Instead, Obama said the United States should start by sending astronauts to an asteroid, a controversial plan known as Asteroid Redirect Mission, which was scrapped earlier this year by the Trump administration.

"Since the beginning of his administration, President Trump has taken steps to refocus NASA on its core mission of space exploration," Deputy White House Press Secretary Hogan Gidley said in a statement earlier Monday.

"He will change our nation's human spaceflight policy to help America become the driving force for the space industry, gain new knowledge from the cosmos, and spur incredible technology," Gidley said.

Both Trump and Vice President Mike Pence, who chairs the National Space Council, have repeatedly expressed interest in human lunar missions during the past year.

Pence, during the first meeting of the National Space Council in October, said the moon will be "a stepping-stone, a training ground, a venue to strengthen our commercial and international partnerships."

On Monday, Pence said signing this space policy directive will allow America to "lead in space once again" although Trump said "we are (already) the leader and we're going to stay the leader."

Monday's ceremony coincided with the 45th anniversary of the Apollo 17 mission, the last time humans walked on the moon. The last living astronaut of the Apollo 17 mission, Harrison "Jack" Schmitt, was also at the event at the White House.

When speaking of the moment Schmitt landed on the lunar surface on Dec. 11 of 1972, Trump turned around and shook hands with him, saying: "today, we pledge that he will not be the last."

Then, Trump asked: "I suspect we'll be finding other places to land in addition to the moon. What do you think, Jack?"

"Yes, we should. Learn from the moon," Schmitt replied.