What item helped activate Eagle's broken liftoff circuit breaker?

A felt-tip pen helped activate Eagle’s broken liftoff circuit breaker. Buzz Aldrin used the pen’s nonconductive tip to press the recessed mechanism that armed the lunar module’s ascent engine before departure from the Moon.

The breaker’s knob had been damaged while the astronauts were working inside Eagle, probably after Aldrin’s portable life-support equipment struck the panel. The broken knob meant the astronauts could not operate the breaker normally, raising the frightening possibility that the ascent engine could not be armed for the trip home.

Engineers in Houston considered the problem and recognized that something narrow could reach the remaining mechanism. Aldrin’s felt-tip pen fit into the opening and allowed him to close the circuit. Telemetry then confirmed that the breaker was in the required position, so the ascent preparations could continue.

The story is often simplified as “a pen started the rocket.” More precisely, the pen actuated a damaged circuit breaker in the engine-arm circuit; it did not replace the engine controls or provide thrust. NASA later added protective guards to help prevent similar damage on subsequent lunar missions.