THE M25 OF OUTER SPACE

DEEP beneath the Cheyenne Mountain in the US Rockies, computer terminals flicker with a dazzling array of numbers. A large screen at the front of the command centre shows a map of the world - land in green, sea in blue - with little white dots marking objects of interest. It looks vaguely like a ship's bridge, partly because the complex was designed by a naval architect. But the vessels it tracks have never been near water.

The US Space Surveillance Centre is where the upper reaches of the sky and the depths of the Earth are joined. Round the clock, subterranean technicians keep track of every orbiting object bigger than a golf ball, making sure they don't crash into each other and warning us when they tumble towards the planet.

The idea that Earth's satellites might begin smashing into each other like cars on a fog-bound motorway might seem ludicrous. It is not. True, the less travelled routes are virtually empty, but the most popular ones are beginning to look like the M25 at rush-hour on the Friday before a bank holiday weekend.

"There have been no major collisions, but it's getting increasingly crowded up there," says Andrew Wilson, editor of the prestigious Jane's Space Directory, an annual report on what is going on over our heads. Countries in the Far East are starting to squabble over orbital slots, he says, and the fighting is likely to get worse.

When Sputnik ushered in the Space Age in 1957, its only companion was the Moon. Today, the Space Surveillance Centre tracks 8,847 objects. The "box score" (a term borrowed from baseball) shows that 3,500 belong to America, 3,800 to the Russians and the rest to the European Union and other countries. Tens of thousands of smaller objects - nuts and bolts from space craft, flecks of paint - are thought to be whizzing around Earth. One estimate puts the number of objects under 1cm at 35 million.

The objects circling the Earth fall into two broad categories. First there are active satellites busilly transmitting data to ground stations. Second, there is junk and debris that has been intentionally discarded like a litterbug's empty pop can, or has accidentally fallen off a space craft like a loose exhaust pipe.

Though space is vast, the places of greatest use to humans are limited. The most valuable is geostationary orbit, where satellites circle the planet in exactly 24 hours, thus seeming to remain stationary above the same point on the ground. Its use was first suggested in the 1940s by Dr Arthur C Clarke - best known now as the author of 2001: a Space Odyssey - but that orbit has room for only 180 satellites, given present technology. Some of these - over the Pacific Ocean, for example - are of little value. But those above Western Europe, New England and California are worth a fortune.

The first three rules of real estate apply as much in space as they do on Earth - location, location and location. One orbital slot auctioned by the US government fetched $682m (pounds 455m) from MCI, about four times as much as the telecommunications satellite that will be put there. Another slot with a poorer view of America fetched a fraction of that amount.

The major concern about geostationary satellites is not that they require a lot of elbow room to avoid bumping into each other. They regularly report their position to their controllers on the ground, and if they did get too close together, small booster rockets could be activated to shunt them on to different courses. Their number is limited instead by radio technology. The advantage of geostationary orbit, sometimes called the Clarke Ring, is that satellites placed in it can receive signals from the same base stations and rebroadcast to the same receivers all the time. If they were closer together than two degrees of arc, their signals would interfere with one another.

The second category, space junk, poses more of a threat of collision. Such items of debris are not transmitting, so have to be tracked by radar and telescope. If they get too close to something else, they can't be manoeuvred by remote control. "There are quite a lot of things in low Earth orbit," says Alan Johnstone, professor of space science at the Mullard Space Science Laboratory near Dorking. "They include dead satellites, parts of rockets and debris from explosions." There is even a spanner dropped by an astronaut during a space walk. No one is quite sure how many crashes there have been so far, but "some examples where space craft have failed could have been due to an impact." In one case a fleck of paint cracked, but did not hole, a window on an American space shuttle.

The more objects that go up, the more will eventually come down. The most recent case was China's FSW1, a one-ton spy satellite that crashed into the sea last month. It was not the first. In 1978 a nuclear-powered Russian satellite, Cosmos 954, blazed a radioactive trail across Canada, while America's 20-ton Skylab tore a swathe across Australia's Outback in 1979. Chunks of Salyut 7, a Russian space station, have crashed into South America.

Calculating which satellites will hit Earth is difficult; it depends on their mass, shape, composition and speed. It also depends on the 11- year sun spot cycle. When the sun is at its most active, the Earth's atmosphere heats and expands, dragging more satellites down but also burning more of them up. The chances of anyone being hit by a falling satellite are remote, as people occupy only a tiny fraction of the planet's surface. The only casualty confirmed to date was a Cuban cow hit by a falling rocket motor in the 1960s. But if one did hit a city it would be a disaster. FSW1, a primitive craft believed to have a heat shield made of oak planks, could have dug a crater 20ft deep and 30ft in diameter if it had hit land in one piece.

