BASIC OBSERVING SERIES

by Greg Burnett (gburnett)

Meteor Observing

[This article was first published in Star Stuff, the newsletter of the Ford

It is estimated that tens of thousands of tons of meteoric material impacts the Earth each day. The vast majority of this material is dust, but some is in lumps of appreciable size, which we call "meteoroids." [Information recently declassified by the U.S. Government indicates that the entry of large meteors into the Earth's atmosphere is much more frequent than originally thought. The data comes from equipment intended to monitor for nuclear explosions. -- G.B.] When a fast-moving meteoroid encounters the Earth's atmosphere, it is heated to incandescence by air friction at an altitude of 60-80 miles, and we see the phenomenon we call a "meteor." If the object is large enough to survive re-entry, it will arrive at the ground and become a "meteorite."

Most of the meteors we observe in the sky are about the size of sand grains. The very brightest, called "fireballs," and the ones that explode, called "bolides," are generally about the size of marbles. Few of these ever reach the ground. Rarely, a meteor of golf-ball size or larger will arrive and become a meteorite. It is estimated that, on average, one meteorite impacts the Earth's surface per square kilometer per year. Obviously, most fall into the oceans. The rest are lost among the naturally occurring rocks and into the soil. Only when a very large meteoroid is observed arriving, or when one breaks into a shower of fragment, do we have a reasonable chance of recovering any of the material. (Many meteorites are also recovered from Antarctica, where they "float" to the surface of the ice.)

It was once thought that most meteors were following hyperbolic orbits and that they arrived at the Earth from somewhere far beyond the Solar System. We now understand that virtually all meteoric material is debris from comets, and while there are many "sporadic" meteors, most meteors occur as showers associated with a particular comet. In many cases the parent comet is a known periodic comet that is regularly observed. A meteor shower occurs when the Earth passes through the swarm of debris lying along the comet's orbital path. Since the meteoroids of a particular swarm are all moving in the same direction along the comet's orbit, they enter the atmosphere along essentially parallel paths. When we see them from the ground, their paths do not appear parallel, but seem to originate from a particular point in the sky. This point is called the "radiant" of the shower. Because the orbit of the meteor swarm is consistent, the shower will recur each year when the Earth passes through it, and the radiant will appear in a consistent place in the sky. Consequently, most major showers are named after the constellation in which the radiant appears, for examples the Perseids each August and the Leonids each November. There are over a hundred known showers, but only about a dozen major showers are commonly recognized.

Meteor observation is important for gaining an understanding of how comets lose material, how it spreads out along the orbit of the comet, the composition of comets, and the relationship between comets and asteroids. Even though most serious meteor observations are now made by radar, the amateur astronomer can still make a contribution. Amateur observations provide important data that helps define meteor swarm orbits and radiant locations, and can help predict major meteor storms. Several organizations collect amateur observations. Two of them are the American Meteor Society (Dept. of Physics and Astronomy, State Univ. of New York, Geneseo, NY, 14454) and the International Meteor Organization (Physics Dept., Univ. of Western Ontario, London, ON, N6A 3K7).

In order for amateur observations to be useful when combined with many others collected from different areas, all must be conducted according to certain guidelines. The most common amateur observation of meteors is simply to count them, but it must be done right. Position yourself comfortably where you can view an unobstructed area of sky for at least an hour or so. If some of your view is obstructed by trees or buildings, record how much (e.g. 5%; If it's more than about 20%, choose another location.). Record your viewing period to the nearest minute, including start and stop times, not just the duration. Record the faintest star visible at your location (i.e. the "limiting magnitude"). There are charts available to help with this. The limiting magnitude can change during the night, so check it at least every hour, and record changes if necessary. If clouds temporarily obstruct your view, either record them as obstructions, or if they are more severe, stop your viewing session until they pass. Record the center of your viewing area. You should observe an area between the zenith and 40-50 degrees altitude. If you submit your observations for compilation, include the latitude and longitude of your location.

During your observing session, count the number of meteors seen. You should count sporadics and shower members separately. This requires knowing the location of the radiant in order to distinguish between them. You should also observe whether the shower meteors are fast or slow, faint or bright, whether they leave glowing trails along their path, called "trains", and what colors you observe, if any. Count only the number of meteors within your field of view; do not combine you count with anyone else's or count meteors that you did not actually see (in spite of "ohs" and "ahs" from your fellow observers!). It is also helpful to observe before and after the predicted peak of a shower, in order to help define the extent of the meteor swarm. If these guidelines are followed, your count will be statistically valid when combined with counts of other who followed the same procedure.

There are other, more rigorous methods for meteor observing. Some observers record the actual path of each meteor seen and its brightness by comparing with nearby stars. This requires substantial dedication and practice. Another technique is to coordinate multiple observations from different locations separated by 50-100 miles. In this way the actual path of the meteor can be reconstructed in three dimensions. There have been several projects aimed at meteorite recovery through path triangulation, but none have yet been successful. The best way to obtain a meteorite is to buy one from a collector. You can then hold a piece of the primordial Solar System in your hand.