Apollo 11 astronauts Buzz Aldrin and Michael Collins have reunited on the eve of the 50th anniversary of humanity’s first moon landing.

They gathered in the Oval Office with US President Donald Trump, who got a rundown on his administration’s plans to get astronauts back on the moon by 2024 and then on to Mars in the 2030s.

“We’re bringing the glamour back” to the space program, Mr Trump said.

Both sons of the late Neil Armstrong, the first man to step onto the moon on July 20, 1969, also attended, as well as first lady Melania Trump, Vice President Mike Pence and NASA Administrator Jim Bridenstine.

The moon versus Mars debate as astronauts’ next destination arose again on Friday.

.@AstroMCollins: "As I look around this room . . . I wish it were large enough to hold 400,000 people. Because at its peak, that's how many people were employed by NASA on the quest for the moon . . . I'm just thrilled to represent them." pic.twitter.com/QYgKN7pX0l

— The White House 45 Archived (@WhiteHouse45) July 19, 2019

The president asked if astronauts could get to Mars without first going back to the moon.

Collins, 88, who circled the moon alone in the command module while Armstrong and Aldrin descended in the Eagle, told the president that he supports going directly to Mars and bypassing the moon.

“It seems to me Mars direct, who knows better than these people?” Mr Trump noted.

Mr 
Bridenstine, though, stressed the importance of the moon as a training ground and noted that because of the planetary alignment, launches to Mars can occur only every 26 months and even then the trip is seven months each way.

“What happens if you miss the timing? They’re in deep trouble?,” MrTrump asked. “You don’t want to be on that ship.”

Just had an excellent meeting with President Donald Trump! We discussed America’s future in space, ways to address space challenges, and the need to keep exploring beyond the horizon. Keep America Great in Space!! #Apollo50 #ApolloXI https://t.co/zv2LgoCheD

— Dr. Buzz Aldrin (@TheRealBuzz) July 19, 2019

Aldrin, meanwhile, said he’s disappointed with the state of human space exploration during the past 10 or 15 years.

“We were able to achieve so much early,” the 89-year-old said.

Aldrin, whose specialty was orbital rendezvous, doesn’t like NASA’s idea for a small space station around the moon, called the Gateway, from which to stage lunar landings and, eventually, Mars trips.

He noted that the Apollo 11 command module and attached lunar module went straight into lunar orbit and even separated and re-docked around the moon.

“We have the No. 1 rocket right now in the US and we have the No. 1 spacecraft, and they cannot get into lunar orbit with significant manoeuvreing capability,” Aldrin pointed out.

The president and space experts talked about future Americans living and working on the moon during today's White House event marking the 50th anniversary of the Apollo 11.

"That's a market," NASA chief Jim Bridenstine said. "It could be for tourism. It could be for resources" pic.twitter.com/GwbKaoVfgO

— POLITICO (@politico) July 19, 2019

Mr Trump directed Mr Bridenstine to listen to the “other side”.

Aldrin and Armstrong, who died in 2012, landed on the Sea of Tranquility at 4:17 pm on July 20 in 1969.

“Houston, Tranquility Base here. The Eagle has landed,” Armstrong radioed.

Armstrong was the first to climb down the ladder, stepping onto the lunar surface at 10:56 pm.

His “That’s one small step for man, one giant leap for mankind,” is arguably the most famous space line of all time.

-AAP