NASA has bumped an astronaut off an upcoming spaceflight, a rare move for the space agency so close to launch.

Astronaut Jeanette Epps was supposed to rocket away in early June, and would have been the first African-American to live on the International Space Station. Late Thursday, NASA announced it was pulling Epps off the mission but didn't disclose why.

She's been replaced by her backup, Serena Aunon-Chancellor.

Epps is returning to Houston from Russia, where she'd been training to fly with a German and Russian. NASA spokeswoman Brandi Dean said Friday it was a decision by NASA, not the Russian Space Agency.

African-Americans have visited the space station, but Epps would have been the first to live there. NASA assigned her to the flight a year ago.