Research in meteor science was well established before the space age. A big worry was the potential threat meteoroids could pose to satellites or humans in space. But experience has shown the threat was mostly slight.

Caption In 1966, the Earth passed right through the trail generated in 1899 (then 2 revolutions old), causing the most spectacular meteor display of the century. In 1999, the Earth again passes near the 1899 trail (now 3 revolutions old), but further from the trail's centre than in 1966. So a good display is expected (estimated peak Zenithal Hourly Rate 1000), but certainly not as remarkable as in 1966. The encounter with the 3-revolution trail in 1999 is centred at about Nov 18, 02:10 UT, so that the best places to see meteors from this trail are Europe, the Middle East and Africa. The moon has set for observers in most of these regions at that time. But although 1999 should be reasonable, a better display is expected (not at the same longitudes) in 2001. Image Copyright David Asher, Armagh Observatory For more dust trail details see Leonid dust trails at Armagh Observatory

However, several times each century, meteor activity has increased to intense levels, called storms. Although storms are related to the dust released from specific comets, their occurrence has been largely unpredictable. With their potential catastrophic consequences for satellites, much attention is now being focused on meteor storms.

Meteor storms result from Earth passing through dense narrow streams of debris from comets. Comet Temple-Tuttle produces the Leonid meteor storms. Storms have been largely unpredictable, but closely related to the orbital period of the parent comet; they occur within a few years of the comet passing Earth. The most intense Leonids occurred 1833 and 1966. The 1966 storm was the only one within the space age. No satellite damage occurred, but few satellites were in orbit then.

As comets approach the Sun, the increasing solar radiation sublimates ices which are swept away by the solar wind. Fine dust grains are released with the gasses.

The size of particles in a comet's visible dust tail are too small to produce naked-eye meteors. Particles causing visible meteors are only slightly affected by solar radiation wind and begin in orbits close to the comet's. The particles eventually scatter around the orbit.

The traditional approach to predicting meteor storms involved examining the correlation between comets' orbital geometry, and the historical dates and times of storms. The best this approach could do was suggest likely storm-years, and estimate peak intensity to within a day, or maybe a few hours. This is inadequate to assess the potential risk or organise evasive action.

New theories

To improve, it's necessary to look at how particles evolve from comet- ejection to collision with Earth's atmosphere. Dust ejected as a spherical cloud each time the comet is nearest the Sun, becomes a narrow dust trail, elongated along the original comet orbit. The precise position of the dust trail in relation to the Earth can be calculated for each year, a direct hit with the a trail being necessary to produce a storms.

We know the solar wind has some influence on the particles producing visual meteors. The radiation counteracts the Sun's gravitational attraction, so particles orbit the Sun more slowly. This is why storms occur in years shortly after passage of the parent comet. The exact time lag for the main bulk of the particles depends on the range of masses and ejection velocities, and the number of orbital revolutions before encounter.

David Asher and I have checked Earth's predicted times of approach to these dust trails. We could back-predict the peak times to within 5 minutes of the observed times, for all years when the maxima were timed (1866, 1867, 1869, 1966 and 1969). We also developed a model of dust trail density; our model closely fits the observed rates for all storm years (1833, 1866, 1867, 1869 and 1966).

We believe we can predict storms to within 5 minutes, and perhaps 50% in visual rates. Our model predicts the main activity will occur between 1999 and 2002, with the highest rates in 2001 and 2002. We predicted no activity from dust trails in 1998.

Predictions & countermeasures

Although no storm occurred in 1998, we still saw impressive Leonid fireballs (bright meteors). The explanation came in a separate study; as comet Tempel-Tuttle is in an orbital resonance with Jupiter, any particles ejected at low enough velocity (generally big ones), also inhabit this resonance zone. The zone only covers about 1/30 of the comet's orbit, so encounters with Earth only occur within a year of the comet passing. Detailed calculations showed the Earth had a direct encounter with large resonant meteoroids ejected from Comet Tempel-Tuttle in 1333.

The highest activity over the next few years will be due to encounters with young dust trails: * 1999, Nov. 18 02:08 UT 3-rev ZHR 1,200 * 2001, Nov. 18 18.19 UT 4-rev ZHR 13,000 * 2002, Nov. 19 10.36 UT 4-rev ZHR 25,000 Additional dust trail encounters in these years will be at different times with lower rates.

The geometry of encounter with the dust trails results in the more southerly parts of Earth experiencing the peak first. It then moves roughly northwards at around 600 km/min., leaving the northern extreme about 22 minutes later.

Given this model, and the small uncertainty in the predicted maximum time, there are specific consequences in mitigating the threat to satellites.

For a satellite that can be maneuvered to any point in its orbit at the maximum time, and having the appropriate orbital geometry, the two obvious strategies are to place the satellite in the "Leonid shadow" produced by the Earth, or at the point in the orbit furthest from the center of the Leonid dust trail. With the maximum of a Leonid storm being around 30 minutes, the period in shadow can be very significant. If the satellite orbit is oriented perpendicular to the dust trail, the rates furthest from the dust trail can be a small fraction of those closest to the dust trail. Combinations of these, and additional strategies, can result in a substantial reduction of the overall risk.

[Robert McNaught works in the Research School of Astronomy and Astrophysics at the Australian National University, studying the discovery and tracking of Near-Earth objects. He previously worked in optical satellite tracking.]