Sat collision highlights growing threat

By Paul Rincon Science reporter, BBC News

There are some 17,000 man-made objects orbiting our planet

The collision between a US and Russian satellite in space highlights the growing importance of monitoring objects in orbit.

It also shows that there are still major capability gaps in current systems set up for this task.

There are about 17,000 man-made objects orbiting Earth, and the tally is constantly increasing. More debris also increases the risk of collisions between objects.

Intact satellites share Earth's orbit with everything from spent rocket stages and spacecraft wreckage to paint flakes and dust.

The diffuse mist of junk around our planet is the legacy of 51 years of human activity in space.

At orbital velocities, even small pieces of space debris can cause serious damage to spacecraft.

And both the Americans and Russians have networks of tracking stations to monitor objects.

The US military operates 25 centres around the world involved in space surveillance, including one at RAF Fylingdales in the UK.

Debris field

Radars are generally used to track objects in low-Earth orbit, while optical telescopes are used to observe objects further away from the Earth.

Radio frequency technology - a form of electronic surveillance - can be used to assess whether or not satellites are active.

China and the European Space Agency (Esa) are developing their own systems.

An advanced ability to monitor space debris, satellites, near-Earth objects and solar activity is known as Space Situational Awareness (SSA). Many experts see this as a step to space traffic control.

Although there have been some near-misses and a few minor collisions, this is the first time two intact satellites have crashed into each other.

The latest incident has produced the worst field of space debris since China destroyed a defunct Fengyun satellite with a missile in January 2007.

That incident, designed to test an anti-satellite weapon system, produced more than 2,000 separate fragments of debris.

On Christmas Eve, the US Air Force notified Esa that a European weather satellite called Metop might be threatened by a piece of debris from the Chinese A-Sat test.

Ultimately, no action was taken. But the event highlighted Europe's near-total reliance on the US military for knowing what is going on in space.

The International Space Station (ISS) flies at an altitude of about 350km (220 miles), well below the point, some 790 km (490 miles) up, where the Russian and US satellites collided.

Nasa has said it considers the threat to the ISS to be low, but that the orbiting outpost could carry out a collision avoidance manoeuvre if necessary.

Paul.Rincon-INTERNET@bbc.co.uk