Space isn’t quite as empty as you might think. This week, a commercial satellite and an old Russian satellite collided some 800 km above Siberia. The crash, which officials said was unprecedented, created a debris field of at least 600 pieces. That adds to already massive collection of man-made bits and pieces orbiting the Earth that other spacecraft, like the International Space Station need to be very careful to avoid. U.S. officials say they track nearly 18,000 objects that are 3.9 inches across, or larger. With that much junk up there, it’s amazing this doesn’t happen more often.