The definition of a comet is a small gaseous body in planetary space consisting of a head and a tail. The name, from the Latin word coma, or “hair,” comes from its appearance. Comets obey the laws of gravity, as do the planets. All, or almost all, comets are part of the solar system. The motion of a comet, however, is different from that of the planets. The planets move in the same direction, in almost the same plane, and in nearly circular orbits around the sun. Comets travel in highly eccentric orbits in both directions, and their orbital planes can be inclined at any angle. Long-period comets (periods of 200 years or more) come from distances a thousand times greater than the farthest planets, and their orbital planes are inclined at virtually any angle. Short-period comets (periods of less than 200 years) come from the region near to or somewhat beyond the outermost planets, move in direct orbits, and their orbital planes are inclined at small angles. At a great distance from the sun, comets usually have no tail, but they may have a faintly visible head. The head grows and the tail develops as it nears the sun.

The term “meteor” comes from a Greek word that was applied to various phenomena in the atmosphere, but today refers to the arrival in the upper atmosphere of a solid particle from space. In the strict sense a meteor is the luminous trace produced as the particle disintegrates. Meteors are popularly known as shooting stars or falling stars; extremely bright ones are often called fireballs.

The number of meteor visible to an observer varies, but under good conditions, away from city lights and in the absence of bright moonlight, an observer may see five to ten meteors per hour. Most of these meteors will be considerably fainter than the brighter stars, with illumination times of about 1 second. Meteors are most frequent after midnight, because late at night an observer is on the forward side of the earth in its motion around the sun and the field of view tends to sweep up more particles. An observer can only see meteors within 300 miles, but but the number of meteors visible somewhere on the surface of the earth is several hundred per day.

Beginning in the 1940's, radar allowed astronomers to obtain echoes from meteors. The solid particles themselves are too small to detected, but they produce a column of electrically charged gas that reflects a radar beam. Radar, unlike photography, is as effective in daylight or in a cloudy sky as it is on a clear night. Radar can also study meteors too small to be recorded photographically. While astronomers more easily determine a visible meteoroid's precise direction in the sky with photographs, they measure its distance and velocities more accurately with radar.

The velocity at which meteoroids enter the atmosphere varies from 7 miles per second to 44 miles per second. The first value corresponds to the velocity an object would acquire falling toward the earth under gravity. This velocity is the same velocity a rocket must have to escape the earth's gravitational pull. The escape velocity from the sun is 26 miles per hour, while the escape velocity of the earth around the sun is 18 miles per second. If the earth meets a meteoroid head on, the maximum speed relative to the earth can therefore be as high as 44 miles per second for a meteoroid that comes from a distant region of the solar system. Any particle with greater velocity must have come from interstellar space. Lack of evidence for such thing strongly supports the idea that meteors are part of the solar system.The height at which a meteor first becomes luminous or detectable by radar depends on its entry velocity. For fast meteors this height can exceed 70 miles, and many of them disentegrate completely bove 50 miles. The slowest meteors may not be visible until slightly below this height, because their collisions with atmospheric particles are less violent. They therefore require a more dense level of the atmosphere to produce light. Meteors comparable to the brightest stars usually have small masses, typically about 0.01 ounce. Large meteors usually survive longer and reach lower end heights. They may slow down considerably due to atmospheric drag.

FUN FACT Observations from space show that each day the earth collects about 100 tons of microscopic dust particles that are too small to prod visual meteors.

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