Measurement Of Meteoroid Orbits

Even though the likely sources of most meteoroids entering Earth's atmosphere are known, the most direct way to determine the number and types of meteoroids coming from each of these sources is by measuring their orbits. When two or more observers at well-separated locations document the same meteor in the sky and determine its coordinates, the direction in which the meteoroid was moving in space before it encountered Earth—i.e., its radiant—can be estimated reasonably well by triangulation. To determine the meteoroid's orbit, however, also requires ascertaining its speed.

This latter requirement was satisfied in the 1940s with the introduction of wide-field astronomical cameras specially designed for studying meteors. Each camera was equipped with a rotating shutter that interrupted the light to the photographic plate at a known rate. The resulting breaks in the photographed meteor streak permitted calculation of the speed of a meteor along its path. The position of the meteor's trajectory with respect to the stars photographed on the same plate also was measured accurately. Information from such observations made at two or more stations could then be combined to calculate precisely the orbit of the mete-oroid before it encountered Earth. About the same time, special radar instruments also were applied to the study of meteors generally fainter than those observed photographically.

A very significant development in meteor science occurred about two decades later. This was the establishment of large-scale networks for photographing very bright meteors, or fireballs. These networks were designed to provide all-sky coverage of meteors over about a million square kilometres (386,102 square miles) of Earth's surface. Three such networks were developed—the Prairie Network in the central United States, the MORP (Meteorite Observation and Recovery Project) network in the Prairie Provinces of Canada, and the European Network with stations in Germany and Czechoslovakia. The most complete set of published data is that of the Prairie Network, which was operated by the Smithsonian Astrophysical Observatory (later merged into the Harvard-Smithsonian Center for Astrophysics) from 1964 to 1974.

Apart from measuring meteoroid orbits, one of the goals of the fireball networks was to determine probable impact areas based on the observed meteor paths and recover any surviving meteorites for laboratory studies. This would enable a comparison of the inferences of theory regarding the density and mechanical strength of meteoroids with "ground truth" provided by the study of the same meteorites in the laboratory. The networks compiled data that became the basis for a new outlook on meteor science and the sources of meteoroids, but the goal of recovering meteorites had only limited success. Only three meteorites were recovered, one by each of the networks. All three meteorites were ordinary chondrites, the most abundant type of stony meteorite.

In spite of this meagre recovery record, the study of the recovered meteorites not only confirmed that they came from the asteroid belt but also led to an improved understanding of what happens to meteoroids when they enter and travel through the atmosphere. This enabled better estimates of the physical properties of meteoroids, allowing researchers to distinguish between meteors resulting from dense, meteorite-like objects and meteors resulting from less substantial objects that, for instance, might come from comets. Prior to this effort, studies of meteors by astronomers and of meteorites by geochemists tended to be pursued as independent scientific fields that had little to contribute to each other.

RESERvOIRS OF METEOROIDS IN SPACE

Most of the mass of the solar system resides in its larger bodies, the Sun and the planets. On the other hand, a smaller fraction of the mass of the solar system is in objects of such small size or in orbits so eccentric (elongated) that their physical survival or orbital stability has been in jeopardy throughout the history of the solar system. Many of these bodies are now found in the asteroid belt, between the orbits of Mars and Jupiter. Small bodies would have been distributed throughout the early solar system, but most were rapidly swept up by the planets during a period that ended about four billion years ago. The flux of material from space that now falls on Earth (in the range of tens of thousands of tons per year) and other planets pales in comparison with this early intense bombardment. Since that time, large impacts have become relatively rare. Nevertheless, when they do occur, the results can be dramatic, as in the case of the impact of Comet Shoemaker-Levy 9 with Jupiter in 1994 or the impact of an asteroid or comet thought to be responsible for the extinction of the dinosaurs and other species at the end of the Cretaceous Period 65 million years ago.

For an object to hit Earth, it must be in an orbit which crosses that of Earth. In the case of the rocky asteroids and their fragments, a limited number of processes can put these bodies into Earth-crossing orbits. Collisions can inject material directly into such orbits, but a more efficient process involves gravitational resonances between asteroidal material and planets, particularly Jupiter. Dust particles can also be moved into an Earth-crossing orbit from the asteroid belt through interactions with solar radiation. In the outermost solar system, icy objects in the Kuiper Belt and Oort cloud, which are believed to be the source reservoirs of comets, are perturbed into orbits that ultimately become Earth-crossing through gravitational interactions with Neptune or even with passing stars and interstellar clouds. When such a frozen body travels inside Jupiter's orbit and approaches the Sun, it gives off gas and sheds small particles, typically taking on the characteristic appearance of a comet. Some of these particles remain in the vicinity of the comet's orbit and may collide with Earth if the orbit is an Earth-crossing one.

The asteroid belt and the outermost solar system, therefore, can be thought of as long-lived though somewhat leaky reservoirs of meteoroidal material. They are long-lived enough to retain a significant quantity of primordial solar system material for 4.567 billion years but leaky enough to permit the escape of the observed quantity of Earth-crossing material. This quantity of Earth-crossing material represents an approximate steady-state balance between the input from the storage regions and the loss by ejection from the solar system, by collision with Earth, the Moon, and other planets and their satellites, or by impacts between meteoroids.

In addition to cometary and asteroi-dal sources, a very minor but identifiable fraction of the meteoroidal population comes from the Moon and Mars. From studies of meteorites, scientists believe that pieces of the surfaces of these planets have been ejected into space by large impacts. They also know from the deep-space missions of the Galileo and Ulysses spacecraft that dust particles from outside the solar system are streaming through it as it orbits the centre of the Milky Way Galaxy. These interstellar grains are small and are traveling at high velocities (about 25 km [15 miles] per second) and thus have high kinetic energies, which must be dissipated as heat. Consequently, few are likely to survive atmospheric entry. Even if they do survive, they are rare and hard to discriminate among the much more abundant asteroi-dal and cometary dust.

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