8. THE NATURE OF METEORS
In answer to an exam question, a freshman astronomy student wrote:
A meteor is the flash of light
Made by a falling meteorite
As it rushes through the air in flight—
I hope to gosh this answer’s right!
Doggerel or not, the student’s definition correctly stated the true distinction between the two terms, and the teacher marked his off-beat answer correct.
Defined in more scientific terms, a meteor is the streak of light (usually of brief duration) that accompanies the flight of a particle of matter from outer space through our atmosphere. This particle may be as small as a tiny dust grain or as large as one of the minor planets which are called asteroids. Fortunately for the inhabitants of the earth, most of the meteor-forming masses encountered by our globe are of the “small-fry” variety!
As the rapidly moving particle plunges earthward through denser and denser layers of atmosphere, the air molecules offer ever-increasing resistance to its passage. This resistance heats up the meteorite body until it glows. Technically speaking, it becomes incandescent. The meteor is this incandescence. We see it as a darting point. Or as a ball of white, orange, bluish, or reddish light. But the material object that produced this light is the meteorite. The distinction between these two terms—meteor and meteorite—we must emphasize again and again because people continue to use them incorrectly, as, for instance, when they keep saying “meteor crater” instead of “meteorite crater.”
The majority of the meteors we observe represent the heat-induced “evaporation” of exceedingly small fragments of cosmic matter. The smallest meteor-forming bodies reach the surface of the earth only as the finest of dust particles or as microscopic droplets of solidified meteorite melt.
These residues descend slowly through the atmosphere and may be carried for great distances. Afterwards, they may be found scattered so widely and uniformly on the ground that their presence in any given locality cannot be accounted for by the fall of any specific meteorite. This is a fact that, for example, one school of modern Russian meteoriticists overlooked when they were dealing with tiny granules of meteoritic dust that had been recently found at Podkamennaya Tunguska. These scientists tried to identify the tiny granules with the meteorite that had fallen there, June 30, 1908. But the members of the latest (1958) Russian expedition to that region about the impact point of 1908 clearly recognize the widespread character of meteoritic dust. So they reject the theory that such dust found in the Podkamennaya Tunguska area is specifically connected with the meteorite that fell there a half century ago.
If sizable chunks of meteoritic material enter the atmosphere, they may produce exceptionally large and brilliant meteors. A spectacular meteor is generally known as a “fireball” if it is as bright as Venus or Jupiter. It receives the French term bolide if, in addition to showing great brilliance, its flight is accompanied by detonations like the alarming sounds heard at the time of the Ussuri and Norton meteorite falls.
COURTESY OF UNIVERSITY OF NEW MEXICO PRESS A bright Giacobinid meteor, photographed from a B-29 during the shower of October 9, 1946. See [p. 115].
The term “shooting star,” which is often applied to meteors, in newspapers and magazine articles, is a misnomer. A meteor is not a distant sun (that is, a star) in rapid motion, for the whole path of the meteor lies close at hand within a restricted zone of the earth’s atmosphere.
The word “meteor” comes from the Greek word meteōra, which once applied to any natural occurrence in the atmosphere—for example, rainbows, halos, auroras, and so forth. Nowadays, the word “meteor” is used in a much more specialized sense than it was by the ancient Greeks. We have a specialized word, meteoritics, for the study of meteors and meteorites. No one should confuse meteoritics with meteorology, which is the science of things other than meteors and meteorites, in the atmosphere—for example, clouds, storms, air currents.
The region in which meteoric phenomena take place was long the subject of controversy. Some persons felt that meteors were nearby, like lightning. Others said that they moved at the distances of the remote fixed stars. This controversy on the whereabouts of meteors became heated, although it could have been settled quickly by a simple experiment you can try out for yourself.
Hold a pencil against the tip of your nose and look at it first with your right eye closed and then with your left eye closed. Repeat this experiment with the pencil held at arm’s length. In the first case, the pencil will seem to shift position very greatly; in the second, although the same base line (the distance between your eyes) is used, the pencil will seem to shift position only slightly.
Such an apparent shift in position is called a parallactic displacement, or, simply, parallax. The notion of parallax is of the greatest importance in most branches of astronomy, and it leads (with proper instruments and a little mathematics) to exact determinations of the distances of remote objects.
