NASA astronaut Jeanette Epps was on track to become the first African-American crew member on the International Space Station this year, but the space agency announced today that she has been pulled from her mission for unspecified reasons. She was supposed to launch as part of Expedition 56/67 in June 2018.

Instead, NASA astronaut Serena Auñón-Chancellor will be taking Epps’ place. Like Epps, Auñón-Chancellor was one of the 14 astronaut candidates that NASA selected out of 3,500 applicants in 2009. Epps came to NASA by way of a PhD in aerospace engineering and seven years at the CIA as a technical intelligence officer. Auñón-Chancellor is a medical doctor with certifications in internal and aerospace medicine.

NASA’s announcement didn’t explain why the agency pulled Epps from the mission. “A number of factors are considered when making flight assignments; these decisions are personnel matters for which NASA doesn’t provide information,” said Brandi Dean, a spokesperson for NASA, in an email to The Verge. Now, instead of taking what would have been her first flight to orbit, Epps will be working out of the Astronaut Office at NASA’s Johnson Space Center in Houston. She could still be assigned to future missions, the space agency said in a statement.

Other African-American astronauts have visited the Space Station, including Leland Melvin who encouraged Epps to apply for NASA’s 2009 class. But Epps would have been the first to become a long-term crew member, living and working on the ISS for months at a time.