Meteors and meteor showers

Meteors are those streaks of light produced by small solar-system bodies (meteoroids) entering the Earth’s atmosphere. Fragments from asteroids produce sporadic meteors, while debris left along the orbit of a comet causes meteor showers. Both provide information about the origins of the solar system, especially if they reach the ground and are recovered as meteorites.

Overview

Scientific study of meteors and their relation to meteorites did not start until the beginning of the nineteenth century. Earlier meteorite falls were observed, with stones recovered, but most witnesses were ridiculed, and “sky stones” were treated with suspicion. In the Christian Bible, Joshua 10:11 records a battle in which the enemy was defeated by “stones from heaven,” which may have been meteorites. Acts 20:35 refers to the image of Diana of Ephesus standing on a stone that fell from heaven.

Anaxagoras, Plutarch, and several Chinese recorders from as early as 644 BCE described stones falling from the sky. A stone preserved in a corner of the Kaaba in Mecca fell in the seventh century. The oldest authenticated meteorite in Europe, a 120-kilogram stone that fell in Switzerland in 1492, remains preserved in a museum. Despite this evidence, much doubt remained among European scientists. When a stone fell near Luce, France, in 1768, it was studied by French chemist Antoine Lavoisier and two other French scientists, who concluded that it was an ordinary stone struck by lightning.

In 1794, Czech acoustic scientist Ernst Chladni published an account of numerous reported meteorite falls, giving strong evidence that some of them must be of extraterrestrial origin. He stated that the flight of such an object through the atmosphere caused the bright, luminous phenomenon known as a fireball. Chladni found few supporters for this idea, as most held to Aristotle’s view that comets and flashes of light across the sky were atmospheric phenomena (the word “meteor” comes from the Greek word for things related to the atmosphere, as in meteorology). Chladni’s cosmic theory of meteors was finally confirmed in 1798 by two students at the University of Göttingen, H. W. Brandes and J. F. Benzenberg, who had read his book. They made simultaneous observations of shooting stars from two locations separated by several kilometers and used a simple triangulation method to show that the light flashes originated at least eighty kilometers above the ground from objects moving several kilometers per second from a source beyond the atmosphere. Most doubts about meteorite falls were removed after the physicist Jean-Baptiste Biot reported an unusual fall of two or three thousand stones at L’Aigle in 1803, which eyewitnesses said was preceded by a rapidly moving fireball and explosion.

On a clear, dark night, a diligent observer may be able to witness, on average, six meteors per hour. More are visible after midnight than before, increasing to a maximum just before dawn. In the 1860s, Italian astronomer Giovanni Schiaparelli, famous for his discovery of Martian canali (channels), explained the increase in meteors at certain times resulting from the Earth’s orbital and rotational motion. Before midnight, the observer is on the trailing side of the Earth’s motion and can see only those meteors that overtake the Earth. After midnight, an observer is on the leading side of the Earth’s motion and will intercept meteors in front of it. Thus, meteors will appear brighter because they enter the Earth’s atmosphere at a higher velocity. Because the Earth’s orbital velocity is about thirty kilometers per second, and the escape velocity from the Sun at the Earth’s orbital distance is about forty-three kilometers per second, solar-system objects should range in speed from thirteen to seventy-three kilometers per second. Because no meteors have been observed with a faster speed, it is believed that they come from within the solar system rather than from interstellar space.

Most meteors become visible about 100 kilometers above the Earth’s surface and are completely consumed when they reach about seventy kilometers, although a few larger ones reach about fifty kilometers. Most meteors range in size from a few microns up to several millimeters. Survey estimates indicate that about twenty-five million meteors are bright enough to be seen over the entire Earth in any twenty-four-hour period. Telescopic surveys suggest that several billion meteoroids enter the Earth’s atmosphere every twenty-four hours, with an average total mass of about 100,000 kilograms. Most of this is consumed in the atmosphere as meteoroids are heated by friction to incandescence, but on average, about 1,000 kilograms per day are deposited on the Earth as meteorites.