For our purpose, we need not go into all the interesting but complicated details. Our experiment with the pencil shows that if a meteor was close by, like a blinding bolt of lightning, then, as seen by a pair of observers separated by only a few blocks, the meteor would show a large parallax. But if this meteor was as far away as the stars, it would show no parallax at all, no matter how widely the pair of observers were separated on the earth.
There were many clever scientists among the Greeks, and it is quite possible that a pair of them actually tried out this simple parallax experiment on the meteors and so were able to prove that these beautiful light effects occurred in the high but not too distant layers of the atmosphere. The earliest calculations of meteor heights that are so far known, however, were made in Bologna, Italy, in 1719 and 1745—long after the heyday of Greek science.
The meteor heights found by the Italians were quite low in the atmosphere, probably for two reasons. First, the visual (unaided-eye) observations they had to use were made by eyewitnesses stationed so close together that accurate fixes were impossible. Secondly, these visual observations must have related only to the very brightest and therefore lowest portions of the luminous paths of the meteors through the atmosphere.
In 1798, two German students operating from carefully chosen and widely separated stations began the systematic observation of meteors for parallax. They found that the height of appearance of most meteors lay between 48 and 60 miles above the earth’s surface. It is now known that most meteors, as observed with the naked eye, appear at about 70 miles and disappear at about 50 miles above the surface of the earth. These figures, obtained from visual work, still stand in spite of the development of such modern techniques as photographic and radar recording of meteor paths.
Rarely, meteors may appear at heights of 150 or more miles and fireballs may penetrate to within a few miles of the earth. The average meteors, however, appear and disappear within a well-defined, high-altitude zone in the atmosphere. Fortunately, this atmospheric zone serves us as an effective shield against the constant bombardment of the smaller and much more numerous particles from outer space.
In earlier times, scientists thought that the particles becoming visible as meteors must be tiny dense masses of iron or stone like the material composing the recovered meteorites. Most modern investigators, however, believe that the typical meteor-forming particles may be small loosely bound-together “dust-balls”; that is, fluffy clusters of matter held together by frozen cosmic vapors, generally referred to simply as “ices.” In any event, these masses are usually very small, ranging perhaps from the size of a pinhead to that of a marble.
Because we cannot collect the tiny masses that are seen only as meteors, it is impossible to determine their composition by ordinary laboratory methods. The best we can do is to observe and record carefully the light these masses give off when they become incandescent in their plunge through the atmosphere.
We can examine this meteor light by using the spectroscope and spectrograph. Through these specially designed instruments we can make the meteor light reveal the chemical elements present in the incandescent masses. Each such element sends out light rays as characteristic of its nature as fingerprints are of the individual who made them. Photographs taken of these characteristic light rays are called spectrograms, and what might be termed the “fingerprints of light” recorded on these spectrograms are known as spectra—which is the plural of the word spectrum. If the source of light is a meteor, the photograph shows a meteor spectrum.
From a study of a considerable number of good quality meteor spectra, scientists have found that the principal elements in the masses responsible for meteors are iron, calcium, manganese, magnesium, chromium, silicon, nickel, aluminum, and sodium.
As we have already noted, the resistance encountered by meteor-forming particles as they dash through our atmosphere is so great that they become incandescent and vaporize. These small bodies must therefore be in very rapid motion.
Before we attempt to find out the nature of the paths in space followed by meteorites, we must take into account the fact that these bodies are observed from a station—the earth—which is itself in rapid motion. You may have noticed that on a still day, when rain drops fall vertically downward, the streaks they leave on the windows of a swiftly moving car are not vertical but almost horizontal. Obviously, it would be wrong to say the rain drops are falling from left to right or from right to left when they are actually falling almost straight down, and it is only the forward motion of the car that makes them leave horizontal streaks.
Diagram showing meteorite moving along a “closed” (elliptic) orbit, e, which intersects the earth’s orbit, E. Held by the gravitational attraction of the sun, the meteorite is a permanent member of the Solar System.
Similarly, neither the apparent speed nor the apparent direction of motion of a meteorite with respect to the moving earth is significant. The important factor is the meteorite’s velocity with respect to the sun at the time the meteorite is picked up by the earth.
Diagram showing meteorite moving across the earth’s orbit, E, along an “open” (hyperbolic) orbit, h. The meteorite is traveling at such high velocity that it will pass right through the Solar System and back out into space unless it should chance to collide with the earth or another planet. The sun, however, in any case is able to change materially the direction of motion of the transient visitor to our Solar System.
