Flying debris: Even ISS astronauts can produce space junk. Here is a toolbag that has been floating through space since being “dropped” by an astronaut in November 2008.

Some pieces of space debris are so large that they do not completely burn up upon reentry into the atmosphere. An example is the 250 kilogram (550 pound) main propellent tank of a rocket. The stainless steel structure survived reentry almost intact in 1997.

A hole made by a piece of orbiting debris in the panel of a NASA “SolarMax” satellite.

Potentially damaged ESA satellite “Envisat.” This earth-observation satellite may have been damaged by a piece of debris from the satellite collision.

The Hypervelocity Ballistic Range at NASA’s Ames Research Center in California. Here, a photograph shows an “energy flash” caused when a projectile hits a solid surface at up to 17,000 mph (27,500 kph). Such tests are used to simulate what happens when a piece of space debris hits a spacecraft.

An artist’s rendition of a US “Iridium” commercial satellite. On Feb. 10, 2009, a satellite of this make crashed into a derelict Russian military satellite, spraying clouds of debris into space.

A graphic from the European Space Agency shows how space debris is spread around the planet and at different heights. Large amounts of debris can especially be found at 800, 1,000 and 1,500 kilometers above sea level. There are an estimated 17,000 objects being tracked in Earth’s orbit.