Resigning NASA astronaut candidate Robb Kulin is pictured here.

(NASA)

At a Glance

Astronaut candidate Robb Kulin said he is resigning from NASA for personal reasons.The last time a NASA astronaut candidate resigned was in 1968.Kulin was among the more than 18,000 applicants for the 12-member class of 2017.

For the first time in 50 years, NASA astronaut candidate has resigned, the space agency said this week.

After completing the first year of a two-year training period, 35-year-old astronaut candidateRobb Kulin decided to hang up his space suit for "personal reasons," NASA spokesperson Brandi Dean confirmed to weather.com. His resignation is effective Aug. 31.

It's arare occurrencefor an astronaut candidate to leave the job, especially since more than 18,000 apply for 12 available spotseach year.

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Space historian Robert Pearlman told Ars Technica the last time an was in 1968.

"ChemistJohn 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," said Pearlman."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.'"

A native of Alaska, Kulin was among the agency's 22ndclass of "ascans," as they are called within the agency, which includes seven men and five women. He came to the program that began in 1959 after serving as amechanical engineer forthe SpaceX rocket company.

After the mandatory two-year training, candidates are eligible to join missions to the International Space Station or, perhaps, future missions to the moon and beyond.

"Hopefully, I will one day fly on a vehicle that has components ," Kulin said during a press conference for the June 2017 class announcement.