Send US astronauts back to the moon

US President Donald Trump directed Nasa on Monday to send Americans to the moon for the first time since 1972, in order to prepare for future trips to Mars.

"This time we will not only plant our flag and leave our footprint," Trump said at a White House ceremony as he signed the new space policy directive.

"We will establish a foundation for an eventual mission to Mars and perhaps someday to many worlds beyond."

The directive calls on Nasa to ramp up its efforts to send people to deep space, a policy that unites politicians on both sides of the aisle in the United States.

However, it steered clear of the most divisive and thorny issues in space exploration: budgets and timelines.

Space policy experts agree that any attempt to send people to Mars, which lies an average of 140 million miles from Earth, would require immense technical prowess and a massive wallet.

The last time US astronauts visited the moon was during the Apollo missions of the 1960s and 1970s. On July 20, 1969, US astronaut Neil Armstrong became the first man to walk on the moon.

Trump, who signed the directive in the presence of Harrison Schmitt, one of the last Americans to walk on the moon 45 years ago, said "today, we pledge that he will not be the last."

The better known Buzz Aldrin, the second man on the moon after Armstrong and a fervent advocate of future space missions, was also present at the ceremony.