Meteors and Meteorites

Meteor showers occur when Earth passes through the tube-like structure of meteoroids left in the wake of a comet. Such meteoroid tubes, or as they are more commonly called meteoroid streams, are formed after a comet has made many repeated passages by the Sun. Meteoroid streams are composed of silicate (i.e. rocky) grains that were once embedded in the surface ices of a parent comet. Grains are released from a cometary nucleus whenever solar heating causes the surface ices to sublimate. New grains are injected into the meteoroid stream each time the comet passes close by the Sun.

The individual dust grains (technically meteoroids once they have left the comet) move along orbits that are similar to that of the parent comet. Gradually, over the course of several hundreds of years, the meteoroids form a diffuse shell of material around the whole orbit of the parent comet. Provided that the stream meteoroids are distributed in a reasonably uniform manner, a meteor shower will be seen each year when the Earth passes through the stream (Fig. 1). The shower occurs at the same time each year because the position at which the meteoroid stream intersects Earth's orbit does not vary much from one year to the next. There are long-term variations, however, and the days during which a shower is active will change eventually.

When Earth passes through a meteoroid stream, the meteoroids are moving through space along nearly parallel paths. Upon entering Earth's atmosphere, however, a perspective effect causes the shower meteors to apparently originate from a small region of the sky; this region is called the radiant. (Fig. 2).

The radiant is typically just a few degrees across when projected onto the night sky. A meteor shower is usually, but not always, named after the constellation in which the radiant falls on the night of the shower maximum. The Orionid meteor shower, for example, is so named because on the night of the shower maximum (October 21st) the stream radiant is located in the constellation of Orion. Some meteor showers are named after bright stars. The Eta Aquarid meteor shower, for example, is so named because on the night of the shower maximum (May 3rd) the radiant is close to the seventh brightest star in the constellation of Aquarius (by convention the brightest stars in a constellation are labeled Figure 2. Illustration by Hans & Cassidy. Courtesy of Gale Group. after the Greek alphabet, and accordingly, the seventh letter in the Greek alphabet is eta).

Probably the best known meteor shower is the one known as the Perseid shower. This shower reaches its peak on the night of August 12th each year, but meteors can be observed from the stream for several weeks on either side of the maximum. The shower's radiant first appears in the constellation of Andromedia in mid-July, and by late August it has moved into the constellation of Camelopardalis. The radiant is in the constellation of Perseus on the night of the shower maximum.

The steady eastward drift of the radiant across the night sky is due to the motion of Earth through the Perseid meteoroid stream. The nearly constant year-to-year activity associated with the Perseid meteor shower indicates that the stream must be very old. Essentially the Earth encounters about the same number of Perseid meteoroids each year even though it is sampling different segments of the stream. Since 1988, however, higher than normal meteor rates have been observed about twelve hours before the time of the traditional shower maximum (August 12th). This short-lived period (approximately half an hour) of high activity is caused by new meteoroids which were ejected from the stream's parent comet, Comet Swift-Tuttle, in 1862. Comet Swift-Tuttle last rounded the Sun in late 1992, and it is expected that higher than normal meteor rates will be visible half-a-day before the time of the "traditional" Perseid maximum for the next few decades.

Meteorites vary greatly in size and so do the craters they make. This tiny crater is the result of a micro-meteor impact on lunar rock. This image, as viewed, is several hundred times the crater's actual size. JLM Visuals. Reproduced by permission.

Another meteor shower known as the Leonid occurs every year in November, caused by the tail of comet Tempel-Tuttle, which passes through the inner solar system every 32-33 years. Such a year was 1998; on November 17 and 18, 1998, observers on Earth saw as many as 200 meteors an hour. The shower was so intense that it generated widespread concern about the disruption of global telecommunications and the possible damage or destruction of space telescopes. Partly as a result of careful preparation by satellite and telescope engineers, however, concerns appeared to be minimal.