Introduction to Meteors

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Updated February 14, 2004

An Introduction to Meteors

Introduction

Almost everyone has seen a shooting star. Fewer have heard a scientific explanation of what they saw: an interplanetary particle colliding with Earth's atmosphere at extremely high speed being vaporized in a streak of light. The technical term for shooting star is a meteor, for its progenitor interplanetary particle a meteoroid.

Meteoroids are distributed in space in two primary ways. Individual particles are randomly dispersed; when one of sufficient size collides with Earth a sporadic meteor can be seen. Such are visible to a single observer at a rate of 1-5 per hour on any given night. Many of the very brightest meteors, known as fireballs or bolides, are of this category. Occasionally one is large enough to withstand its fiery passage through Earth's atmosphere and hit the ground. Remnants which may be recovered are known as meteorites.

However, meteoroids also occur in related groupings known as meteoroid streams. There are loose, asymmetric rings of particles orbiting the Sun. In general these streams resemble the highly-elliptical orbits of comets, and indeed this is no accident; such meteoroids are in fact debris from periodic comets, or in exceptional cases asteroids, dislodged from the parent body during its repeated passage near the Sun. If the meteoroid stream crosses Earth's orbit, the result will be a meteor shower.

Showers occur on very predictable dates when Earth returns to the same position in its own orbit. Meteors related to the shower will appear to radiate from a single point, the radiant, but can be seen all over the sky. Typically meteor showers are named after the constellation in which the radiant is located. A list of major showers appears here.

Each meteor shower has its own characteristic duration, numbers of meteors (typically standardized to a zenith hourly rate), speed, average brightness, likelihood of residual luminous streaks or trains, and even colours.

Over the eons, meteor streams spread out over a much broader orbit than that of the parent comet, in many cases millions of km across. It can take Earth several days or weeks to plough through them, resulting in a long lasting display; in such cases a range of dates is given, with the peak of the display occurring very near the mid-point of the stream. In some cases the parent comet has had its orbit altered over time by interactions with planets, primarily Jupiter, leaving behind discrete threads or filaments of material. The related meteor stream may be considered as a bundle of such threads, resulting in increases in meteor activity as Earth passes through the individual filaments.

Meteoroids will also spread out throughout the entire orbit over very long periods of time. However, in some cases involving an active comet, meteoroids tend to be bunched fairly closely around the comet itself in a meteoroid swarm. The best known example of this is the Leonids, whose parent comet, Tempel-Tuttle, has an orbital period of 33 years. When the comet makes a pass of the Sun, as it did in 1998, Earth is more likely to pass through such a swarm, resulting in a series of meteor storms over the subsequent few years.

History

Meteors have been observed throughout recorded history and before. While there have been many cultural myths and legends about them, scientific study of them is fairly recent, with much work still to be done. That they might have an extraterrestrial origin was posited by none other than Sir Edmund Halley after observing a fireball in 1686. A century later Ernst Chladni's study of two meteorites suggested neither was of terrestrial origin. Although heavily criticized, Chladni's theory was vindicated a few years later when a pair of students observed meteors from two different locations, and observed the same ones in different areas of the sky. Using triangulation, they estimated meteors became visible at an average height of 97 km; the derived velocities of several km per second suggested they originated outside Earth's atmosphere.

On November 13, 1833, a great Leonid storm was observed in eastern North America. Although he was wrong on some of the specifics, Denison Olmsted correctly concluded that a cloud of particles was responsible.

The 1860s saw the birth of modern meteorics. Daniel Kirkwood suggested that meteor showers were caused by the debris of old comets. He was proven spectacularly right within five years. First, the derived orbit of Comet Swift-Tuttle, discovered in 1862, was found to agree very closely with the radiant of the well-known Perseid shower. Then Comet Tempel-Tuttle was discovered in late 1865, and its orbit compared favorably with that of the Leonids, particularly in its 33-year period that was already associated with the Leonid storm activity. Another great storm was indeed observed in November 1866, and the causal relationship was verified.

The orbits of most significant showers have subsequently been associated with known comets, or in isolated cases (notably the December Geminids) asteroids. Others are thought to have been caused by comets which have disintegrated completely or at least beyond visibility.

By the mid-20th century, new methods of observation had been developed, including photographic, radio-echo, and radar detection. These more precise methods have been used to calculate radiants, velocities, and orbits.

Visual Observation of Meteors

Meteor watching is like fishing, only easier; one can simply relax and enjoy the canopy of the sky while waiting for a bite. It is easy for the observer to feel connected to the cosmos, the link between the heavens and the Earth being provided by the meteors themselves.

Meteor showers provide a most accessible and rewarding observing experience, requiring no optical aid whatever. "Equipment" requirements include a comfortable lawn chair or mattress, a sleeping bag, bug spray, and warm beverages. Clear skies are a must, preferably (but not necessarily) dark, rural skies. Bright moonlight will hamper observations, which is dependent on the luck of the calendar from shower to shower and from one year to the next.

Meteor observing can be done alone, but is most enjoyable when done in a group with family or friends. Limited knowledge of basic concepts such as constellations and star magnitudes is helpful, which can be learned on the spot if a more experienced observer is present.

Observers are encouraged to count the number of meteors they see, and record them at regular intervals. A wristwatch with a timer function and a microcassette recorder are useful accessories.

Radio detection of meteors

Although the rudimentary method originated in 1931, radio methods really came into their own in 1947, both on the amateur front as well as a major study undertaken by the Jodrell Bank in England. From 1958-67 much important work was done in this area by the now-defunct Springhill Meteor Observatory in Ottawa. This continues to be an area of sporadic professional research and continuous amateur monitoring.

The basic method is very simple. As a meteor streaks through the atmosphere, some 10% of its energy is released as light; the remainder is dispersed in a trail of ionized air, or ionization train. This cylinder of electrified air will reflect radio waves of the FM and UHF frequencies. Unlike AM, such signals do not conform to the curvature of the Earth, and a receiver beyond the direct line of sight to the transmitter cannot pick up its signals unless they are reflected in some manner. For example, an FM receiver in Edmonton tuned to a station in Calgary will normally receive nothing but static and white noise. However, when a meteor occurs in the right area of the sky, the ionization train will reflect the signal for up to several seconds. This can be detected by listening to the receiver, or by connecting it to some sort of recording device.

Radio detection has several distinct advantages over its optical counterpart. Whereas visually observing meteors requires clear, dark, and preferably rural and moonless skies, radio observation can be done day and night, in fair weather and foul, in the comfort of a home office or classroom environment. It is particularly useful for observing daytime showers, which occur when the meteoroid stream is on that portion of its orbit which is outbound from the Sun. Such showers cannot be observed visually, and were indeed discovered using radio methods. Many of the best visual showers occur at least partly during daytime hours, so observations can be extended through radio. Also, a percentage of meteors observed visually can be correlated with radio observations, providing a valuable comparison and cross-reference of observations and data. For more information on radio meteor detection follow this link.

Sources

* Meteor Showers: A Descriptive Catalog, Gary W. Kronk * RadioScience Observing, Joseph J. Carr * Observing Comets, Asteroids, and the Zodiacal Light, Stephen Edberg and David Levy * Observe Meteors, David H. Levy and Stephen J. Edberg * The Astronomcial Companion, Guy Ottewell * Observer's Handbook 2001, Rajiv Gupta, Editor * Cosmic Collisions, Dana Desonie * Target Earth, Duncan Steel * Handbook of Space Astronomy & Astrophysics, 2e, Martin V. Zombeck * Planetary Sciences, Imke de Pater and Jack J. Lissauer

 

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