make its first launch -- unmanned -- on Feb. 9, will mark the first time in history NASA has sent astronauts to space on systems "owned, built, tested and operated by private companies".
NASA retired its Space Shuttle Program in 2011, and says working with Boeing and SpaceX will allow American astronauts to launch to the ISS from the US using American-made rockets for the first time since the closure.
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