An American astronaut, selected to prepare for space missions, decided to reject the training and leave the course, becoming the first candidate to resign in 50 years, NASA confirmed Monday, according to the Houston Chronicle newspaper.

Until recently, future astronaut Robb Kulin, a 35-year-old Alaska native, will leave training on August 31. His decision, he said, is motivated by “personal reasons”, without other details being known.

Brandi Dean, official visiting spokesperson for NASA’s Lyndon B. Johnson Space Center, said the space agency will not replace Kulin and will leave in only 11 members, six men and five women, the 2017 astronaut selection.

Kulin began his training in Houston in August 2017. He requested three times to become a member of the astronaut team before being finally adopted. Thus, after being only one of the approximately 18,300 applicants, he managed to enter the new team of NASA astronauts, composed of 12 people. Now his case is the first, since 1968, in which a person selected for training leaves the ranks of the astronauts still before getting their admission on space flights.

That year, two astronauts, Brian O’Leary and John Llewellyn, left the class of 1967. O’Leary resigned in April 1968, due to what he called lack of space flight prospects. Llewellyn, on the other hand, resigned a few months later, claiming that he could not progress in piloting planes.