In his new book Team of Vipers, former Trump communications official Cliff Sims said that he was with the president on April 24, 2017 when Trump called astronaut Peggy Whitson for setting a new record for spending the most amount of time in space. Everything went well until Trump started asking Whitson about Mars and how soon humans could get there, according to the book, which was earlier reported on by Intelligencer. Whitson responded by referring to Trump’s own directive in a bill outlining a trip to Mars, saying that a human flight wouldn’t happen until the 2030s.

“Well, I think we want to do it in my first term or at worst in my second term,” Trump said, according to Sims. “So I think we’ll have to speed that up a little bit.”

Trump’s comments were captured by The New York Times at the time, which said that he said them “perhaps jokingly.” Sims, however, believes Trump was serious.

Next, Sims said Trump brought up the issue with NASA administrator Robert Lightfoot Jr. Trump told Lightfoot that he wanted to go to Mars by the end of his Presidential term. Lightfoot was then forced to discuss how difficult it is to go to Mars and all the challenges the U.S. would face in doing it. Trump wasn’t done, according to Sims.

“But what if I gave you all the money you could ever need to do it?” Trump asked, according to Sims. “What if we sent NASA’s budget through the roof, but focused entirely on that instead of whatever else you’re doing now. Could it work then?”

Much to Trump’s dismay, according to Sims, Lightfoot said it was impossible.

It might be even more difficult now: 95% of NASA employees have been furloughed during the current government shutdown, delaying future space missions.