in interplanetary space, solid bodies (meteoroids) that are smaller than the asteroids and comets and move around the sun. On encountering the earth, meteoroids give rise to meteors and fall to the earth’s surface as meteorites. Minute meteoroids scatter sunlight intensively and are observed in the form of the zodiacal light.
The orbits of several tens of thousands of meteoroids have been determined from photographic and radar observations. By far the greater number move in elliptical orbits around the sun. Meteoroids having indisputably hyperbolic orbits, which would indicate that they came to our solar system from interstellar space, have not been observed. Meteoric matter is concentrated in the plane of the ecliptic and has mainly direct motion; that is, it moves in the same direction as the planets.
The motion of meteoroids is determined by the gravitational attraction of the sun and planets and by nongravitational forces that arise as a result of the interaction of meteoroids with electro-magnetic and corpuscular solar radiation (for example, light pressure and the Poynting-Robertson effect). Light pressure may drive very small meteoroids, measuring less than 10-4 cm in size, from the solar system. Under the action of the Poynting-Robertson effect, the size and eccentricity of the orbit decrease gradually (the smaller the meteoroid and the dimensions of the orbit, the faster this occurs), as a consequence of which the meteoroid approaches the sun in a spiral. En route to the sun, the meteoroid may be captured by planets; capture by Jupiter is predominant. The “barrier” created by Jupiter can be overcome only by very small meteoroids. The lifetime of meteoroids in the central part of the solar system (within the orbit of Jupiter) is much less than the age of the solar system, and consequently the meteoric matter here must be continually replenished. Different sources of meteoric matter are possible and include the debris of disintegrated comets and broken up asteroids and very small meteoroids arriving from the outer regions of the solar system.
The vast majority of large meteoroids have orbits similar to those of comets (chiefly of short-period comets) and are apparently formed as a result of the disintegration of comets. The range of orbits of smaller meteoroids observable only by radar methods is more complex; however, the lower accuracy and high selectivity of radar observations of meteors make it impossible to draw an unambiguous conclusion concerning the origin of these bodies.
About one-half of the bright meteors observed photographically occur in meteor streams; the rest are sporadic meteors. Among fainter meteors the proportion found in meteor streams is decreasing.
V. N. LEBEDINETS