More than half of all meteors are called sporadic because they appear at any time and from any direction in the sky. The remaining meteors are associated with meteor showers that appear to radiate from a common point in the sky called the radiant. They move along parallel paths but appear to diverge from the radiant—much like the divergence of railroad tracks when viewed in perspective. Meteor showers recur annually, about a tenfold increase over the usual sporadic rate. They are named for the constellation in which the radiant appears. Annual showers occur when the Earth crosses a meteoroid stream that fills the orbit of a comet, while periodic showers occur less frequently when the Earth crosses a meteoroid swarm in the wake of a comet. The most spectacular intermittent meteor showers are the Leonids, whose radiant is located in the constellation Leo. Historical records as far back as 902 CE mention the Leonids. A spectacular display on October 14, 934, is described in Chinese, European, and Arabic chronicles. The Japanese recorded a six-hour display in 967, and Chinese records continued to describe them every thirty-three years for several centuries.

The modern study of meteor showers began with the famous naturalist Baron Alexander von Humboldt. He observed the Leonids by chance during a trip to South America in 1799 in a two-hour display of hundreds of thousands of meteors. Humboldt was the first to suggest that these meteors might originate from a common point in the sky. The greatest Leonid display in the nineteenth century was observed in the United States and Canada on November 12, 1833. About one thousand meteors per minute were counted, and the appearance of the radiant was confirmed. The following year, two Americans, D. Olmstead and A. C. Twining, suggested that the annual Leonids were caused by the Earth passing through a cloud of meteoroids each November. A few years later, German astronomer Heinrich Wilhelm Olbers proposed that the more intense periodic meteor showers of 1799 and 1833 were caused by a denser swarm of the Leonid meteoroid stream. In 1864, H. A. Newton of Yale College reached the same conclusion independently and showed a period of recurrence of just over thirty-three years from historical records, beginning with the shower of 902. Their prediction of a spectacular display in 1866 was confirmed. Later, English astronomer John Couch Adams, who theorized the existence of the planet Neptune, succeeded in computing the Leonid stream orbit.

In the 1860s, other meteoroid streams were identified and traced back through history. Records back to the tenth century in England recorded meteor showers associated with the festival of St. Lawrence (August 10), known as “the tears of St. Lawrence” but now identified as the August Perseids from their radiant in Perseus. In 1861, the American astronomer Daniel Kirkwood, who later discovered gaps in the asteroid belt, suggested that meteor showers result from debris left in the wake of a comet through which the Earth occasionally passes. In 1866, Schiaparelli announced that the August Perseids appeared to occupy the same orbit as Comet Swift-Tuttle (1862 III). Soon after, Urbain Le Verrier and C. A. F. Peters identified the November Leonids with Comet Tempel-Tuttle (1866 I), which had a recurrence period of thirty-three years. The May Aquarids and the October Orionids have been associated with Halley’s comet. The greatest naked-eye meteor observer was W. F. Denning, who published an 1899 catalog of several thousand radiants, mostly of minor meteor showers of less than ten meteors per hour, based on more than twenty years of observation.

Like comets, meteor streams may be perturbed by planets into new orbits. Those with high inclinations to the ecliptic plane (the plane of the Earth’s orbit) or in retrograde orbits (opposite to the Earth’s motion) are least affected, such as the Leonids, Perseids, and Lyrids. After the Leonid display of 1866, the main body of the stream passed close to Jupiter and Saturn. Its associated comet could no longer be found, and only a few meteors were observed in 1899 and 1933. The comet was found again in 1965, and then, on the morning of November 17, 1966, the Leonids returned with meteors as bright as Venus. Viewed from the western United States, they reached a maximum rate of more than two thousand per minute before dawn, producing the greatest meteor display in recorded history.

Only the bright fireball meteors, sometimes brighter than the full Moon, are produced by meteoroids large enough to survive passage through the Earth’s atmosphere and fall to the ground as meteorites. Almost all these are sporadic meteors; even among the fireballs, less than 1 percent yield meteorites. Dozens of meteorites fall to the surface of the Earth each day, but very few are recovered. About 95 percent of falls (seen falling and then recovered) are classified as stones (about 75 percent silicates and 25 percent iron), but 65 percent of finds (whose associated meteors are not observed) are irons (90 percent iron and 8 percent nickel) because irons are easier than stones to identify on the ground as meteorites.

