“Bullet hole – a small stone from the universe went through our solar array,” tweeted space station Commander Chris Hadfield yesterday. “Glad it missed the hull.”
Hadfield photographed the small puncture in the array caused by either a tiny meteoroid or a piece of man-made space junk. There’s plenty of both to go around. Earth gets peppered by over 40 tons of asteroid dust and grit every day.
Man-made orbital debris from decades of rocket launches plus a considerable amount of additional space trash from two anti-satellite tests (in 1985 by the U.S. and 2007 by China) and a 2009 collision of an Iridium communications satellite and Russian military satellite orbit Earth across a wide spread of altitudes.
Some 20,000 pieces of debris larger than 2 inches (5 cm) are tracked by NORAD radar and at least 500,000 pieces 1/2-inch (1 cm) and larger occupy low Earth orbit between 99 and 1,200 miles high. It’s here where the space station circles the planet at over 17,000 mph 250 miles overhead.
For perspective, a marble-sized object moving with a relative speed of 10,500 mph (17,000 km) to a satellite or ISS would deliver as much energy as a small hand grenade.
The space station proper is protected by its hull from small hits by millimeter-sized objects. Presumably this “bullet” was small enough to not pose a danger. Had it struck Earth’s atmosphere, the bit of debris would have burned completely as a meteor. In orbit, the “skin” that protects astronauts is thin in comparison.