It was one small step for man, that led to one giant sale of a pen, but it's not just any felt-tip.

It's the pen that helped save the famous US Apollo 11 moon mission, and it's been sold for a whopping $857,600 (£638,238) at Sotheby's in New York.

The plastic marker was the unexpected hero for astronauts Neil Armstrong and Buzz Aldrin when they landed on the moon back in 1969.

But how did the pen help, and what was the Apollo 11 mission? Read on to find out.

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The plastic marker pen helped Neil Armstrong and Buzz Aldrin back off the lunar surface.

When returning to their space capsule after their first moonwalk, Buzz noticed that the switch for the engine control circuit breaker had broken off and was on the floor.

By sticking his pen in the hole where the switch should have been, he was able to reactive the engine and so the astronauts were able to return home!

The broken switch was also included in the sale with the pen.

Buzz Aldrin, Neil Armstrong and Michael Collins, before their mission to the Moon

The Apollo 11 mission is famous for being the first time people walked on the Moon.

Astronauts Neil Armstrong, Buzz Aldrin and Michael Collins were the brave crew that embarked on the journey, which took just over 8 days.

Neil Armstrong was the first man on the Moon, which is where his famous words came from: "That's one small step for man, one giant leap for mankind."

Buzz Aldrin shortly followed Armstrong, and the mission was transmitted and watched by around 650 million people.

In total they spent 21 days on the Moon's surface, before returning to Earth.