By Paul Rincon
Science reporter, BBC News
The Strela 2M Russian satellite looked broadly similar to this one
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 significant capability gaps in current systems set up for this task.
There are about 17,000 man-made objects above 10cm in size that orbit Earth - and the tally is constantly increasing. This in turn raises the risk of collisions between objects.
Richard Crowther, an expert on space debris and near-Earth objects, told BBC News: "It is unfortunate but inevitable, first that we would see such a collision in Low Earth orbit, and secondly - given the number of Iridium satellites in its constellation - more likely that the Iridium system would be affected rather than single satellite systems."
The commercial Iridium satellites comprised a network of 66 spacecraft up until the accident. According to Dr Crowther, from the UK's Science and Technology Facilities Council (STFC), the group also occupies "a very crowded altitude of low- Earth orbit".
Nevertheless, some say the incident was extremely unusual. 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.
"Even with a constellation of 66 satellites, if you do the geometry there is still a huge amount of space," said Patrick Wood, head of satellites at EADS-Astrium, Europe's biggest space company.
The craft were travelling at thousands of kilometres per hour when they crashed into one another.
"They were travelling so fast, the timing of any collision would be extremely difficult to predict. But also, the sheer probability of two spacecraft hitting each other is incredibly low," Mr Wood explained.
But Richard Crowther says the debris could now threaten other Iridium satellites: "Unique to the Iridium system is that all the remaining 65 satellites in the constellation pass through the same region of space - at the poles.
"So the debris cloud that is forming as a result of the Iridium satellite breakup will present a debris torus of high (spatial) density at 90 degrees to the equator that all the surviving Iridium satellites will need to pass through."
Intact satellites share Earth's orbit with everything from spent rocket stages and spacecraft wreckage to paint flakes and dust.
There are some 17,000 man-made objects orbiting our planet
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.
Russia's space surveillance facilities include an "Okno" optical tracking system near Nurek, Tajikistan, and a "Krona" long-range radar and optical tracking centre at Storozhevaya in south-west Russia.
China and the European Space Agency (Esa) are developing their own systems.
Experts would almost certainly have been tracking the orbits of these two spacecraft. But in addition to the probability of a collision being vanishingly small, information on the positions of objects in space is only approximate, not exact.
Radars are generally used to track objects in low-Earth orbit, while optical telescopes are often 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. This discipline is known as signals intelligence, or SIGINT for short.
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.
But co-operation on such matters between countries are complicated by defence sensitivities. Sharing data can betray the capabilities of a country's sensors.
The latest incident has produced the worst field of space debris since China destroyed a defunct Fengyun 1-C 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.
Nasa has said it considers the threat to the International Space Station (ISS) to be low, but that the orbiting outpost could carry out a collision avoidance manoeuvre if necessary.
The 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.
But experts say the Hubble Space Telescope and Earth observation satellites at higher orbits and therefore closer to the collision point, could be at greater risk of damage.
"The problem we've got is that there is a debris cloud which is in two slightly different orbits because of the spacecraft. The challenge now is to accurately track that debris cloud," said Patrick Wood.