THE 70 men who act as traffic cops for the world's satellites are in no danger of getting clobbered. To reach their posts they have to take a bus half a kilometre into the granite heart of Cheyenne Mountain, through three sets of 25-ton steel blast doors (designed in the 1950s to withstand a direct hit from a hydrogen bomb, though it is no longer considered "nuclear survivable"). Inside the 4.5-acre, man-made cavern is a cluster of windowless, three-storey steel buildings lit only by fluorescent lights. "It's fairly sterile," says Lt Col Jim House, one of two officers in charge of the surveillance centre. "Lots of exposed metal."

Lt Col House is an infantry officer who had no experience of space until he took a four- month course to prepare for his current command. He is over 6ft tall, a sharp dresser with a professional military bearing. His grey uniform carries a crest showing a silhouette of the Mountain. He is responsible for the 90 per cent of orbiting objects that are dead - those that are no longer transmitting data to Earth.

By contrast, his partner - the bespectacled Lt Col Michael Muolo - is a career missile man with almost 19 years experience. His crew is responsible for the remaining 10 per cent of Earth's orbiting retinue - the telecommunications, scientific and spy satellites - that is still active. His uniform, in Air Force blue, bears a crest showing the Earth with a flying tiger, a tribute to a Second World War plane.

The Space Surveillance Centre was set up in 1971 to ensure that the Pentagon could tell the difference between a nuclear missile attack from the Soviet Union and a satellite that has fallen innocently out of its orbit. With the collapse of Russian communism, the threat of intercontinental ballistic missiles has been replaced by that of atomic terrorists. But the centre has evolved a new role - advising the world's space powers where not to point their rockets. "We're the book keepers," says Muolo. "Every week we update the space catalogue, a chronological list of everything that goes into space and everything that comes down."

The data that goes into the catalogue comes from a network of 23 optical and radio observatories around the globe, including a radar base at RAF Fylingdales near Whitby, North Yorkshire. More than 60,000 checks are made each day, and the longest an object goes without being looked at is a week. If the satellite is active or within 25km of a route to be used by the space shuttle, it is checked more often. Nasa and a consortium of other countries, including Britain, are currently planning an international space station which - if congressional budget cuts and inter-governmental squabbling ever allow it to get off the ground - will require even more vigilance. "We'll run more than 8,000 object orbits through the computers to see if any of them come close," says Lt Col House.

While the chances of collision might be remote, the consequences could be dramatic. A head-on collision between two objects, no matter how small, each travelling at 15km per second, releases a huge amount of kinetic energy. Even particles so small they would be considered dust can be a hazard (see panel opposite).

While junk of all sizes can be almost anywhere in space, working satellites tend to be found in specific orbital shells. The strict hierarchy is a bit like the concentric spheres the Greek astronomer Ptolemy proposed, incorrectly, 19 centuries ago to explain the apparent movement of the Sun and stars and planets. Different types of satellite are usually found at altitudes best suited for their function, explains Janes' Mr Wilson. The factors that enter into the equation are speed - the higher they are, the slower - and their need to be close to the surface. Some 2,440 satellites flying above 150km are said to be in deep space or high Earth orbit; the rest, 6,400 of them, are in near or low Earth orbit.

At the top of the pecking order are a handful of scientific probes that scan the universe without the interference of Earth's atmosphere. However, the most famous of this type, the Hubble Space Telescope, is at a much lower altitude so that it can easily be serviced by the space shuttles, which have a ceiling of 620km.

Below them are the telecommunications satellites in geostationary orbit, 37,100km high. Almost 90 of them are in orbit at any given time, though the practical limit at the moment is closer to 180, up from 120 a few years ago. It is by far the most crowded of Earth's orbits. Mostly they are used as telephone and television relays. Like all satellites, they have a limited life span, usually less than a decade. When they run out of power, a small rocket booster gives them a final nudge into a higher orbit to clear the way for their replacements.

Not far below geostationary, at 20,000km, are the GPS satellites, a network that gives off coded signals that can be read by Earth-based stations to give exact locations. Developed as an aid to the US military, they are now widely used by ships; some companies are now trying to get them installed in cars, in combination with computerised maps and route planners.

