Have you ever wanted to know more about those strange probes hanging from three of the Lunar Modules’ four legs?
In this article, I will dig into those “tentacles” named “Lunar Surface Sensing Probe.”
They were mounted on the bottom of the lunar module’s landing gear’s footpad.
They were essentially five-foot-long “tentacles”: Once one touched the Moon’s surface, a moon surface “contact light” lit up.
They were indicating to the astronauts that they were close to the surface and that they should cut the DPS engine.
You can hear Buzz Aldrin call, “Contact light” during the Apollo 11 landing.
How Was The Sensing Probes Retained?
The lunar surface sensing probe appended to each landing gear footpad is an electromechanical device.
The sensing probes are retained in the stowed position, against the primary strut, to landing gear deployment.
Mechanical interlocks are released throughout the deployment, allowing spring energy to extend the sensing probes so that the probe head is approximately five feet below the footpad.
When any sensing probe touches the Moon’s surface, pressure on the probe head will close the circuit that tells the astronauts to shut down the descent engine.
This shutdown point determines Lunar Module velocity at contact. It is a tradeoff between landing gear design weight and the descent engine’s thermal and thrust reactions near the lunar surface.
Each sensing probe has indicator plates attached to it, which, when aligned, indicate that the sensing probes are fully extended.
Apollo 11 Originally Four, Then Only Three
Initially, all four legs of the lunar module had contact probes (refer, e.g., to AS09-21-3199).
The contact probe was removed from the lunar module’s leg containing the egress ladder out of fear that it would bend and jut up toward the ladder, ready to puncture a pressure suit.
Starting with Apollo 11, only three legs sported probes.
So, originally a probe had been attached to all four footpads.
Still, Neil Armstrong had pointed out the possibility that his descent down the ladder might be impeded by a considerable length of a metal probe that had been bent in some unexpected way during the landing.
The probe below the ladder was therefore removed from Eagle and all subsequent landers.
Each mission’s Commander flew the Apollo Lunar Module.
The Lunar Module Pilot was tasked with watching the equipment and providing the Commander with all the information needed to fly the spacecraft.
How Long Was The Lunar Surface Sensing Probes?
After parting from the Command Module (CM), the first lunar landing approach was flown by computer using Doppler radar.
During a 30-second burn to decrease speed and reduce altitude to 50,000 feet (or 15,240 meters), 260 miles (or 418.5 kilometers) from the lunar landing site.
The following burn brought forward and vertical velocity to near-zero with the crew on their backs.
At 700 feet ( or 213 meters) altitude and 2000 feet (or 610 meters) from the landing target, the Lunar Module was tipped up so the astronauts could see the planned landing target.
Manual control was enabled for the LM Commander to make any required corrections.
The sensing probes measuring 5.6 feet (or 1.73 meters) extended beneath 3 of the four footpads and would indicate when surface contact was made and would start engine shutdown.
Furthermore, of the ten Apollo Lunar Modules that flew in space, none returned to Earth.
The Descent Stages all remain on the lunar surface where they landed.
The ascent stages were either deliberately crashed onto the lunar surface, burned up in the Earth’s atmosphere, or, in the case of the Lunar Module “Charlie Brown” from the Apollo 10 mission, remains in a solar orbit.
What Does Apollo 11 “Contact Light” Mean?
Three of the four landing pads of the Lunar Module were fitted with a 5.6 feet (1.73 meters) sensing probe used to detect contact with the lunar surface.
When one of these probes made contact, it illuminated the cockpit’s contact light, telling the pilot to manually turn off the descent engine.
The lunar module would fall the remaining distance without thrust to reduce lunar materials’ kickback, which could have damaged the module.
Four Facts About Lunar Surface Sensing Probe:
- It was a probe for sensing contact with the lunar surface as the Lunar Module landed on the lunar surface.
- It is made of an aluminum tube with a switch at the end and a spring-driven mechanism for rotating the probe to the extended position.
- The switch would initiate two blue “Lunar Contact” lights on the Lunar Module’s control panel, signaling the astronauts to shut down the descent engine to complete the landing.
- The tube is encased with insulation made of gold-colored Kapton foil and black Inconel foil for the side facing the Lunar Module descent engine. A little access port near the end of the tube admitted access to the switch.
What Did The Crew Say During The Apollo 11 Moon Landing?
Apollo 11 Moon landing: Last minute
- Mission Control: 60 seconds. (This is the amount of time the Eagle has left before the ‘bingo’ call).
- Buzz Aldrin: Lights on. 60 feet. Down 2.5. Forward. Forward.
- Buzz: 40 feet, down 2.5. Picking up some dust.
- Buzz: 30 feet, 2.5 down… shadow.
- Buzz: 4 forward. 4 forward. Drifting to the right a little. 20 feet. Down a half.
- Mission Control: 30 seconds.
- Buzz: Drifting forward just a little bit. That’s good.
- Buzz: Contact light.
A sensor probe hanging from the feet of the Lunar Module “Eagle” has touched the surface, setting off a light inside the lunar module.
That’s it. I hope you enjoyed this article about Apollo Lunar Module’s “Lunar Surface Sensing Probe.”
If you want to know the history of the Lunar Module, head over to this article called; “The Birth of Apollo Lunar Module LM.”