Answer: Satellite Antennas

If you gaze up at the night sky long enough, you’re bound to see an interesting thing or two. Shooting stars, occasional atmospheric disturbances, and every once in a great while, a brilliant flash that might catch an ordinary viewer off guard.

These bright flashes aren’t a natural phenomenon, however, but a side effect of the large network of 66 communication satellites that make up the Iridium satellite constellation (that provides global service to satellite-based phones, pagers, and integrated transceivers). The vast majority of satellites in the night sky are completely invisible to us thanks to their small size and lack of large reflective surfaces. The satellites in the Iridium constellation, however, feature large door-sized antennas covered in a polished and highly reflective material.

Most of the time, the alignment of the satellites is off just enough so that they don’t reflect sunlight towards Earth, but when they do, the reflection shines down like a giant flashlight. For a brief moment, they are the brightest thing in the night sky—each flash is visible over a 6.2 mile (10 kilometer) diameter area below. The flashes can be so bright they even, rarely, appear to viewers during the daytime who happen to be looking at the right place at the right time in the sky above.