Lasers and Lunar Landers

Yesterday I wrote a post about the lunik 17 moon lander and how it was the vanguard of today’s robotic exploration of the solar system, but even though the hulking Russian craft is now into its 45th year, and has long since stopped functioning, it is still being used for science today. Admittedly it is playing quite a passive role in that process but it is true nonetheless.

One hundred years ago this month one Mr A. Einstein published his General Theory of Relativity. It remains the best tool we have for explaining gravity and much of the universe on the macro scale. It is also one of the most heavily tested theories to have existed and so far it has withstood every test we have subjected it to. One way we can test it is if we use it to calculate precisely the way that the earth, the moon and the sun all move in relation to each other. In this context when I say precisely I mean to the millimetre. With that level of precision it is possible to test some of the finer points of the theory like the Equivalence Principles and whether the gravitational constant, G, remains the same as the universe expands. I’m not going to try to create any false tension here: relativity remains entirely unbroken, Einstein was a genius blah blah blah. The issue here is how can we measure the distance of the moon from the earth with sub-centimetre accuracy?

Atop the lunik 17 module was a retroreflector. This is not simply a mirror that reflects light off at the same angle of incidence at which it arrives but a special surface that reflects light back in the same direction it came from, no matter which direction that is. This means that it is possible to shine a laser up at the moon, time how long it takes to receive the reflection back and then it’s just a simple matter of distance = speed x time. Do this multiple times and it becomes possible to very, very, very accurately calculate the distance. The only problem was that, for several decades, no one knew where lunik 17 was.

The APOLLO. Image courtesy of M3long
The APOLLO. Image courtesy of M3long

We knew roughly where it was. It had landed in the Mare Imbrium region of the moon, but that’s over 1000km wide. Lunik was lost. But then, in 2010, Albert Abdrakhimov spotted it in an image taken by the Lunar Reconnaissance Orbiter. Take a look at the image yourself to get a feel for how long he must have spent staring at these pictures. This gave the people with the massive lasers something to point at. More specifically it gave the people at the Apache Point Observatory Lunar Laser-ranging Operation in New Mexico something to point at. They had already been conducting such experiments by bouncing lasers off of reflectors deliberately left for this purpose by Apollo astronauts but, surprisingly, they found that the lunik reflector actually worked better.

Their initial round of experiments were able to pin down the crafts position to an accuracy of 1 metre; follow up work narrowed that to 1 centimetre. Using a combination of the different reflectors on the surface (5 in total; 3 from Apollo missions and 2 from lunik) we are able to measure the distance of the moon from earth to within a few millimetres and that, science fans, is amazing.

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