A little more than a year ago, NASA introduced its newest class of 12 astronaut candidates. These talented men and women were chosen from a deep pool of 18,300 applicants, and after two years of training they were to join the space agency's corps for possible assignment on missions to the International Space Station, lunar orbit, or possibly the surface of the Moon.

However, one of those 12 astronauts, Robb Kulin, will not be among them. On Monday, NASA spokeswoman Brandi Dean confirmed to Ars that Kulin had resigned his employment at NASA, effective August 31, "for personal reasons."

Sources described Kulin as a "family man," confident in his abilities. A native of Anchorage, Alaska, Kulin was a mechanical engineer who came to NASA from the rocket company SpaceX, where he had been a senior manager for flight reliability. "Hopefully, I will one day fly on a vehicle that has components that I got to design," Kulin said in June 2017, during a news conference announcing the new astronaut class.

It is rare for an astronaut "candidate," or ascan in space agency parlance, to drop out of training before completing this work and becoming assigned to the astronaut office, said Robert Pearlman, a space historian.

“The last time that a NASA astronaut candidate resigned before being qualified for a potential spaceflight assignment was 50 years ago, in 1968," said Pearlman, editor of the space history news website collectSPACE. "Chemist John Llewellyn, a member of NASA's sixth group of trainees and second scientist-astronaut group selected in 1967, withdrew from the program a year later after realizing he was not cut out for flying jets. An August 1968 NASA release announcing his departure stated, 'Dr. Llewellyn said it became apparent several weeks ago that he was not progressing as he should.'"

NASA's 2017 class of 12 astronauts, seven men and five women, were the space agency's 22nd group of trainees since the original Mercury 7 astronauts were selected in 1959. The group ranged in age from 29 to 42 at the time. Kulin is now 35 years old.