[MRtecno98]

Astronomers today measure the distance between Earth and the Moon by shining a laser beam towards it and measuring the time it takes to come back.

Now, guess why the beam actually comes back instead of getting absorbed by the lunar surface? Because Apollo 11 left a mirror there half a century ago, it still works

There are people alive, today, that can prove to you that we went to the moon just by shooting a laser beam in the sky, so yes, we did go to the moon.

[seabass-labrax]

I used to think that if it were only slightly off, the return beam might end up intersecting the Earth at somewhere inconvenient, or even missing it entirely. Even more impressively, it took only five minutes to deploy, which is faster than most bathroom mirrors are installed :)

The reason why this isn't a problem is that the device wasn't 'just' a mirror, but rather a retroreflector. This reflects any light back at its source, regardless of which direction the light came from.

If you were really lost in deep space, perhaps you could flash a very bright light (not a laser) momentarily, then look for the return flash from the retroreflector moments later - or at least, hopefully that soon, otherwise you are very far away indeed! A few strategically-placed retroreflectors around the solar system could make an effective triangulation-based location tracker. I wonder if this already exists in some form.