Space station built to weather battering from space rocks

International Space Station Commander Chris Hadfield shared a photo this week of a small hole in one of the solar arrays providing power to the station. The hole was created by what NASA calls Micrometeoroids and Orbital Debris (MMOD). The more lyrical Hadfield called it "a small stone from the universe."

Objects hurtling through space and the threat they pose are well known to the ISS crew. When asked about the scariest thing he'd ever seen in space, Hadfield recalled "watching a large meteorite burn up between me and Australia."

This week's hole in the array is far from the first created by debris. 

Most are small and not noticeable, but this larger one caught the crew's eye. MMOD is a fact of life when orbiting 250 miles up, but the station is built to handle these small objects flying into it.

Hadfield added when tweeting the photo, "Glad it missed the hull."

Unlike the fragile solar arrays, inhabited sections of the ISS are protected by layers of material designed to slow down small debris before it can breach the hull. The crew often hears a "ping" as tiny objects impact. Astronauts returning of the ISS also talk of creaking sounds as the hull expands and contracts as it passes in and out of sunlight 16 times each day.

Last summer a micro-meteorite struck one of the large windows in the cupola section of the station. The strike left a chip but did not penetrate the 4 layers of Silica and borosilicate glass. Here on Earth this sturdy glass is similar to Pyrex you may have in your kitchen now. Protective kevlar shutters were raised while engineers on the ground reviewed photos of the damage. Should replacement ever be required, spacewalking astronauts would fit a pressure cover over the viewport from outside, then replace the window from the inside.

Larger debris presents a different threat to the ISS. Tens of thousands of larger objects, down to a diameter of about 2 inches, are tracked by U.S. military ground stations. The ISS can adjust its orbit height to avoid any of these larger objects when necessary.