Donald Trump, the man in charge of the US nuclear arsenal and the country's Commander-in-chief, says the moon is "a part" of Mars.

In a tweet, Trump criticised his own administration's space policy, firing a shot at Nasa for talking about returning to the moon.

"For all of the money we are spending, NASA should NOT be talking about going to the Moon - We did that 50 years ago," he wrote.

"They should be focused on the much bigger things we are doing, including Mars (of which the Moon is a part), Defense and Science!"

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Mars is located 140 million miles from the moon, which orbits our planet.

Trump's tweet is surprising for more than one reason: on one hand, for claiming the moon is a part of Mars; on another hand, for backtracking on his support of a return to the moon.

Just three weeks ago, he spoke publicly in support of another lunar mission.

"We are going back to the moon," he wrote in a tweet on May 13.

There is wide speculation about Trump's latest tweet. One theory is that he changed his mind about the moon visit after watching Fox News. One hour before his tweet, Fox guest Neil Cavut expressed his scepticism over a potential trip to the moon.

Nasa has not publicly responded to Trump's latest theory.