Interplanetary space is littered with rocks tens of meters in diameter or less. When these meteoroids strike the Earth's atmosphere at high relative speeds they leave visible trails created when the intense heat caused by friction vaporizes them. These are called meteors ("shooting stars").
Perseid Shower, 1996
The left image shows the path of a fireball recorded over Hannover, Germany. The fireball outshines Sirius and the constellation Orion (Ref). The middle image shows a fireball recorded in the 1996 Perseid meteor shower by the Dutch Meteor Society. The curved streaks are star tracks in the timed exposure. The pulsed nature of the fireball track is produced by the camera which is being chopped. The right image shows a fireball observed for over 20 seconds from Kentucky to New York in 1992 that led to a meteorite fall in Peekskill, New York. In this image the fireball is beginning to break up into smaller pieces. Here is a movie of the Peekskill fireball (972 kB; Source).
The meteor shower is commonly named after the constellation in which this radiant is found, and occurs annually during a well-defined time period. For example, the Perseid meteor shower occurs every year from about July 25th through August 18th, with a peak on August 12, and has its radiant in the constellation Perseus. In this shower, the typical maximum number of meteors that can be seen per hour at its peak is about 70, which is 10 times the rate of sporadic meteors.
Here is an observational determination of the radiant for the Quadrantid shower, and an MPEG movie (127 kB, compressed in time) showing 18 meteors of the Quandrantid shower. The movie was obtained from this paper, where a more detailed and technical discussion of photographing meteors and meteor showers may be found.
Meteor showers result when the Earth encounters cometary orbits
Perspective makes the meteors appear to come from a point in the sky
As comets move about their orbits they leave a stream of debris because dust and rocky material is liberated from the head as the ices vaporize. If the earth crosses the cometary orbit, this debris leads to an increased number of meteors (left figure). These periods are called meteor showers. That meteors in meteor showers appear to radiate from a single point in the sky is an optical illusion, as illustrated in the figure on the right. The meteor stream that produces the shower has meteoroids moving on essentially parallel trajectories, but because of perspective (parallel lines appear to meet at infinity) the meteors actually entering Earth's atmosphere on parallel trajectories seem to radiate from a point, as viewed from a site on Earth.
|Some Meteor Showers|
|Meteors / Hour
|Perseids||Aug. 12||68||Comet 1862 III|
|Orionids||Oct. 21||30||Comet Halley|
|Leonids||Nov. 17||10||Comet P/Tempel-Tuttle|
|Geminids||Dec. 14||58||3200 Phaethon|
Here is a more extensive listing of meteor showers and their characteristics.
Such predictions often do not pan out, but there is historical precedent for spectacular outbursts from the Leonids. On November 12-13, 1833, a "tempest of falling stars" was observed in Boston with a rate "half that of flakes of snow in an average snowstorm", while in the 1966 the Leonids produced "a hail of meteors too numerous to count" according to one observer and 150,000 per hour for a 20 minute period according to another (Reference). Newspaper accounts of the time reported that many people were awakened from sleep either by the shouts of neighbors, or the light of the meteors in the sky.
The image on the right depicts a wood-cut engraving by Adolf Vollmy based upon an original painting by the Swiss artist Karl Jauslin, that is in turn based on a first-person account of the 1833 storm by a minister, Joseph Harvey Waggoner. According to Waggoner's account, "... 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 myriad's to the north, east, south and west. Any representation on paper must at best be a very limited idea of the reality" (Reference). Here is another artist's depiction of the view from Niagara Falls during the 1833 shower, and another account of the history of the Leonids.
You are encouraged to follow these predictions of Leonid intensity as November, 1998, approaches. If they are true, it will be a spectacular sight---perhaps the greatest meteor display of the last 100 years. To help you keep track, here is the Leonid '98 Meteor Outburst Mission HomePage, a set of predictions for 1998 Leonid rates, and a history of Leonid showers. For good measure, here is the orbit and present position of the parent comet, P/Tempel-Tuttle.
These studies indicate that P/Tempel-Tuttle is in an unusual orbit for a comet, because it (1) comes close to the Earth's orbit, but (2) has a period sufficiently long that its orbit is only slightly perturbed by the planets. Comets with short periods that come close to the Earth typically do so only a time or two before gravitational interactions with the planets (primarily Jupiter) disturb them into other orbits. But P/Tempel-Tuttle's orbit has been relatively stable for many centuries and meteor storms have been seen for more than 1000 years that we now know are associated with this comet.