By Alistair Quarterman
When Sputnik 1 was launched in 1957, not only did it herald the beginning of the space age but it also marked the start of a cumulative process that has led to the build up of space debris, or junk.
Larger items are tracked and craft manoeuvred to avoid them
This mass of debris now consists of more than 500,000 objects that mostly circle the Earth at altitudes of up to 2,000km.
Just what is space debris?
It can best be classed as any manmade object that no longer has a practical use and includes empty upper-stages of launch vehicles, explosion fragments and dead satellites. Other items include cameras, gloves, wrenches and even a toothbrush lost by astronauts from past space missions.
Any object weighing more than a few grammes and travelling at seven kilometres per second can easily destroy a satellite and pose a serious threat to future space missions.
Just how serious is the problem?
At present, the collision risk is very small; but space agencies and commercial enterprises are concerned that if further debris is created then certain orbits may in the future become too hazardous to use.
Not only can space debris pose a threat to the global network of communication, navigation and Earth-observation satellites in Low Earth Orbit (LEO) but it could also hamper ongoing space projects such as the International Space Station (ISS) which is already protected by impact shield panels.
What is being done to reduce the risks?
Most effort is being aimed at keeping track of larger debris and the prevention of more debris. The US already manages a network of optical and radar sensors that can track and catalogue anything larger than 10cm in LEO. It currently tracks about 10,000 potentially lethal objects.
Such monitoring identifies situations where it might be necessary to perform avoidance manoeuvres. For example, during shuttle mission STS-48 in September 1991, the orbiter had to perform a seven-second burn to dodge debris from the Kosmos satellite 995.
What is the UK involvement?
The UK has been at the forefront of research into reducing the amount and damaging effects of space debris.
The British National Space Centre (BNSC) is an active member of the Inter Agency Space Debris Co-ordination Committee (IADC) which coordinates worldwide research activities and is drawing up a code of conduct for government and commercial groups to reduce further build up of space junk.
The BNSC has also committed funding to the development of a tracking system.
The junk will fall back to Earth - but it can take a long time
The Starbrook sensor, a space surveillance system developed by UK company Space Insight Limited, completed initial tests in 2006.
Using a combination of optical and other detector technologies, the sensor can monitor a sector of space regularly and keep an up-to-date picture of the number and location of typical payload and rocket body objects orbiting the Earth.
The Natural History Museum and the University of Kent have been carrying out impact damage experiments, whilst UK technology firm QinetiQ has participated in a European Space Agency (Esa) study on the response of spacecraft equipment to debris impacts which could help evolve spacecraft design in the future.
Can space be cleaned up?
All of this material will eventually fall back to Earth - but some of the orbit decay times are very long. Some items will stay up for many decades.
Ideas to reduce the amount of debris have included laser "brooms" to sweep debris back into rapid-decaying orbits. Thought has been given to pushing material together into orbital junk-yards which could be used as a resource for future missions whilst being kept out of the way.
But the priority for most international groups is to highlight preventative measures in reducing the future increase in debris and how to mitigate extant debris still in orbit.
A key proposal is the development of convenient ways in which to de-orbit satellites when they have come to the end of their operational lives.
These could either bring them into lower orbits where atmospheric drag will slow them and pull them into a fiery re-entry; or to place them in what are known as "grave yard" orbits where no satellites or spacecraft are operating.
With the growth in the commercial space industry, telecommunications, and with the real prospect of affordable space tourism on the horizon, the control and monitoring of debris will become an increasingly important factor in the planning of future space activity.