The great trash can in the sky

With the profusion of satellites and rocket debris in space, we could be heading for the next big bang, writes Charles Arthur
Later this month, a rocket will be launched from the Vandenburg US Air Force base in California. If you listen hard enough, you might be able to hear a chorus of groans rising with it, coming from radioastronomers around the world. Astronauts may also have cause to grumble, because the space above Earth is becoming increasingly crowded and less useful for pure science.

In fact, within a couple of decades it might be so crowded - and deadly - that we will have cut ourselves off from space entirely.

The Vandenburg rocket's payload will be the first of 66 satellites forming the Iridium global mobile phone system, due to start operations in 1998. Each satellite will be in "low-earth orbit" (LEO), 780 kilometres above the ground - unlike normal communications satellites located in geostationary orbit (so they appear to remain fixed in the sky) more than 40,000 kilometres up. Using a hand-held mobile phone, Iridium users will be able to send and receive phone calls, faxes and e-mail anywhere in the world where they can see the sky.

While that sounds like a terrific idea to the Iridium corporation, which has raised more than $2bn in finance in the past five years, to people like Dr Jim Cohen at the famous Jodrell Bank radiotelescope it sounds instead like very bad news.

"Our sensitivity to interstellar signals will be governed by the satellite signal," says Dr Cohen. "We have been trying to negotiate with them since 1991, yet six years later we've found that there's nothing they're going to change."

The problem arises because the frequency range of 1610 to 1626.5MHz allocated by the International Telecommunications Union (ITU), the business's global governing body, cuts exactly into one that radioastronomers like Dr Cohen use as an astronomical ruler: 1612MHz, the natural frequency of the hydroxyl radical (OH).

Like any molecule, the hydroxyl radical is constantly absorbing and re- radiating energy, and like any molecule, most of the re-radiated energy is emitted at a particular wavelength. For hydroxyl, that is as a radio signal at 1612MHz. Astronomers use that fact to study all sorts of phenomena, including red giant stars, comets and interstellar gas clouds which are forming stars. The hydroxyl signal is one of the best "rulers" for measuring distance available: according to Dr Cohen, whole classes of interstellar objects can only be measured by observing their emissions at that frequency.

Now, Iridium is going to come along and could, potentially, obliterate that. "It will be a problem for half the day," says Dr Cohen, before adding philosophically: "Maybe in the small hours we'll be able to do some useful work."

Could these problems have been avoided? Certainly, and it's not for lack of trying on the part of Jodrell Bank and other radioastronomy organisations, principally in the US (which expects to be affected first by Iridium). They have lobbied and negotiated consistently with Motorola, the US company that is the principal behind Iridium. A last-ditch meeting last November failed to produce any result. "Basically, it seems to be an unresolved difficulty," says Dr Cohen.

Motorola has insisted that radioastronomers "won't be able to hear us". Dr Cohen is less convinced.

Oddly, he found that the Soviet military - whose satellite navigation system, Glonass, also interfered with the hydroxyl frequency - were far more prepared to accommodate them. But Motorola's unyielding stance could have repercussions: "There may be a point where the Russians say, 'Why should we go any further in helping them?' And our concern is that though other people have been appraised of our needs, they may decide to do the same as Motorola." In the worst case, an Iridium user might be based almost next to a radiotelescope - meaning the signal from the satellite would be beamed directly down on to it, completely overwhelming the hydroxyl signal. There is no deal yet agreed with Arecibo Observatory in Puerto Rico, where Iridium users - such as travelling executives - might be more plentiful than in Jodrell Bank's home in Staffordshire.

But it's not only on the ground that the proliferation of low-orbit communications satellites could pose a problem. They pose a very real danger to astronauts and their spacecraft.

Iridium is only the first of the satellite phone systems in progress; by 2001, there will be more than 1,000 satellites in orbits less than 11,000 kilometres up, operated by six different groups. Although they will be distributed throughout a huge volume of space, all it takes is one chance impact by a meteor to turn a useful satellite into a fast-moving cloud of lethal metal.

The chances of that happening are growing. A recent investigation by the US National Research Council estimated that there are now more than 10,000 fist-sized and tens of millions of smaller objects orbiting Earth - debris from the launches of almost 5,000 spacecraft since the first, Sputnik, in 1957.

The problem with those tiny parts is that their relative orbital speed is enormous compared with something emerging from the Earth's atmosphere. Though they are small, their tremendous speed gives them terrific energy - enough to pack a deadly punch. The Space Shuttle's windows have been pitted by flecks of paint from past collisions. Last August, a French military satellite collided with a portion of a 10-year-old Ariane rocket, smashing the satellite's stabiliser. (So far, spacewalking astronauts have remained safe; but fear of impact is nevertheless a strong reason for keeping "extravehicular activities" as brief as possible.)

The worst part of this process is that it could lead to a "cascade", in which the newly formed particles (which will travel at a different speed from their parent) go on to hit another large object, breaking that apart, and so on.

The US space agency Nasa has suggested that wouldn't happen for another 50 years. But Richard Crowther, a satellite expert at the Defence Research Agency in Farnborough, Hampshire, has developed computer models which he says suggest that such a cascade "could perhaps occur in the next 10 to 20 years". The crowding implicit in the proposals of Iridium and its rivals could accelerate that. "They are all at the same altitude and they all pass through the same regions of space." That's unlike geostationary satellites, whose greater distance allows greater latitude for "crowding". But no hand-held phone would be powerful enough to send to a geostationary satellite.

Hence the low-orbiting satellites from the new phone consortia. It remains to be seen whether their investment plans have allowed for the possibility of space junk - and whether they will recoup their huge investments before the whole project is turned into a cloud of silent metal girdling the Earth.