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meteor stream[′mēd·ē·ər ‚strēm]
an aggregate of meteors visible in the atmosphere during the earth’s encounter with a meteor swarm —meteoroids moving in adjacent orbits and having a common origin. Sometimes the term “meteor stream” is also used to designate the meteor swarm itself that gives rise to the given meteor stream. The trajectories of all the meteors in a stream are almost parallel and appear to diverge approximately from a single point—the radiant of the meteor stream. Streams with a large number of meteors are named for the constellation in which the radiant lies or for the nearest bright star. Meteor streams are observed on approximately the same dates annually or once every several years. Several tens of nighttime meteor streams have been identified in the 19th and 20th centuries through visual observations. Radar observations of meteors have permitted the study of daytime streams as well. The orbits of several hundred meteor swarms have been determined by means of photographic and radar observations; most are similar to cometary orbits, primarily those of short-period comets. The orbits of several dozen meteor swarms lie close to the orbits of known comets; the connection between meteor swarms and known comets has been established in about 15 cases. (See Table 1.)
Meteor swarms are formed as a result of the disintegration of the nuclei of comets and initially move as a compact group occupying only a portion of the comet’s orbit. When they encounter the earth, the young compact swarms produce short-lived meteor streams with a very high number of meteors; such streams are called meteor showers. Under the action of gravitational perturbations by the planets, the Poynting-Robertson effect, and other factors, a meteor swarm gradually elongates along the orbit, expands, and finally disintegrates.
|Table 1. Principal meteor streams|
|Stream||Period of activity||Date of maximum||Associated comet|
|Quadrantids||Dec. 27-Jan. 7||Jan. 3-4||231||+50|
|Lydrids||Apr. 15-26||Apr. 21||272||+32||1861 I|
|η-Aquarids||Apr. 21 -May 12||May 4||336||00||1910 II Halley|
|Arietids||May 29-June 19||June 7||45||+23|
|Southern δ-Aquarids||July 21 -Aug. 15||July 29||339||-17|
|Perseids||July 25-Aug. 20||Aug. 12||46||+58||1862 III Swift-Tuttle|
|Draconids||Oct. 8-12||Oct. 9-10||268||+60||1946 V Giacobini-Zinner|
|Orionids||Oct. 14-26||Oct. 21||95||+15||1910 II Halley|
|Leoaids||Nov. 10-20||Nov. 16||152||+22||1866 I|
|Geminids||Dec. 1-17||Dec. 13-14||112||+32|
Some currently observable meteor streams—for example, the Lyrids and the Perseids—have been known for several thousand years. Some meteor swarms that formerly produced active meteor showers—for example, the Andromedids and the Bootids—have moved out of the earth’s orbit because of planetary perturbations.
V. N. LEBEDINETS