Comet
—ACCOUNT OF THE LEONID METEOR SHOWER
OF NOVEMBER 13 AND 14, 1866. ROBERT BALL,
THE STORY OF THE HEAVENS, LONDON, 1900
In a meteor shower, the trails all diverge from the same point in space. This is a circumstance familiar to those who must drive in snowstorms at night. As the automobile plunges through the falling snowflakes, they speed by on all sides, apparently diverging from a fixed point just ahead* —or to the side if a steady wind is blowing. Similarly, a meteor shower occurs when the Earth, in its swift annual motion about the Sun, passes through a swarm of fine particles (see the illustrations on this page).
What is happening was described vividly at the turn of the last century by the British astronomer Robert Ball:
Let us imagine a swarm of small objects roaming through space. Think of a shoal of herrings in the ocean, extending over many square miles, and containing countless myriads of individuals … The shoal of shooting stars is perhaps much more numerous than the herrings … The shooting stars are, however, not very close together; they are, on an average, probably some few miles apart. The actual bulk of the shoal is therefore prodigious; and its dimensions are to be measured by hundreds of thousands of miles.†
To the question of how there come to be shoals of meteors passing by the Earth, an answer was supplied when meteor showers were discovered to be associated with comets—especially defunct comets such as Comet Biela/Gambart (Chapter 5). There is now good evidence—including direct observations from spacecraft—that fine particles abound in the dust tails of comets, and in the coma. Those too large to be removed by radiation pressure and the solar wind continue to move around the Sun as separate microplanets, sharing essentially the same orbit as the parent comet. Because of jetting and the successive shedding of comas, some particles will be moving a little faster and some a little slower than the cometary nucleus. As a result, the particles have slightly different periods around the Sun. A particle somewhat slower than the rest will trail a little bit after one perihelion passage, more after two, and so on. Eventually, the particles would be spread out over the entire length of the orbit, dispersing a little in lateral dimensions as well. Also, small particles feel radiation forces to which big particles are immune, and the gravity of nearby planets also serves to spread the stream of particles. A swarm of small meteors can orbit the Sun shoulder to shoulder in the same orbit with virtually no collisions, like skydivers falling together from an airplane. As a comet slowly dissipates, it fills its orbit with debris.
In most cases, the shoals of meteors in a cometary orbit are entirely invisible to astronomers on Earth. But occasionally, by chance, the orbit of the comet intersects the orbit of the Earth. Since the Earth is in a specific segment of its orbit on every day of the calendar, the resulting meteor shower must occur on a specific day of the annual calendar (this page, bottom). The main meteor streams today are the Perseids, the Leonids, the Orionids, and the Geminids. Also, the Taurids, associated with the periodic Comet Encke. There are two meteor streams associated with Comet Halley (see Appendix 3). Every comet as it dies litters its orbit with debris. Some orbits cross the Earth’s. Meteor showers are the ghosts of comets past and passing.
Ball continued,
The meteors cannot choose their own track, like the shoal of herrings, for they are compelled to follow the route which is prescribed to them by the Sun. Each one pursues its own ellipse in complete independence of its neighbours … We never see them till the Earth catches them. Every 33 years the Earth makes a haul of these meteors just as successfully as the fishermen among the herrings, and in much the same way, for while the fisherman spreads his net in which the fishes meet their doom, so the Earth has an atmosphere wherein the meteors perish. We are told that there is no fear of the [supply of] herrings becoming exhausted, for those the fishermen catch are as nothing compared to the profusion in which they abound in [the] ocean. We may say the same with regard to the meteors.
Just as Tycho Brahe was able to determine that comets passed far beyond the Moon from observations of parallax (Chapter 2), so can photographic observations of the same meteor, made by separated cameras on the Earth, determine how high up the meteor trail is. The typical answer is somewhere in the vicinity of 100 kilometers (60 miles). At this altitude the atmospheric pressure is 0.00003 percent that at the surface of the Earth; Humboldt was entirely right to wonder how so brilliant a trail could be made by passing through air so thin.
Imagine you’re holding a small piece of comet (perhaps only a grain of dust) so it is hovering far above the Earth, and then you let it go. Of course it speeds up as it falls. By the time it reaches the Earth’s upper atmosphere it will be moving at escape velocity, about 11 kilometers (or 7 miles) a second. Ordinarily a cometary fragment will be traveling very fast relative to the Earth before it is attracted by the Earth’s gravity, and will therefore hit it at a higher speed. A particle on a highly eccentric orbit, moving in a retrograde direction so it collides with the dawn hemisphere, can be traveling as fast as 72 kilometers (or 45 miles) per second. By contrast, the typical muzzle velocity of a rifle bullet is about one kilometer per second. When a meteor enters the Earth’s atmosphere it is heated to incandescence by friction with the thin air at an altitude of around 100 kilometers. Spectroscopy of meteors shows spectral lines of iron, magnesium, silicon, and a range of other elements that make up ordinary rocks. Organics and even some ices may be in them before they enter the Earth’s atmosphere, but at least the stony component is still there as the meteors burn and die.
