Reflectors, actually. While normally an irrelevant detail, the distinction is quite important here: lasers reflected on mirrors reflect at a complementary angle, lasers reflected on a reflector reflect right back at the source, allowing us to actually detect it.
The problem is that one has to have research-grade equipment in order to make use of them. Whereas first-hand accounts eyewitnesses who saw the Saturn V rockets go up are available to those who want to do the legwork.
