The Mysterious Degradation of the Apollo Reflector Arrays

Lunar laser ranging experiments have produced a treasure trove of interesting information about the Moon, for example that it is spiralling away from us at a rate of 38 mm per year.

The experiments are simple. Astronomers fire a laser pulse at a reflector placed on the lunar surface by the Apollo 15 mission and then use a telescope to look for the reflection, some 2 seconds later.

The observations are challenging. Of the 10^17 photons that set out towards the Moon in each pulse, only one makes it back, on average. And only then if seeing conditions are good.

When conditions are good, astronomers often take aim at the arrays left by the Apollo 11 and 14 missions which are only a third of the size of Apollo 15’s and therefore harder to see. If the observers are feeling lucky, they might also try for the Russian Lunakhod 2 array (the Lunakhod 1 array hasn’t been seen since 1971).

All in all, astronomers have been taking observations since 1969, first from the MacDonald Observatory in West Texas and later from the Apache Point Observatory in New Mexico . This gives them a substantial database with which to analyse the behaviour of the reflectors.

So how have these reflectors fared in the harsh conditions on the lunar surface over the years? That’s the question addressed today by Tom Murphy at the University of California San Diego and a few buddies. And their analysis poses an interesting problem.

First of all they say that the efficiency of all three Apollo reflector arrays has fallen by an order of magnitude during their sojourn on the Moon. The Lunakhod reflector has fared even worse. When it arrived on the moon in 1973, its signal was 25 per cent stronger than Apollo 15’s. Today it is ten times worse.

What’s happened this gear?

The reflectors consist of an array of cubic prisms that operate by total internal reflection. In addition, the Lunakhod prisms have silvered surfaces and are more exposed. Degradation of this silvering probably explains its relative drop in performance.

But what has caused the degradation of the Apollo prisms? Anything that settles on or damages the optical surfaces of the prisms will reduce the efficiency of the total internal reflections. Murphy and co discuss several possibilities such as micrometeorite damage, lunar dust aggregation and the breakdown of the Teflon mounting rings which may have left deposits on the back surface of the prisms.

Any of these mechanisms could account for the drop but its hard to pin one down.

However, there is another more intriguing puzzle about the laser ranging data. When the Moon is full, the efficiency of all the Apollo reflectors drops by another factor of ten. Murphy and co have ruled out ground-based effects such as the saturation of their photon detectors when the moon is bright.

So why does this happen? One clue comes from the study of returns during total lunar eclipses. Within 15 minutes of an eclipse occurring, the efficiency of the reflectors returns to its normal levels. When the eclipse ends and the Moon is full again, the efficiency immediately drops again.

That strongly points to a thermal effect. When the Sun is low in the lunar sky, its rays cannot directly enter the prisms which are recessed in the arrays. But when the Sun is overhead (which is when the Moon appears full on Earth), its rays travel directly into the prisms. This is probably heating the prisms, distorting them and reducing the efficiency of their reflections.

But why now? The full Moon effect was not a problem in the early days of lunar ranging.

“Dust is perhaps the most likely candidate for the observed degradation,” say Murphy and co. The sunlight is probably absorbed by dust on the optical surfaces which in turn heats the silica prisms.

Dust is known to hover above the lunar surface because of electrostatic forces and micrometeorite impacts probably send a few puffs into the lunar atmosphere on a regular basis.

Interesting work. And one that is of more than passing interest for many astronomers because it has implications for anybody thinking of sending gear to the Moon in future. Various astronomers want to send telescopes to the Moon, particularly the far side because of the tremendous seeing conditions there and its isolation from the Earth. Knowing how the Apollo gear has fared will be crucial when it comes to designing this stuff.

Ref: arxiv.org/abs/1003.0713: Long-Term Degradation Of Optical Devices On The Moon