Dozens of craters apparently formed by large meteorites have been identified around the world. The first such identification was made about 1900 by Daniel Barringer at Canyon Diablo in Arizona. This crater is 1.3 kilometers across and 180 meters deep, with a rim rising forty-five meters above the surrounding plain. About 25,000 kilograms of iron meteorite fragments have been found in and around the crater. It is estimated that the crater was formed by an explosive impact about 50,000 years ago from a sixty-million-kilogram meteorite. In 1908, a brilliant fireball meteor exploded in the Tunguska region of Siberia, leveling trees over a distance of thirty kilometers and killing some 1,500 reindeer. No large crater or meteorite has been found, but its effects were estimated to be equivalent to the explosion of a billion-kilogram meteoroid. In 1972, a fireball meteor with an estimated mass of a million kilograms was photographed in daylight some sixty kilometers above the Grand Teton Mountains before leaving the atmosphere over Canada.

Methods of Study

Information about meteors can be obtained with the unaided eye, but much greater scope and precision results from the use of photographic, radar (radio echo), and space-probe techniques. Modern photographic meteor observations were begun by Fred Whipple in 1936 at the Harvard College Observatory using short-focal-length, wide-angle cameras. These were later replaced by ultra-fast Super-Schmidt cameras that could detect meteors as small as a milligram. To measure the height, direction, and velocity of a meteor, simultaneous photographs of the meteor trail are taken from two stations separated by about fifty kilometers. Each photograph shows the positions of the meteor trail against the background of stars from each station so that its trajectory can be calculated by triangulation. The velocity of the meteor is measured by using a rotating shutter to interrupt the meteor trail up to sixty times per second. The velocity vector and the known position of the Earth in its orbit make it possible to compute meteor orbits.

The density of a meteoroid can be estimated from its deceleration in the atmosphere, showing that most meteoroids are of lower density than meteorites. Statistical studies have demonstrated that meteoroids with the most incredible meteor heights have average relative densities of 0.6, while another group appearing about ten kilometers lower has relative densities averaging 2.1. The few meteoroids that penetrate the atmosphere have average relative densities of 3.7. Several hundred fireball meteors have been photographed, their masses ranging from 100 grams to 1,000 kilograms, including one meteorite fall near Lost City, Oklahoma. Experiments with artificial meteors and theories of meteor burning led to estimated initial meteoroid masses from observed optical effects. Meteors comparable in light to the brightest stars have initial masses of a few grams and diameters of about one centimeter, producing more than a megawatt of power. Meteor showers are produced by the most fragile (lowest-density) meteoroids, and different showers produce meteoroids of other characteristics. Generally, short-period comets exposed more often to the Sun produce higher-density particles than long-period comets because of greater evaporation.

During World War II, it was accidentally discovered that meteors could be detected by radar. The radar method of studying meteors is especially valuable because it can detect meteors in daylight and is sensitive to meteoroids as small as a microgram. This method depends on the fact that meteors separate electrons from atoms, producing ionized gases that can reflect radio waves. Meteor heights can be measured from the time delay of the return signal, and velocities can be determined from the frequency shift (Doppler effect). Observations from three stations are needed to calculate a meteoroid orbit. Several important meteor showers that occur only in daylight hours were discovered by radar, including the Beta Taurids, which are probably associated with Comet Encke. Radar also shows that radiants can be complex structures that appear to overlap and shift positions within a few hours.

Micrometeoroids with masses of a few micrograms or less have been collected by high-altitude aircraft and rockets. Micrometeoroids are fluffy particles containing carbonaceous material different from normal meteorites but consistent with comet theories. They can be studied with microphone detectors in space probes by measuring the intensity of their collisions. The weak structure of these particles indicates that they are gently separated from their parent material, suggesting dust emitted from evaporating ice in a comet rather than violently ejected from high-temperature or colliding meteoroids. Particles of less than a milligram contribute the largest fraction of the total mass swept up by the Earth each day. Rocketborne mass spectrometers have recorded metallic ions (charged atoms) of apparent meteoric origin, and meteor spectroscopy has provided chemical analysis of all major meteor streams. These data indicate significant differences between cometary meteor material and the composition of meteorites.

