Fencing the moon

by Paul Curzon, Queen Mary University of London

Lunar module in landing configuration. Probes below each foot tell when the Lunar Module has almost landed.
Lunar module Eagle from the Apollo 11 moon landing getting ready to land (taken from the command module)
Image by NASA (public domain)

The Apollo lunar modules that landed on the moon were guided by a complex mixture of computer program control and human control. Neil Armstrong and the other astronauts essentially operated an semi-automatic autopilot, switching on and off pre-programmed routines. One of the many problems the astronauts had to deal with was that the engines had to be shut down before the craft actually landed. Too soon and they would land too heavily with a crunch, too late and they could kick up the surface and the dust might cause the lunar module to explode. But how to know when?

They had ground sensing radar but would it be accurate enough? They needed to know when they were only feet above the surface. The solution was a cunning contraption: essentially a sensor button on the end of a long stick. These sensors dangled below each foot of the lunar module (see image). When they touched the surface the button pressed in, a light came on in the control panel and the astronaut knew to switch the engines off. Essentially, this sensor is the same as an epee: a fencing sword. In a fencing match the sword registers a hit on the opponent when the button on its tip is pressed against their body. Via a wire running down the sword and out behind the fencer, that switches on a light on the score board telling the referee who made the hit. So the Lunar Module effectively had a fencing bout with the moon…and won.

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This cs4fn blog is funded by EPSRC, through grant EP/W033615/1.

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