The head of Nasa's human spaceflight programme has stepped down just days before a "historic" launch.
Doug Loverro resigned on Monday, Nasa announced, less than a year after his appointment.
Next week, astronauts Bob Behnken and Doug Hurley will travel to the International Space Station (ISS).
The mission, using a rocket and capsule from private firm SpaceX, will be the first crewed mission to orbit to depart from US soil since 2011.
No official reason for Mr Loverro's departure has been announced, but a leaked copy of an email sent to Nasa employees mentioned a risk taken earlier in the year "because I judged it necessary to fulfil our mission".
"Now, over the balance of time, it is clear that I made a mistake in that choice for which I alone must bear the consequences," the message continued.
While Mr Loverro offered no further explanation, he told the Axios news website that his decision to leave the agency was unrelated to the upcoming launch. "I have 100% faith in the success of that mission," he said.
Mr Loverro was appointed in October last year. His deputy, Ken Bowersox, will become the acting head of human spaceflight.
Next week's launch, which is set for 27 May, will mark the first time that Nasa has launched people from its own territory since the retirement of the shuttles almost a decade ago. The rocket and spacecraft for next Wednesday's flight were both designed and built by SpaceX, the private company owned by billionaire Elon Musk.
Loverro had been expected to chair the "flight readiness review" on Thursday, which will check on preparations and clear Behnken and Hurley for the mission. A replacement will now have to step in.
Nasa has been using Russian rockets for crewed flights since 2011 and recently purchased one more ride for later in the year.
In 2017, US President Donald Trump ordered Nasa to plan a crewed mission to the Moon for the first time since 1972. The Artemis mission will see the first woman and the next man step on to the lunar surface and is due to take place by 2024.
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