Robb Kulin, a 2017 astronaut candidate for NASA, is resigning Friday for "personal reasons," becoming the first candidate to do so in 50 years, NASA officials confirmed Monday.

Kulin, a 35-year-old Alaska native, reported to NASA's Johnson Space Center in Houston for his two-year astronaut training in August 2017. He was one of about 18,300 applicants -- a record number for the space agency -- vying for the 12 spots in the 2017 astronaut class.

The 2017 class application was the third he had submitted to NASA, according to the Anchorage Daily News.

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"It's a long process," Kulin told The Verge last year. "One of the big objectives that the team is going for here at NASA is they're trying to see, 'Hey if I got stuck with this guy on Space Station, is that actually something I'd enjoy?' "

Kulin declined to comment Monday. Brandi Dean, a Houston center spokeswoman, said the space agency will not be replacing Kulin, leaving just 11 members -- six men and five women -- in the 2017 class. His resignation is effective Aug. 31, she added, making Kulin the first astronaut candidate since 1968 to resign before qualifying for spaceflight.

That year, two astronauts from the 1967 class resigned: Brian O'Leary and John Llewellyn. O'Leary resigned in April 1968 due to the lack of spaceflight prospects and Llewellyn resigned a few months later, in August 1968, after failing to make progress piloting jets. Both O'Leary and Llewellyn have died, in 2011 and 2013, respectively.

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Prior to becoming an astronaut candidate last year, Kulin worked for SpaceX, where he primarily worked on the Falcon 9 rocket, for about six years.

Kulin's dream of becoming an astronaut didn't start as a small child like so many rocket men before him. In fact, he didn't set his sights on the stars until he was a mechanical engineering undergraduate at the University of Denver, where he studied the 2003 Space Shuttle Columbia disaster, the Anchorage newspaper reported.

"Maybe you could say I'm not very smart, because what gave me the space bug was actually doing a project on the Columbia disaster back in undergrad — looking at the approach of the investigation and some of the findings there," Kulin told The Verge. "And I guess I'd never really thought much about space until that point. That kind of let me bridge out and think more about space and NASA in general."

Kulin received his bachelor's degree in 2004 and then spent a year in Italy as a Fulbright Fellow. While pursuing his doctorate in engineering at University of California-San Diego — which he received in 2010 — Kulin took multiple three-month leaves to go to Antarctica, where he drilled for ice cores that help scientists understand the history of the planet's climate.

Those excursions -- and many more of his career decisions -- were made with an eye toward donning the white suit and rocketing into space, his Ph.D. advisory at UC-San Diego, Kenneth Vecchio said in June 2017, shortly after the candidate announcements were made.

"Robb has always wanted to be an astronaut, and the excursions to Antarctica were part of his personal preparation for going into space," Vecchio also said. "He knew then what it would take to build a resume to be considered for the space program, and nearly every decision he has made in his career was with an eye toward being selected for the astronaut program, including his desire to work at SpaceX."

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Kulin also is a private pilot, scuba diver and has worked as a commercial fisherman in Alaska. In interviews following the 2017 candidate announcement, Kulin said its important for humans to continue to explore -- especially when it comes to space.

Exploration "continually pushes humanity to learn, adapt and improve — and with our increased perspective on the fragility of planet Earth, it is important we start pushing beyond," he told the University of Denver Magazine in July 2017. "I figured that even if I never fly in space, I would like to be part of making space exploration possible."

Alex Stuckey covers NASA and the environment for the Houston Chronicle. You can reach her at alex.stuckey@chron.com or twitter.com/alexdstuckey.