Washington

On cue: a Leonid meteor captured at 2:07 UT by the European Calar Alto observing team in Spain. Credit: ESA/EST EC

Last week's Leonid meteor storm, as well as being much more intense than last year's, marked the beginning of a new era in meteor forecasting. For the first time, researchers accurately predicted — to within minutes — when the two-day-long storm would peak.

The Leonids occur every November, when Earth's path crosses the tilted orbit of Comet Tempel–Tuttle. The display tends to be most dazzling at 33-year intervals, when the comet rounds the Sun and sheds a new layer of dust and grit to burn up in the Earth's atmosphere.

But forecasting the exact timing and intensity of any meteor shower (as measured by a standardized count known as the zenithal hourly rate, ZHR) is notoriously hard. Last year's Leonids, for example, surprised experts by peaking 16 hours earlier than expected.

Don Yeomans of the US space agency NASA's Jet Propulsion Laboratory, a veteran meteor prognosticator, wrote on his website last month that “like the weather, it is extremely difficult to predict the times and hourly rates of meteor showers”.

Yet David Asher, of Armagh Observatory in Northern Ireland, and Rob McNaught, of the Australian National University in Canberra, nailed this year's Leonids peak almost exactly. Their prediction of 2:08 Universal Time on 18 November was off by five minutes at the most.

Yeomans, basing his forecast on historic Leonids observations, missed by 20 minutes. Most researchers declined to hazard such a precise guess.

Asher and McNaught use computers to model dust emission from the comet, taking into account the rate and speed of ejection, particle size and other factors. They then add repeated gravitational tugs from Jupiter and other planets and watch how the dust disperses.

According to their model, the thin ribbons of dust created during each 33-year passage retain their coherence for centuries. Whereas scientists previously had assumed these independent dust trails follow the same path as the comet, Asher and McNaught found that the orbits are slightly offset.

Knowing their positions can give very accurate peak times, as well as the age of the source meteors in the shower. This year, the Earth passed through a cloud of dust shed from Tempel–Tuttle in 1899. Next year we'll hit the outskirts of the 1733 trail and the 1866 trail.

Asher and McNaught credit several Russian researchers, including E. D. Kondrat'eva and E. A. Reznikov, who pioneered this method of meteor forecasting in the mid-1980s but went largely unnoticed by Western scientists.

Peter Brown of the University of Western Ontario, in London, Ontario, has obtained similar results with his own modelling of the Leonid stream. He also identified the 1899 cloud as the source of this year's shower.

The new modelling work has greatly advanced the art of meteor prediction, says Rainer Arlt of the Astrophysikalisches Institüt Potsdam, who is compiling this year's Leonids observations for the International Meteor Organization. “All of these models are working, no matter what the details of each model are. We're really able to follow the dynamics over 1,000 years without problems.”

Predicting the meteor rate is a much tougher nut to crack, however. This year's ZHR was around 5,000, five to ten times the rate predicted by Asher and McNaught. Yeomans calls their forecast of the peak time “impressive”, but says the real proof will come in 2001 and 2002.

Yeomans and other experts say the Leonids should have pretty much died down by then, while Asher and McNaught are calling for intense storms with ZHRs of 15,000 or more. “That's going way out on a limb,” says Yeomans. “And if they get that right, I'll be the first to congratulate them on a major meteor-shower prediction breakthrough.”