50 Years Later: The pen that saved Apollo 11

If it weren’t for a small felt-tipped pen and the quick thinking of the astronauts, the outcome of the Apollo 11 mission could have ended in disaster.

On July 20, 1969, the first men to land on the moon, Neil Armstrong and Buzz Aldrin, had been on the moon’s surface collecting rock and dust samples for almost three hours.

As they were climbing back into the Lunar Module (LM) in preparation to return home, Aldrin hit the circuit breaker switch with the life support backpack on his suit.

"It's very cumbersome," Armstrong said. "You're like the Frankenstein monster. And you have this big backpack on your back."

The switch (ENG ARM) activates the spacecraft’s ascent engine to lift them off the moon so they could rendezvous with Michael Collins, who was in the Columbia command module orbiting overhead.

"He could’ve picked something that was not very important," Armstrong said. "But he banged into the circuit breaker that controlled the SM engine that got us back into orbit."

Armstrong and Aldrin did not think the circuit breaker would trip on liftoff, but they wanted to be sure.

"It was just insurance. It’s nice to get a little insurance," Armstrong said.

The astronauts notified Mission Control by radio, but they had already disposed of most of their tools aboard to make the module lighter.

Using what they had left, a pen, Aldrin activated the inside switch to engage the circuit breaker and trigger the engine.

"So, I pushed that in. Houston says, ‘Oh we've got a good complete circuit,’" Aldrin said.

Designed by Duro Marker, the pen used to fix the broken switch and bring the Apollo 11 astronauts back to Earth is on display at The Museum of Flight in Seattle, Washington, accompanied by the original broken circuit breaker switch.

This pen is equipped with a pressurized ink cartridge and can function in a weightless environment, underwater and in extreme temperatures—still used by astronauts today.

ABC News' Nate Luna, Christine Theodorou, and Mina Kaji contributed to this report.

Editor's note: An earlier version of this story incorrectly listed the designer of the pen. The pen was designed by Duro Marker.