It could rank as the greatest light-show seen in three decades, but will we be able to see it? The annual Leonid meteor shower is making its usual, November appearance but Ireland and Europe could be on the wrong side of Earth when the performance begins on Tuesday night.
Meteors or "shooting stars" are those ephemeral tracks of light in the night sky, so short-lived you wonder whether you really saw them or just imagined them. They occur when tiny bits of space dust, about the size of a grain of sand, get swept up by Earth as it orbits the sun. The dust-particle flares white-hot as it burns up in the thin atmosphere, 50 miles overhead, producing a trail of light before it vaporises, never getting even close to the surface.
In space, a bit of this drifting debris is called a meteoroid and such things rain down on us all the time, day and night. The light-track they leave is too weak to be seen, except at night - and even then the moon's light can wash them out of visual existence, but they are always there and sensitive radar can detect their presence even on the brightest day.
On an average clear night, an observer might hope to see five to 10 shooting stars in an hour. Add the light of a few nearby street lights and you could halve this - not exactly the kind of night-time entertainment that would appeal to most.
There are times, however, when Earth passes through thicker patches of space dust, which in turn increases the meteor rate. A meteor shower occurs when anything up to 1,000 meteors are seen in an hour. If the rate rises above this, it is known as a meteor storm, but even within this classification there is tremendous variation. A dramatic storm might scatter the night sky with 10,000 meteors in an hour, but a truly tremendous storm could dump 100,000 shooting stars into the upper atmosphere.
Meteor storms at this level are remarkable to watch and worth waiting for, even when there is no more than a slim chance they might deliver, explains Prof Mark Bailey, director of the Armagh Observatory. A true storm is one of the world's great natural phenomenons, he says.
One of the greatest meteor storms in recorded history lit up the night sky over eastern North America during the early morning hours of November 13th, 1833. Impressive is a term inadequate to describe the scene, judging by an eye-witness account from South Carolina:
"I then opened the door, and it is difficult to say which excited me most - the awfulness of the scene, or the distressed cries of the negroes. Upward of 100 lay prostrate on the ground - some speechless, and some with the bitterest cries, but with their hands raised, imploring God to save the world and them. The scene was truly awful; for never did rain fall much thicker than the meteors fell towards the Earth."
The world will be treated to more of the same this week when the source of the 1833 event, the Leonid meteor shower, makes its annual pilgrimage.
The space dirt and dust we see as the Leonids does not arrive each year by chance: rather, it is left there in a dependable and regular fashion by an orbiting comet known as Comet Tempel-Tuttle. This migrant loops around the sun once every 33 years and three months, leaving in its wake a stream of refuse. Earth in its own orbit around the sun passes through this wake every year during November, treating us in turn to another Leonids light show. And once every three decades or so we traverse this dust stream soon after Tempel-Tuttle has passed by, when the dust stream has had little time to thin and disperse. It is at these years and a year or two afterwards that the Leonids can become spectacular and astronomers begin to predict a meteor storm.
Tempel-Tuttle passed us by on February 27th last and is now "well gone", explains David Moore, editor of Astronomy Ireland. The newly-laid dust stream from its wake means expectations are high for a spectacular Leonid storm either this November or on November 18th, 1999.
It is at this point that chance and luck come into play. The plume of material left behind by the comet, with dust ranging in size from a fraction of a millimetre to the size of a pea, is not consistent. There are thick patches and thin and is more "filimentary" than uniform, Moore says. Earth's orbit will bring us close to a dense part of the dust stream, however, and the denser the stream, the better the meteor storm.
The last time this happened was in 1966 when Earth was treated to a 100,000-shooting-star-an-hour display that lasted 40 minutes. The quality of the performance is highly variable, however. The Leonids did not reach storm levels in either 1900 or 1933, and the 1998 display isn't expected to be as dramatic the 1966.
The quality is dependent on whether Earth passes through the densest part of the dust stream or just clips its edge, although there is always enough material in Tempel-Tuttle's wake to ensure at least a pleasant shower-level of shooting stars. Yet even if you ploughed headlong into the dirtiest part of the wake you might still see nothing if you are unlucky enough to be on the wrong side of Earth.
