Meteors, or shooting stars, are pieces of rock and dust meeting a fiery end as they crash into the Earth's atmosphere and burn up. Sometimes they arrive as unexpected random ("sporadic") meteors but most are associated with known comets and asteroids, which follow predictable orbits and so arrive at regular intervals.
Regular arrivals are termed showers, whose names generally reflect the constellation that they appear to originate from. A meteor in a shower can appear anywhere in the sky but if a line is drawn backwards they all meet at a point of origin, called the "radiant", for that particular shower. The individual meteors all travel in parallel tracks and the radiating effect is caused entirely by parallax, just as snowflakes appear to spread out as they come towards you when you drive in a snowstorm.
The Perseid meteor shower is a regular performer in early August, and coming as it does during northern hemisphere summer makes for good viewing.
The Geminids come in early December, conveniently before midnight, just do not let the cold weather put you off.
The Leonids come in mid November and in a cycle of 33 years put on spectacular displays as these meteors are tightly bunched around their parent comet. The last return was around 2000, with some spectacular displays, but away from these returns you have an intervening run of very lean years.
The parent comet of the Leonids is 55P/Temple-Tuttle, which orbits the sun in 33 years and last paid a visit in 1998. The latest return should provide meteor showers or storms on or around 17th November up to 2002.
This shower is still generating a lot of excitement following the return of the parent comet Temple-Tuttle in 1998 on its 33 year orbit. It is the only comet known to produce true meteor storms on a fairly regular basis. The last two years have produced good, though very different, displays and there are good prospects for the next three years.
The comet is young and this means that the peak meteor rate can be very high but for a very short duration, sometimes less than 1 hour either side of the maximum. Therefore even when the comet is near perihelion the prospects of seeing a display from Britain vary considerably.from one year to the next.
Until last year predicting the timing and size of meteor displays was an inexact science and really the best that could be done was to calculate the time and location at which the Earth intersects the comet's orbit.
Following the surprise result in 1998, when the Leonids appeared to come 16 hours early, two astronomers at the Armagh Observatory in Northern Ireland, Asher & Macnaught, developed a model that not only tracks the comet itself but also the orbits and behaviour of all the streams of debris evaporated from the comet on each of its returns. The model was run retrospectively to "predict" past events, and the result showed a satisfying correlation with what actually happened. The first real test was the prediction for 1999, and in the event the model proved spectacularly successful.
|Link to Armagh Observatory Leonids predictions|
The parent comet of the Perseids in 109P/Swift-Tuttle, which orbits the sun in about 130 years and last paid a visit in 1992. Although it is several years since perihelion and the meteor rates drop off as the comet recedes into the outer solar system, there is material spread over a long section of its orbit. Therefore it is expected that meteor rates of at least 60 an hour should be visible annually on or around the 12th August for a good many years to come. The material is also dispersed sideways to a greater degree than most meteor showers, so a lesser number of meteors can be seen up to 2 weeks before and a week after the maximum.
According to calculations by Esko Lyytinen, using the same methods as were employed so successfully for the Leonids, it looks like 2004 should give high, possibly even storm rates centred on 11 Aug at 2054GMT, best visibility of which will probably be from the longitude of central Asia. This is caused by gravitational perturbations, mainly by Jupiter, swinging the trail of debris from Temple-Tuttle's 1862 return (1 revolution ago) into the path of the Earth. A similar movement of the 1479 debris, from 4 revolutions ago, looks likely to produce a storm on 12 Aug 2028 at 0530GMT, which will be best seen from the USA.
The 1992 return caused a scare as the orbital calculations predicted a possible collision with the Earth on the next return in 2126. Swift-Tuttle's orbit is curiously hard to predict accurately, as it seems to be affected more than most by the reaction force of material evaporating from the nucleus when it is near the sun. However, it now seems that it will pass closest to the Earth on 12 Jul 2126, a month before the Earth reaches the same spot, so will miss by 0.15AU (about 13 million miles). The following return still looks very worrying as closest approach is predicted for 10 Aug 2261, with the Earth just 2 days from the intersect point.
|Report for 2000||Julian Taylor|
|Link to American Meteor Society for a full history of the Perseids|
|Link to Armagh Observatory Perseids pages|