On Sunday, a group of amateur astronomers in San Antonio fired a one-watt blue laser up at the ISS and scored a direct hit, a first in ISS history. And instead of getting hunted down and arrested (which is what usually happens when you shoot lasers at flying things owned by the Feds), they got a congratulatory email from an astronaut.
Don Pettit, one of the lucky few up on the ISS, has been working with the San Antonio Astronomical Association for weeks to try and help them flash the ISS with a laser from the ground. Successfully doing this involves having a bright enough laser that's properly focused, knowing exactly where to point it and when, tracking the station as it moves, and most importantly, having an astronaut standing by on the station looking out a window with a camera.
Lots of people have tried to flash the ISS before this, and none of them have been successful. Pettit explained why it's so hard to do in a blog post last month:
Ironically, when earthlings can see us, we cannot see them. The glare from the full sun effectively turns our windows into mirrors that return our own ghostly reflection. This often plays out when friends want to flash space station from the ground as it travels overhead. They shine green lasers, xenon strobes, and halogen spotlights at us as we sprint across the sky. These well-wishers don't know that we cannot see a thing during this time. The best time to try this is during a dark pass when orbital calculations show that we are passing overhead. This becomes complicated when highly collimated light from lasers are used, since the beam diameter at our orbital distance is about one kilometer, and this spot has to be tracking us while in the dark. And of course we have to be looking. As often happens, technical details complicate what seems like a simple observation. So far, all attempts at flashing the space station have failed.
Getting Sunday's attempt to work required a lot of coordination, and since the ISS only sends and receives email two or three times a day (!), that meant weeks of back-and-forth. But as you can see from the above pic, they managed to do it, successfully flashing the space station with a one-watt blue laser and a white spotlight as it passed overhead.
We should probably mention that unless you're emailing directly with an astronaut, firing powerful lasers into the sky at random is not a very good idea at all, and you run the risk of causing an international incident. So just stick to popping balloons and stuff, m'kay?