Science: Lock up your satellites

The Leonids are coming. Or, more precisely, we are coming to the Leonids - a meteor shower caused by a comet's trail of dust left in the Earth's orbit. On 17 November, our planet will pass through a sparse trail of dust containing mote-sized particles, most smaller than those in a puff of cigarette smoke.

That might sound harmless enough. But all over the world, and above it in the orbiting Mir space station, scientists and engineers are worrying that those harmless-sounding objects could do real damage to communications satellites and anything else in their path. There is a precedent: in 1993 the European Space Agency's (ESA) Olympus satellite was hit four times in two minutes during a different meteor shower, the Perseids, and stopped working.

The Russian Space Agency already has plans to order the two cosmonauts now on board Mir, Gennady Padalka and Sergei Avdeyev, to take refuge in the Soyuz "escape" craft on the morning of the 17th, after they realign the ageing space station to reduce the area of its solar panels in the path of the oncoming storm. "Of course, we are anxious," Victor Blagov, the deputy chief of Mir flight control, told Space News this week. If there is a suggestion that a Leonid meteor might puncture Mir's hull, "they will have to move to Soyuz, even if our calculations show there is one chance in one million."

Similarly, operators of commercial and scientific satellites in high orbits, such as Luxembourg's SES (which runs the Astra satellite used to beam Rupert Murdoch's Sky TV to Europe) and Intelsat, which has a global satellite network for telecommunications, are making plans in case one of their "birds" is damaged. SES is not expecting it, according to Yves Feltes, a spokesman for the organisation, but the possibility is not being ruled out completely. Just to be safe, the Hubble Space Telescope is being turned to put its back to the direction of the potential bombardment. America's National Aeronautics and Space Administration (Nasa) has banned Shuttle spacewalks for the duration of the shower.

Why such a fuss about the Leonids? They are the leftovers from the trail of Comet Temple-Tuttle, which takes an elliptical orbit around the Sun, making its closest approach every 33 years. As it gets nearer, the heat begins to melt the comet's heart of ice and dust, releasing tiny particles. These are left in the path of the Earth's orbit, where we swing into them every year. Thus, the Leonid meteor shower is an annual event. Amateur astronomers mark it in their calendars as a time when the night skies can come alight with tiny streaks as the dust particles burn up in the atmosphere. This year, the Leonid shower is expected to begin on 13 November, peaking on the 17th and dying away on the 20th.

Every 33 years that show leaps in intensity, caused by the passage of the parent comet - which went past the Sun in February. (There is no risk of our colliding with the parent comet because of this difference in timing.) The number of particles leaps a hundred-fold: it truly is a "meteor storm".

The event of 1833 produced a wood-cut engraving that has been described as "the beginning of modern meteor astronomy". An eyewitness, Joseph Harvey Waggoner, a minister of the Seventh Day Adventist church, wrote of the display: "It appeared so grand and magnificent as to be truly exhilarating. It was a sight never to be forgotten. It stands as vividly printed on my memory today as it did a month after it occurred... it is not possible to give in a picture a representation of the stars falling at all points of the compass at once. But they fell in myriads to the north, east, south and west. Any representation on paper must at best be a very limited idea of the reality."

The last time that the Leonids peaked was 32 years ago, in 1966: that is because the year after Comet Temple-Tuttle passes the Sun, our orbit bisects the trail of remaining dust, rather than moving straight into it (as happens in years when the comet has just passed). But it is in the year that the comet has just passed that the number of particles it leaves behind is at a maximum.

In 1966, the Leonids were still just a free, natural light show; there were hardly any satellites in orbit (the first, the Soviet Union's Sputnik, was only launched nine years before). Now, though, there are thousands - for telephone and data links, environmental monitoring, global mapping and location and, of course, military use. Simply, there are more satellites to be hit - which makes it likelier that some, at least, will be.

What gives the Leonids, and all other meteors, their potentially devastating effect is not their mass, but their velocity. The Earth is moving towards the dust stream at a relative speed of 71 kilometres per second, or about 160,000mph. At that speed, a particle with a mass of 0.1g carries the same energy as a one-ton car moving at 160mph. If it hits a solar panel or vital electrical element, or the hull of a spacecraft, the effects can be devastating - which explains the wariness of the crew and operators of Mir.

One organisation, the quasi-governmental organisation CRESTech in Canada, has set up radar monitoring sites in Australia and Mongolia to monitor the storm, and for three hours at its peak will be offering updates every 15 minutes, with hourly updates at times around that. "The Leonid showers are not very long, but they're intense," Richard Worsfold, CRESTech's director of business development for space systems, explains. "There could be up to 10,000 meteoroid particles per hour, and while they're not big, they moving so fast they could blast a hole in something."

CRESTech is offering a service to commercial operators, in which for $50,000 (pounds 29,400) it will provide those updates in real time. It has already signed deals with the ESA, the Canadian Space Agency, Hughes Aerospace (a big American maker and operator of satellites), Finland's Department of Industry and the US Air Force. "The USAF wants to know because it has hundreds of satellites up there," Mr Worsfold says.

The most vulnerable part of a satellite tends not to be its hull, but the solar panels that gather energy to keep it running. A fast-moving particle can punch a hole in them. Though there is a transparent anti- meteor layer three millimetres thick on Mir's panels, one has a hole 10 centimetres (4 inches) across which dates back to 1991, according to Mr Blagov; though exactly which event caused it is not known. The worry is that although most of Mir's panels will be unaffected, enough damage could be caused to reduce the station's energy supplies to below a safe level. Alternatively, the particles could generate an electrostatic charge that could damage electronic circuits. And with satellites, there is no way to repair problems - the only option is replacement.

Most operators are therefore taking precautions, moving their satellites so that the solar panels present the smallest surface areas facing the oncoming storm, which will produce a few thousand meteoroids per hour at its peak. Intelsat aims to orient the panels so they are parallel to the expected flow, as will Eutelsat and Inmarsat, which each operate a number of commercial telecommunications satellites.

That should suffice for this year, but as the evidence of the 1966 event - when the shower was more intense the year after the comet's passage - shows, it could be next year which actually proves to be the most hazardous, because of tiny variations in the Earth's orbit. Next year's Leonids could prove to be the Millennium Bug of satellites.