This factor enables us to determine in which of two possible kinds of path the meteorite was moving before it was “fielded,” as we might say in baseball, by the earth. This factor tells us whether the meteorite was moving about the sun in a relatively short, closed, oval-shaped path or, instead, was following an indefinitely long, open path which began in the depths of space and would have returned there if the collision with the earth had not prevented.
Either type of path is technically called an orbit. The closed orbits are what the mathematicians term ellipses; the open orbits, hyperbolas.
To scientists, the nature of the orbits followed by meteorites is most important, especially in efforts to determine the mode and place of origin of these bodies. To rocket engineers and astronauts, it also matters a good deal whether the meteorites encountered on flights through space are traveling sedately along closed orbits about the sun or are zipping swiftly along open orbits.
The greater the speed of these cosmic “hot-rods,” the more dangerous they are to space travelers. For example, a mere grain of nickel-iron moving at 40 miles per second is quite as lethal as a .50-caliber machine-gun slug, which, relatively speaking, is traveling at only a snail’s pace.
As our earth moves along its orbit about the sun, meteoritic bodies can run into it from any direction. The direction from which they do approach strongly influences the speed of these bodies as they plunge through the earth’s atmosphere. A meteorite moving slowly about the sun in the same direction as the earth and chancing to catch up with our globe more or less from behind will have an observed speed of only a few miles a second. For example, the speed calculated from Harvard meteor-photographs of one such not-too-spectacular “rear-end” collision amounted to no more than 7.3 miles per second, just about the speed a rocket must acquire to escape from the apron strings of Mother Earth.
Meteor shower. Earth and particle-swarm passing through the intersection of their orbits at nearly the same moment.
In contrast to such a “rear-end” collision, the speed observed would be far greater if the meteorite happened to collide exactly “head-on” with the earth. For, in this case, the orbital speed of our planet would be added to that of the meteorite about the sun. As an example, suppose that at the earth’s average distance from the center of our Solar System, the speed of a meteorite with respect to the sun were 32.23 miles per second. (This speed was actually found for the mass that produced one of the first meteors photographed simultaneously by the Harvard stations at Cambridge and Oak Ridge, Massachusetts.) Then if such a meteorite ran “head-on” into the earth, the speed observed for it in the atmosphere would be over 51 miles per second. And mathematics would show that the orbit of this meteorite with respect to the sun was a wide open hyperbola.
If the orbit of the earth and the orbit of a swarm of particles of cosmic matter intersect, and if the earth and the swarm pass through this intersection in space at nearly the same moment, multitudes of meteors appear. We then say that a meteor shower takes place. The position of the point at which the particle-swarm crosses the earth’s orbit about the sun fixes the date of the meteor shower.
Because the particles that make a meteor shower are moving through space along parallel paths as they come into the earth’s atmosphere, the meteors all seem to shoot out from a single small area in the sky. You may have seen something like this in the case of the sunrise or sunset effect known as “the sun drawing water.” In this more familiar phenomenon, the sun’s disk is the area from which shafts of sunlight radiate out in a beautiful, if somewhat irregular, fan-like pattern. The area from which the meteors of a given shower seem to come is the radiant of that shower.
Meteor showers are named for the constellation in which their radiant lies. The suffix “-id” (Greek for “daughters of”), or some modification of this suffix, is added to the name of the constellation from which the meteors seem to radiate. The Orionid radiant, for example, is in Orion, the Hunter; the Leonid radiant is in Leo, the Lion; and the Lyrid radiant is in Lyra, the Harp. Exceptions to this rule do occur, however. Astronomers may refer to a shower sometimes appearing on the night of October 9 as the “Giacobinid” shower in honor of the comet Giacobini-Zinner, which is associated with this particle-swarm.
Radiant of a meteor shower. Generally not a point but a small area, here intentionally exaggerated in size. Solid arrows represent plotted paths of observed meteors. By extending these paths backwards, observer can determine the radiant.
In the course of each year, the earth passes through a number of particle-swarms of varying densities. Some of the resulting meteor showers, like the Leonids and Giacobinids, are very feeble in most years, but sometimes produce spectacular displays.