Big particles the size of your fist, or larger, streak through the Earth’s atmosphere, heat up by friction with the air, and char, melt, or burn off a thin crust. This process protects the interior of the meteorite just as the ablation shield on a spacecraft protects astronauts during reentry. The remainder of the object survives passage through the Earth’s atmosphere and reaches the ground, where, when they are recovered, they are called meteorites.
A still active comet, through successive perihelion passages, has begun to break up, and fine cometary debris now litters its entire orbit. The particles have also spread out from the width of the original cometary orbit. The comet itself passes well behind the Earth’s orbit in this diagram, but the debris intercepts the Earth’s orbit. Thus, on a given date of each year, the Earth runs into the cometary debris, producing a meteor shower.
The Earth does not pass through most meteor streams left by decaying comets. In a few cases where the debris in the cometary orbit intersects the Earth’s orbit, meteor showers occur. Here small segments of the meteor-strewn orbits of several comets are shown where they intercept the orbit of the Earth. Each intersection, for the debris from a different comet, corresponds to a particular date in the year. Diagrams by Jon Lomberg/BPS.
Very small particles are able to radiate their heat away quickly—because their area is so large compared to their mass—and so they never melt. They simply slow down at around 100 kilometers altitude, where they contribute to the rare “noctilucent” (literally, bright at night) clouds that reflect sunlight back to the nighttime Earth. Gently, they fall for years through a barrage of bombarding air molecules which tend to keep them suspended. Eventually, they enter the circulation of the lower atmosphere and are carried down to the Earth’s surface. They are called micrometeorites.
Particles of intermediate size are too small to survive the charring of even a thin crust, and too large to radiate all their frictional heat away and float gently down. They burn up entirely during entry. These are the meteors.
With radar techniques and a network of fast cameras, it is possible to measure how the meteor decelerates when it enters the Earth’s atmosphere, and how it flares. Mass and density information is derived. Typical visual meteors are millimeter sized—no bigger than a small pea. A fireball as bright as the brightest star typically weighs less than a hundred grams (about an ounce). A porous object presents more area than does a denser object with the same mass, and so it dec
elerates differently. In this way, densities are determined for the meteors of different showers. For example, the Geminids are meteors with the density of ordinary terrestrial materials, around one gram for every cubic centimeter. Most meteors, however, have much lower densities. Comet Giacobini-Zinner is the presumed source of the Draconid (or Giacobinid) meteor shower—once formidable, now not very impressive—approximately on the evening of each October 9. The density of the Draconids is very low, possibly as small as 0.01 grams per cubic centimeter. To maintain such fragile structures these meteoroids could not have been violently ejected from their parent body. So there seem to be at least two populations of objects that enter the Earth’s atmosphere—one very much like the meteorites that are recovered, and the other, very fragile porous structures that are unlike macroscopic objects made on Earth. There is probably a continuous range of bodies of intermediate density as well.
A swarm of cometary debris falling into the Earth’s atmosphere. Very small particles float down as a fine mist, large chunks survive entry and arrive slightly scorched at the Earth’s surface, and particles of intermediate size burn up as meteors. Painting by Don Dixon.
With modern photographic and radar techniques it is possible to calculate the speed and direction from which a meteor comes, and then to extract from this data the meteor’s orbit. Sporadic meteors—those unconnected with meteor showers—tend to lie in the ecliptic plane and go around the Sun in the same direction that the planets and the short-period comets do. The shower meteors, on the other hand, have much larger eccentricities and orbital inclinations, although some certainly lie in the ecliptic plane. Here again, a kind of natural selection has occurred. Short-period comets with small inclinations produce meteor streams that tend to be disrupted by Jupiter’s gravity, and fragments in a range of orbits are produced—some of which become sporadic meteors. But comets with large orbital inclinations tend to avoid Jupiter, so the meteor streams they produce tend to remain intact for much longer. Of more than 40,000 meteor trails studied, not a single one has an orbit that originated beyond the solar system.
Only three meteorites have ever been recovered that are fireball remnants: Lost City, Pribram, and Innisfree, each named after the locale near which it was recovered. They tend to be ordinary stony meteorites that derive from the asteroid belt, interior to Jupiter’s orbit.
Those bright meteors that arrive from beyond Jupiter have been given the stirring name of transjovian fireballs. As determined from their entry characteristics, they are as fragile as the most delicate meteor known. If a sizable piece of such material were gently placed on the table before you it would collapse under its own weight. It is possible that the spaces in these silicate dust balls were originally, on the parent comet, filled with ices and organics.
