Space is big, but it can still get crowded up there. The recent collision between two orbiting satellites, including one Iridium satellite, was just an accident waiting to happen, according to debris scientists at Johnson Space Center in Houston. The fact that there even exists such a job as a space debris scientist shows that the problems are very real. The dangers are not so much to us on Earth - falling debris largely breaks up by the intense heat on accelerating through the earth's atmosphere - but rather to other fully functioning satellites. How many satellites are there in space orbiting Earth? And more importantly, how many satellites does it take to cause a space crash?

The most up-to-date data comes from CelesTrak which is funded by the Center for Space Standards and Innovation, located in Colorado Springs. As of writing there are over 13,000 satellites in orbit and over 20,500 satellites have decayed since 1957. Looking carefully at the data it appears that there are just under 3,500 satellites that are both functioning and in their correct orbit compared to nearly 10,000 that are classed as debris but haven't yet decayed. So 75% of the satellites orbiting the Earth are junk!

Just to get a sense of what a collision would mean let us look at how fast these satellites are travelling. The Moon, which is at an average altitude of 385,000 km from Earth travels at a mean straight-line speed of 3,600 km/h. A geo-synchronous Earth orbit satellite such as GEO is at 35,800 km and travels as 11,000 km/h, whereas the International Space Station is only 380 km from Earth and whizzes around at an astonishing 27,600 km/h. At these speeds, even a small piece of debris can cause serious damage to satellite instruments and sensors and larger pieces can even shunt the satellite out of orbit.

The collision quoted above happened at an altitude of about 800 km, so the real concern is that as the debris decays and falls towards Earth it may hit either the ISS or the Hubble telescope. Telecommunications satellites are usually in geo-synchronous orbits so that they appear to an observer on Earth to be in a fixed position in the sky. As seen above, they are at high altitude and do not move relative to each other, so the likelihood of two communications satellites colliding is very low. But other satellites such as spy satellites are in lower faster orbits so they can cover the whole Earth taking snapshots as they pass overhead.

The Iridium satellite phone system has 66 (now 65) satellites in fast low-altitude orbits so that they can give good reception even in the least populated areas of the globe. Users of Iridium make telephone calls in direct communication with the satellites rather than through a local mobile phone mast. Their biggest client is the US military. Luckily, there is some in-built redundancy in the system to safeguard the network against just such an accident. So what are the world's space agencies going to do about all this space junk?

The Inter-Agency Space Debris Coordination Committee was founded to monitor space junk and advise member space agencies on actions to mitigate the problems this causes to future projects. The 27th meeting of the IADC is scheduled for late March and this recent collision will no doubt be high on their agenda. But, just as it is difficult to force polluters to clean up their terrestrial accidents, it is likely to be just as difficult to get a consensus on cleaning up space.