One of only a dozen astronauts selected from more than 18,000 applicants to train with NASA has become the first to quit the training programme for 50 years.
Robb Kulin, 35, a former SpaceX engineer from Alaska, is the first astronaut candidate since 1968 to have departed from the training.
The last candidates to have left before becoming fully-fledged astronauts were John Llewellyn and Brian O'Leary, each of whom NASA said was "not progressing as he should".
NASA said that Mr Kulin was leaving the programme due to "personal reasons".
A spokesperson for NASA stated that the agency does not intend to replace the Alaskan, leaving 11 candidates within its 2017 intake group.
NASA's 2017 intake is potentially the group of astronauts who could one-day take the helm for the agency's next manned missions to the moon.
Earlier this year, a team of scientists at the agency confirmed that water ice exists in the darkest and coldest parts of the moon's poles.
The researchers believe that if there is enough ice sitting at the surface of the moon – within the top few millimetres – the water could be accessible as a resource for expeditions.
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Although there are currently no planned manned missions to the moon, Donald Trump has ordered the Pentagon to create a US "space force", which he said would become the sixth branch of the American military.
It is not yet clear what military training for astronauts may involve.