Earth's orbit about the sun brings us through the wake but Earth is also turning on its axis. The best Leonids view is obtained those in the "front seats" - people on the part of Earth facing directly into the dust stream. If the stream is very wide at the point at which Earth passes through, then more people will get a chance of front-row seats as Earth turns, but if the stream is narrow then the meteor storm will deliver only a short, sharp shock, as in 1966. This is great for those sitting in the front, but a total bore for those staying up all night in the hope of a show that never materialises. And such are the chances you take if you decide to watch for a Leonids storm. Ireland will be perched on the "side" of Earth when we encounter the dust stream, Moore says. The Far East is expected to be in the front row at this stage and should have the best view, and possibly a memorable storm. "We are just being rotated into the firing line," at about 10.30-11 p.m., he says, but at this stage the best might have past.
There is hope, however. Orbital dynamics is well understood and the paths of Earth, Tempel-Tuttle and the dust stream can be predicted, Prof Bailey explains. How dense the dust will be - and so how spectacular the meteor shower will be - is very difficult to predict. There remains a chance the densest part of the stream will not be reached until we are in the front row and so we will get the best the Leonids have to offer. This chance will have Bailey, at least, wrapped up against the cold and hoping for the best on the night of the 17th.
And there is always next year. "Europe is in the firing line for 1999," Moore says. It will be our turn to be in the front row and we will be in for a treat - so long as the weather holds clear. Miss this and you are finished, however. A gravitational perturbation by Jupiter in 2029 will tug Comet Tempel-Tuttle's orbit away from Earth and not until 2098 or perhaps 2131 will there be another chance of a Leonid storm, according to the American Astronomical Society.
There is one group, however, which will be in a front row of a different kind when the Leonids come - the 500 or so communications, weather and military satellites that orbit Earth. The meteors we see burn up dozens of miles overhead and never get near us. But for those spacecraft in orbit above the protective atmosphere, there is nothing to slow down the incoming particles. The communications systems which guide our aircraft, signal our pagers and keep our mobile phones working are about to get a sand-blasting by the Leonids.
The US Department of Defense and the National Aeronautics and Space Administration have been studying the possible risk to satellites for some time. They conclude - perhaps optimistically - that there will be an "elevated" though not serious risk to spacecraft exposed to the Leonids.
There are several factors to consider. Earth travels at about 18 miles a second as it orbits the sun, so the speed of the incoming dust particles varies depending on whether they are moving in our direction or coming at us head on. Some penetrate the atmosphere at no more than nine miles a second but those coming straight at us can reach a spectacular 45 miles a second. The Leonids are at near head-on, so they will pepper the satellites at the upper end of this range.
Meteors are tiny specks of material but at such speeds they pack a terrific punch. A particle no bigger than a grain of sand carries 5,000 times the energy of a rifle bullet when travelling at 40 miles a second, so the satellites will truly be in the firing line.
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Prof David J. Fegan, UCD professor of experimental physics, likens the risk to being on a golf course. If there are few golfers about - comparable to when the Leonids are at low ebb and the counts are low - then the chance of hitting another golfer is low. Throw hundreds of golfers onto the course hitting balls in all directions and the chances of a dangerous blow rise significantly. "Some of these particles could plough right through and do lots of damage," he says. "They can deliver quite a punch."
Most commentators believe a satellite or two could be knocked out by the Leonids, but they have no way of telling, since passing through such a meteor shower is a new experience for the space agencies. "We haven't had a meteor storm since before we had a lot of satellites. No one would be surprised if a satellite got zapped. It is uncharted territory," Prof Bailey says. However, it is unlikely to mean wholesale extinction. "I don't think anyone would expect there would be no satellites left the next morning."
Even so, many satellites are being positioned to their least vulnerable profiles face into the dust stream. The Hubble Space Telescope is also being turned so that its sensitive lenses are not exposed to the sand-papering effects of the Leonids. With a typical communications satellite costing $200 million to build and launch, the precautions are a patently good idea.
Encounters with meteor showers are not uncommon, however. There are 11 regular showers caused by comets that orbit close to Earth and we pass through these dust streams right through the year, even if they are not very spectacular to watch. Cosmonauts orbiting in the Russian Mir space station have had first-hand experience of a buffeting by a meteor shower.
Each August, the Perseid meteor showers fall as we pass through the dust stream left by Comet Swift Tuttle. The cosmonauts were able to hear the sounds as the tiny particles hammered away at the walls of their spacecraft. They lived to tell the tale.