The more important recognized meteor showers are:
| NAME OF SHOWER | DATE OF MAXIMUM |
| Quadrantids | January 1-3 |
| Lyrids | April 21 |
| Eta-Aquarids | May 4-6 |
| Perseids | August 10-14 |
| Giacobinids (Nu-Draconids) | October 9 |
| Orionids | October 20-23 |
| Leonids | November 16-17 |
| Geminids | December 12-13 |
Certain daytime streams are also known to be active during June and July. These daytime showers are, of course, invisible in the glare of sunlight, but they can be picked up by radar devices like those used in World War II to spot enemy airplanes.
Some meteor showers have been splendid enough to make a place for themselves in the historical record. Examples are the Leonid returns of 1833 and 1866, and the Giacobinid showers of 1933 and 1946. During these displays, meteors fell in a veritable fiery snowstorm, several hundred meteors sometimes appearing within a minute.
Not every annual return of a meteor shower is spectacular, however, since conditions may not be favorable each year for a brilliant display. After all, both parties to a traffic collision at an intersection must try to pass through the intersection at the same time. Our earth, like a well-managed train, always goes through the intersection on schedule, but the particles responsible for meteor showers are much more erratic. They may be early or late—or they may not show up at all. Of the meteor showers seen annually, the Perseids are the most dependable. The Leonids put on their best shows at intervals of 33 years (1799-1800, 1832-33, 1866, etc.). The Giacobinids at intervals of 6½ years (1933, strong; 1939-40, poor; 1946, magnificent).
If you plan to observe a meteor shower, here are some suggestions. You will need:
Acquaintance with the stars, both faint and bright, in the region containing the radiant of the shower.
Comfortable reclining lawn-chair.
Warm clothing (including blankets) for winter showers or summer ones at high elevations.
A patient family that will not only approve of your observing but will help you get up to watch after midnight, when most showers are at their best.
A corner of your back yard (or sun roof) where you can shade your eyes from street lights and other illumination.
Timepiece, preferably with radiant dial.
Sit back and watch Nature put on her show. Any records you make may have some scientific value even if you note only these two things: Hourly number of meteors seen. Condition of the sky (clear, hazy, cloudy, etc.) during each hour of your watch.[6] At present, we know of only one instance in which it seems probable that a meteorite came to earth during a meteor shower. The Mazapil, Mexico, iron meteorite fell at 9:00 p.m. on November 27, 1885, during a return of the now very weak Bielid meteor shower. Scientists still cannot decide whether or not a mere coincidence was involved in this case.
As we have already mentioned, most of the cosmic particles rushing into our atmosphere evaporate and do not reach the earth at all except as the tiny congealed droplets and spherules of their own melt. Some cosmic particles, the micro-meteorites, are so tiny that they “stall” rather than fall down. These minute objects do not melt or disintegrate and so preserve their original cosmic form unchanged. Scientists have developed various methods for the collection of both of these types of material in order that at least rough estimates of their rate of accumulation on the earth can be made.
One of the simplest methods of collecting this so-called “meteoritic dust” is to expose a sticky glycerine-coated glass microscope slide for at least a 24-hour period in a protected spot well away from locations where any industrial contamination is in the air. At the end of the period of exposure, the “catch” on the slide is examined microscopically, and the individual trapped particles are counted and classified. Meteoritic dust is also carried down to the ground by rain, snow, and hail and can therefore be obtained by filtering rainwater or melted glacier-ice, snow, and hail.
Such collection efforts have been plagued by the difficulty of identifying the particles. How can a collector be sure that the dust he has trapped, even though magnetic and possibly even in part metallic, does not come from some smelter or other industrial plant? Because of such uncertainties, the current estimates of the annual deposit of meteoritic dust for the world range from approximately 20 tons to several million tons. We need improved collection and identification techniques if we are to obtain trustworthy figures.
Recent analyses of rainfall records indicate that the infall of meteoritic dust produces at least one interesting weather-effect. These analyses show that rainfall peaks often occur some 30 days after the appearance of important meteor showers. Apparently, as meteoritic dust particles from the meteor showers filter down through the cloud systems in the lower layers of the atmosphere, the individual particles serve as centers about which atmospheric moisture condenses to form raindrops. The time lag of approximately a month is considered to be due to the very slow rate of fall of such tiny particles. It looks very much as if Mother Nature had beaten man to the idea of “seeding” the clouds to produce rainfall!