LRRR (Laser Ranging Retroreflector) Experiment

The LRRR was one of the two scientific experiments that were a part of Apollo 11, and was the first deployed by Buzz Aldren.  It is the one being carried on the right.  The other was a passive seismic experiment.

What an incredible thrill to watch this piece of  hardware, that had been in our laboratory, being deployed on the lunar surface!

This is what it looks like (you can see the Astronaut's foot prints in the soil).  It was a very simple device, containing 100 "cat's-eyes" mounted in an aluminum pallet, and pointed towards the earth.  Each "cat's-eye" was a high-purity fused-silica cube, that was machined to contain a circular face, with a small flange, that was then held in the pallet by low out-gassing O-rings.  The road signs that reflect car light back to you as you drive along are a familiar form of a low-quality retroreflector (cat's-eye).  It is different from a mirror in that you see your reflection even if the "mirror" is not directly facing you.  This is important since it is impossible to exactly align the array to a fixed location on earth, and also because the moon wobbles.  The retroreflectors are also called "corner-cubes" because that's how they are formed - take a cube, and then mill off a face with a corner of the cube under the face.

The goals of the design were to enable the retroreflectors to withstand the launch vibrations, allow them to become relatively thermally isolated from the pallet containing them once placed on the moon, and to enable a long-term passive operation in an environment of about a 350 degree temperature variation throughout the month and constant bombardment from highly-energetic solar particles as well as micro-meteorites.  Also, it had to be light weight and be very simple and quick to deploy.  (In other words, if the experiment looked like something that any old dummy could put together, we succeeded!)

The reason the corner-cubes had to be thermally isolated from the pallet is that each cube needed to be isothermal (uniform temperature).  Temperature gradients in the quartz change the optical characteristics, so that the laser beam that arrived from the earth (already low energy by now because of beam spreading) would spread further upon "reflection."  Spreading of the beam weakened the signal at the receiving telescope back on earth.  With an optically perfect retroreflector the beam strength would already be less than 1/100,000,000,000,000,000 of the original strength..

My part in the experiment  was in the thermal design. 

The way the experiments works is this.  A high-energy and short pulse laser beam is sent towards the moon.  Then an attenuated return signal is awaited. The time difference between the sending and receiving of the signal is the measure of the distance the beam has traveled.  Imagine a beam that is sent towards a target that is only about 24 inches wide and is 224,000 miles away.  The signal is then "reflected" and sent back another 224,000 miles to a telescope that has an aperture of a few feet.  Because of the precision of the experiment, the distance from the telescope to the LRRR can be measured with an accuracy of about 3 cm.  That is like measuring the distance between Cape Porpoise and Los Angeles to an accuracy of 0.014 inches! 

The experiments worked and continued to provide data for decades.  Practical applications included precise measurements of continental drift, study of the effects of gravity on the lunar orbit, studies of the changes in the earth's rotation rate, and a test of general relatively. 

This was a time of tremendous excitement.  We had pride in making one of the first two experiments deployed on the moon.  It was almost like being there.

As a boy I had dreamed of becoming the first man to land on the moon.  This was a way of realizing the dream.