Considerably lower are the much faster orbits used for Earth Sciences. At heights of around 1,300km, these satellites can take clear pictures of the terrain. Many of them are in polar orbits, calculated to allow them to take images of the same patch of the surface at the same time on each pass, so that changing shadows do not complicate comparisons.

At the lowest level are the spy satellites, typically between 200km and 500km above Earth. Some wander even lower to take close-ups of areas of particular interest to their military controllers. They are said to have a resolution fine enough to read the number-plates on cars, though governments will not confirm this. Another advantage, used particularly by the Russians, is that it is relatively easy for satellites at these altitudes to drop canisters containing film, rather than having to risk their encrypted transmissions being intercepted by allied monitoring stations such as Britain's Government Communications Headquarters (GCHQ).

Crowding on the orbital motorway is about to get worse with the introduction of complicated networks of satellites designed to work with mobile phones. The communications satellites currently orbiting in the Clarke Ring have two main disadvantages: first, there is a noticeable delay in the time it takes messages to travel the 74,200km round trip, resulting in people talking over each other as if their voices were echoing in a cave: second, and more significant, the satellites are too far away to be reached directly from mobiles. As yet, cellular calls have to be routed through a ground- based station first.

Several companies now plan to launch low- flying satellites that will weave a web around the Earth so that every mobile phone will be within range of one or another at any given time. As one satellite passes over the horizon, another will rise, and automatically take over the handling of the call. Motorola's planned Iridium network will have 66 working satellites, travelling in orbits that pass over the north and south poles at an altitude of 780km. The first in the estimated pounds 3bn project will be launched at the end of this year. Another proposed system, Odyssey, claims to have patented the orbits it wants to use, says Mr Wilson.

Space could become even more like a motorway, or at least an American interstate highway, if contentious proposals to put up orbiting advertising billboards ever go ahead. It is already technically feasible to put up a satellite that would unfurl a banner visible from the ground. To do so would be expensive, and might persuade more people to boycott the advertised product than buy it. "It would be an economic problem more than a technical problem," says University of Kent physicist Dr John Zarnecki, who works on European Space Agency missions. A plan to celebrate the centenary of France's Eiffel Tower in 1989 with an orbiting display was dropped following widespread protests. "If I weren't an astronomer I'd have thought it would be quite fun," says Zarnecki.

Astronomers are fiercely opposed to the idea of hoardings in space because they would make observations more difficult. Satellites already reflect light that obscures stars, and give off electromagnetic radiation that can mask the dimmer emissions of distant galaxies. At the moment the problem is slight. "You'd have to be really unfortunate to have a satellite pass over the exact piece of sky you were interested in," notes Dr Zarnecki. But orbiting ads would make the problem far worse, hiding not just pinpricks but whole arcs of the sky.

Concern about the proliferation of objects in orbit has already led to positive, pollution-reducing steps. Nasa and other space agencies routinely drain the last dregs of fuel from spent booster rockets discarded in orbit after it was discovered that the bulkheads between tanks in the second stages of Delta rockets can corrode, leading to an explosion. A fine mist of hydrocarbon fuel molecules is far less dangerous than the mangled shrapnel from an explosion. "You'd much rather have one big lump than 10,000 small ones," says Dr Zarnecki.

Better technology is also helping to ease the looming traffic jam. The spacing between geostationary communications satellites is expected to get smaller as their radio and television transmissions become more focused. Satellites are also being built with more relays, so they are capable of re-broadcasting a wider range of frequencies.

The European Space Agency's Ariane rockets, which used to carry two satellites on each launch, can only handle one of the larger variety. The combination of more orbital slots, each with a satellite that does more work, should allow capacity to keep growing for at least another decade. Larger, longer- lasting satellites have a secondary effect; they require fewer launches that might end in disaster and debris.

But the biggest change in the last decade has undoubtedly been the collapse of the Soviet Union. One of the main consequences of the fall of communism has been the sharp reining in of Russian ambitions in space. ThoughRussia is still the major player, the number of launches it has made since 1989 has declined steadily, making up for the increased use by late entrants into the space race. There were a total of 116 launches into orbit in 1990, says Mr Wilson, but just 75 last year.

In the future, better radar and telescopes being developed for military and scientific purposes could enable the Space Surveillance Centre to keep track of ever smaller objects. But experts agree that the best long- term solution to the problem of orbital pollution, as with its Earthly cousin, is prevention. Space holds enough risks without more being inadvertently added by the careless parking of wrecks.