Radioactive-dating techniques indicate that most meteorites have existed as solid bodies for about 4.5-4.7 billion years, close to the estimates for the Earth, Sun, and Moon ages. This suggests that all the solar system's matter condensed at approximately the same time. Cosmic-ray dating from the unusual isotopes produced in a meteorite by cosmic rays colliding with atoms in its crystalline structure usually indicates only a few million years since its formation, presumably by some fragmentation process from a giant asteroid. Fine bands are also observed in such meteorites, similar to those in metal crystals subjected to sharp collisional shock. This finding has led to the idea that meteorites probably come from asteroids that were shattered in collisions.

National Aeronautics and Space Administration’s (NASA) Double Asteroid Redirection Test (DART) mission in 2022 created the first human-made meteor shower called the Dimorphids. The mission’s intent was to assess the agency’s asteroid deflection technology by intentionally crashing a spacecraft into an asteroid at 13,645 miles per hour (6.1 kilometers per second). The meteor shower was estimated to be able to last up to one hundred years.

Context

Meteors and meteor showers are not only interesting as visual phenomena but also provide one of the most important sources of information about asteroid and comet composition and deterioration and clues to the solar system's origin. Fortunately, most meteors are caused by tiny particles (less than one gram) and are completely vaporized high in the atmosphere. Meteors enter the Earth’s atmosphere with solar-system speeds and random inclinations to the ecliptic (plane of the Earth’s orbit); thus, most meteors are associated with comets or asteroids with small inclinations.

A cometary origin for most meteors is supported by the phenomenon of meteor showers, which can be traced to particle swarms in various orbits with random inclinations around the Sun. Many of these showers can be associated with comets or former comets. They appear to be caused by particles released when solar radiation evaporates cometary ice. These particles either concentrate in a swarm of meteoroids behind the cometary nucleus or eventually become distributed in a stream around the entire orbit of the comet. Annual meteor showers occur when the Earth crosses a meteoroid stream, while more intense periodic showers occur when the Earth passes through a meteoroid swarm. The densities and compositions of these meteoroids are also consistent with a cometary origin.

The few meteoroids large enough to survive their passage through the atmosphere and yield meteorites are associated with the rare fireball meteors. Their trajectories tend to have low inclinations to the ecliptic, similar to asteroids. The metal's crystalline structure in meteorites indicates that most were formed at high temperatures and slowly cooled over several million years. Thus, they did not appear from icy comets but probably originated with asteroids. Calculations show that the rocky outer shell of an asteroid would insulate its hot metallic core, causing it to cool at the prolonged rate suggested by the crystalline structure of iron meteorites. Furthermore, the cooling rate in a planet is too slow to fit this observed crystal pattern.

Meteoroids in asteroidal orbits enter the atmosphere with an average velocity of about twenty kilometers per second. Most are slowed rapidly by the atmosphere. If they survive as a meteorite, they fall to the ground at free-fall speeds and cool rapidly since most of the hot surface material is swept away. Meteoroids larger than about one million kilograms (ten-meter-sized) strike the ground with most of their initial velocities, producing impact craters. A fifty-meter, one hundred-million-kilogram object moving at high speed can create a one-kilometer-wide cavity, causing widespread devastation by its shock waves and throwing dust into the upper atmosphere, with marked effects on climate and life on Earth. Some evidence from large craters and geological layers of meteorite debris suggests the possibility that kilometer-sized objects strike the Earth about every twenty-six million years, coinciding with significant extinctions of life forms. One attempt to explain these data theorizes that the Sun has a dim companion star in a twenty-six-million-year eccentric orbit. At its closest approach to the Sun, its gravity would disturb many comets in the outer solar system, causing some of them to strike the Earth.

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