What is space junk and where does it come from?
There are millions of pieces of rubbish orbiting Earth, from defunct satellites, launch vehicles and rocket stages to nuts and bolts and even flecks of paint. In 2007 the crew of the International Space Station (ISS) jettisoned a fridge-size tank containing ammonia that was part of the cooling system.
How much is out there?
There are roughly 18,000 pieces of man made debris at least 10cm (4in) in size orbiting Earth. Nasa believes there are more than 200,000 objects between 1cm and 10cm and tens of millions smaller than that. The debris orbits the Earth at up to 25,000mph. Orbital debris is considered the biggest threat to a space shuttle.
How common are collisions?
There have been four other collisions, but they involved parts of spent rockets or small satellites and were considered minor. The collision between an Iridium commercial satellite and a Russian satellite launched in 1993 and believed to be non-functioning was the first high-speed impact between two intact spacecraft. Nasa expects more such events.
What has been the effect of the collision?
The US Space Surveillance Network has detected the two massive debris clouds after the satellites collided nearly 500 miles over Siberia on Tuesday. Early radar measurements have detected hundreds of pieces of debris.