Three Apollo missions left retroreflectors on the moon. How was it decided which missions would leave them?
Although you only need a single retroreflector to measure the distance from the Earth to the moon.
But it turns out that having measurements from three widely-spaced retroreflectors allows researchers to accurately measure what is known as lunar librations — that is the way in which the moon’s movement changes in relation to the Earth.
To make these measurements accurately, you want reflectors that are widely spaced in both longitude and latitude.
If you look at the Apollo landing sites, Apollo 11, 14, and 15 provide a nice spread in both longitude and latitude:
Once you have widely-spaced retroreflectors, there’s not much point in adding more.
The retroreflectors essentially last forever — they’re passive devices that don’t require any sort of power supply or chemicals in order to operate.
So unless one of them gets hit by a meteorite, they should be usable for a long, long time.
And there was a good reason not to take more than they needed. In space, Weight Is Everything.
There was an endless list of experiments that the Apollo mission planners would have liked to take to the moon, but they were severely limited in the weight they could take.
Every experiment they decided to take was at the expense of others that they couldn’t.
Once they had three retroreflectors on the moon, no one wanted to waste a mission’s precious weight budget on a redundant piece of equipment.
Incidentally, the Apollo 11 and 14 retroreflectors were the same size (2,116 square centimeters), but the Apollo 15 retroreflector was much bigger (6,825 square centimeters).
All three retroreflectors are still operational, but the vast majority of distance measurements (for which you only need one retroreflector) are made using the Apollo 15 retroreflector since the measurements can be made with a smaller laser.
If you are interested in similar topics like this one, you can read my article about “Apollo 11 Science Experiments”.