On May 5, 1960, the Soviet Premier, Nikita Khrushchev, made a brief announcement that an American airplane, four days earlier, had violated Soviet airspace and had been shot down. A little later on May 5, the newly formed U.S. National Aeronautics and Space Administration issued a related bulletin that revealed, for the first time to many, that there was a new kind of aircraft called the U-2. It could fly very high. A research plane of this sort, in the course of studying “meteoro-logical conditions at high altitudes,” NASA said, had been missing over the “mountainous and rugged” area of Lake Van, Turkey. Perhaps it had accidentally strayed over the border into the U.S.S.R. The airplane was described variously as a “flying test bed,” and a “flying weather laboratory.” It had been used by NASA, among other purposes, to determine “the concentration of certain elements in the atmosphere.” State Department spokesman Lincoln White said “there was absolutely no deliberate attempt to violate Soviet airspace, and never has been.” Three days later, Mr. Khrushchev announced that the U-2 airplane had been shot down near Sverdlovsk, more than 2,000 kilometers from Lake Van. Its pilot, Francis Gary Powers, and some of his photographic equipment had been recovered in one piece. Powers admitted to working for the U.S. Central Intelligence Agency on one of a series of daring espionage overflights of the Soviet Union. The plane, it turned out, had no equipment for analyzing the atmosphere, but Khrushchev displayed some of the equipment that Powers acknowledged bringing with him—including a pistol with silencer; a poison capsule to swallow if captured; 7,500 rubles in Soviet currency; French, West German, and Italian money; three watches; and “seven gold rings for ladies.” Khrushchev asked,
A lithograph by Honoré Daumier, entitled “Comet of 1847.” However, the woman’s urgency suggests that the visitor is streaking across the sky, in which case it would be not a comet but a meteor. Since we do not know beforehand the precise position of each meteor, it is much more sensible to use a very wide angle telescope rather than the spyglass variety depicted here. From the collection of D. K. Yeomans.
Why was all this necessary in the upper layers of the atmosphere? Or maybe the pilot was to have flown still higher to Mars, and was going to lead the martian ladies astray. You see how thoroughly American pilots are equipped before setting off on a flight to take samples of air in the upper layers of the atmosphere.
The American response, making no reference to the NASA cover story of only a few days earlier, much less Mr. White’s assurances, talked about the importance of obtaining data about Soviet military capability because of the closed nature of Soviet society. The incident is historically important for a number of reasons, including its scuttling of the scheduled summit conference between Khrushchev and the American President, Dwight Eisenhower. It also threatened the integrity of the spanking new National Aeronautics and Space Administration, which at Eisenhower’s explicit request was to have been dedicated to peaceful research in science and technology.
The U-2 was designed for photographic reconnaissance from an altitude too high to shoot down conveniently. But as Soviet surface-to-air missiles improved, U-2’s were mainly supplanted for intelligence work by reconnaissance satellites. Once they became obsolete for espionage, U-2’s began being used for science in earnest. It is therefore a mildly ironic footnote to history that, many years later, the U-2 was the key to fundamental—even trailblazing—discoveries in what might properly be called upper atmospheric research. But it is better described as, for the first time in human history, capturing pieces of a comet and bringing them home for examination. The driving force behind this program has been Donald Brownlee of the University of Washington in Seattle.
A U-2 takes off from NASA’s Ames Research Center at the Moffett Field Naval Air Station near Mountain View, California. The aircraft has immense wings for its size; it looks a little ungainly, as if it were a cross between a glider and a jet aircraft. Attached to a pylon mounted on the wing are covered collecting surfaces of sticky silicone grease. The surfaces are not opened to the airstream until the U-2 reaches an altitude of some 20 kilometers. The aircraft flies more or less at random, because it cannot know where in the stratosphere there might be concentrations of fine cometary or meteoritic dust. For every hour that it flies, ramming the sticky plate into the air in front of it, it collects about one big particle (more than ten microns across, still invisible to the naked eye), and many smaller ones. The plate is then automatically covered, the airplane swoops down out of the almost black sky and the day’s catch of stratospheric dust is examined through the microscope back on Earth.
It rarely happens that a small particle lifted a little off the Earth’s surface one day finds itself in the stratosphere. Such particles tend to be rained out or carried out before they reach high altitudes. There is a natural barrier to circulation between the lower and the upper atmosphere. (This restriction is removed in a nuclear war, but that is another story.) So as long as humans are somewhat restrained about polluting the atmosphere with fine particles, and we are not looking just after a major volcanic explosion, the stratosphere should act as a useful natural catchment region for extraterrestrial particles falling to the Earth from space.
You take the sticky plate off the wing of the U
-2 and put it under the microscope. You count the number of particles of various sizes, you photograph them, you try to do some chemical analysis—although this is difficult because there are so few big particles. For the same reason, you try to avoid destructive testing. There are many other scientists in line to examine this material after you. Between examinations, the particles are stored at the old Lunar Curatorial Facility at NASA’s Johnson Spaceflight Center in Houston, where the rocks returned from the Moon by Apollo astronauts are also kept. There is, at this institution, the only person in the world with the official title “Curator of Cosmic Dust.”
One kind of dust you find is very simple—roughly spherical particles of pure aluminum oxide, collected at 20 kilometers altitude. How did they get there? These particles have been generated by solid fuel rockets—largely of American, Russian, and French manufacture—as they accelerate through the stratosphere on their way to more distant places. The population of aluminum oxide particles in the stratosphere is growing, but it still represents only a distraction. The sticky plates have captured far stranger particles.