Both the chemical structure and the velocity of the meteoric body help determine its apparent color. As the burning object plunges through the atmosphere and vaporizes, the chemical elements produce their typical colors. At higher velocities, atmospheric friction heats the body to higher temperatures and whitens the color; as the body slows down and becomes less hot, it is apt to appear redder.
In a few instances astronomers have been able to photograph the color spectrum of a meteor in flight, to analyze the spectral lines and determine exactly what elements were present[[V-10]]. As a rule, however, the chemical content must be found from a laboratory analysis of recovered meteorites. Some meteors do contain traces of copper, and free nodules of pure copper have been found in several meteorites [[V-5], p. 81]. Magnesium occurs in fairly high percentages in most meteorites and the amount is unusually high in green meteors[[V-11]]. It produces a color almost identical with that from copper. Seeing the green of a vaporizing meteor, no observer could tell whether the color came from copper or from magnesium unless he could photograph the spectrum or make a chemical analysis of the meteorite.
The color displayed by the New Mexico fireballs may have come from copper, but more probably from magnesium. Another possible source is frozen nitrogen. Laboratory experiments relating to problems of satellite re-entry[[V-12]] have shown that when frozen nitrogen vaporizes, it emits a brilliant green glow whose wave length is almost identical with that of the New Mexico fireballs, as judged from the paintings made by witnesses. One of the prevailing theories suggests that meteors of this type may be icy “cometoids”—cometary debris, chunks of ice, and frozen gases (including nitrogen) at very low temperatures. When they enter the earth’s atmosphere and are slowed down to speeds of several hundred miles an hour, they become heated and vaporize, and the surface alternately melts and refreezes; the vaporizing nitrogen would produce the green color seen in the fireballs. Such a process would account for the color, the short lifetime, and the lack of fragments of the New Mexico meteors.
To summarize: Meteors can exhibit the particular green color shown by the New Mexico fireballs. It can result from copper, magnesium, or frozen nitrogen, which can normally occur in meteors.
2. Speed and trajectory. Meteors vary widely in their velocities and flight paths. They plunge from space into the earth’s atmosphere at speeds estimated to range from seven to forty-five miles a second relative to the earth—from 25,000 to more than 150,000 miles per hour. Members of a particular meteor stream usually show a characteristic velocity. The Perseids, for example, travel at high speed, some thirty-six miles a second, while the Geminids saunter in at a mere twenty-one miles a second. Most of these “falling stars” become visible to us when they have descended to around sixty or seventy miles above the earth. Flashing down in a steep path, they usually burn up and vanish by the time they have fallen to around fifty or forty miles. The larger the meteor’s body, the longer its life and the lower its point of disappearance. Most meteors maintain a straight course as they descend toward earth. A typical path is that photographed by Smithsonian astronomers in New Mexico on the night of November 23, 1960 (see [Plate IIIa]). Some fireballs have been reported to change course after exploding. More probably, the witness is actually observing the shifting pattern of the smoke cloud left by the meteor. The Puerto Rico fireball of January 12, 1947, left an erratic trail of this type, which was photographed ten to twenty minutes after the meteor had disappeared (see [Plate IIIb]).
The original entrance velocity, angle of entry, size, and chemical structure all influence the shape of a meteor’s path and its time of survival. The apparent angle of descent as seen by the observer depends on the distance and the direction the object is moving relative to the observer. When the meteor travels parallel to the observer’s line of sight, it seems much slower than when it passes the line of sight at right angles. The greater the distance between the observer and the meteor, the slower its apparent motion[[V-13]].
Some meteors move very slowly; traveling at an almost leisurely rate, they soar through the sky on a long, level path almost parallel with the earth. The slow fireballs in the great meteor procession of 1913 maintained a horizontal course over a distance of several thousand miles, from western Canada to Brazil[V-14].
Astronomical records show that green meteors are usually slow. Some 230 persons reported to the American Meteor Society that on November 28, 1953, at 6:30 P.M., a fireball moved slowly through the sky from Massachusetts to Pennsylvania. Described as blue-white-green, changing to orange-yellow-red, it was huge, disk-shaped, and vanished silently without depositing fragments [[V-1], p. 273]. On May 15, 1954, at 11:22 P.M., more than 100 persons observed (and reported) a slow-moving fireball, blue-green changing to red, of luminosity so great that it woke sleeping people. Toward the end of its course it seemed to stop, spiraled a couple of times, and then simply vanished without leaving fragments [[V-8], p. 336].
To summarize: Meteors can travel at low velocities and in apparently horizontal paths.
3. Size and brilliance. Giant meteors of great luminosity have been recorded throughout history. Some fireballs have been visible to observers throughout an area of thousands of square miles. Typical descriptions are: dazzling, like an airplane falling in flames, bigger than the full moon, of blinding brilliance, so bright it turned night into day, like the headlight of a locomotive, as big as the setting sun but three times as brilliant.