Reading practice 8

Comets are solid masses of dust and frozen gases with diameters of only a few kilometers that revolve in highly eccentric orbits around the Sun. As a comet approaches the Sun, a very small portion of the frozen matter evaporates. This creates a shroud of gas and dust, called a coma, enveloping an area up to a million kilometers around the solid nucleus of the comet. Solar winds and radiation pressure from the Sun can blow the material of the coma away from the comet's nucleus, creating a tail, which₁ is sometimes longer than the distance from the Earth to the Sun. However, the appearance of comets is misleading; they cast no light of their own. Though the comas and tails of the brightest comets can be seen with the naked eye in cities with heavy light pollution, the nucleus of a comet cannot be detected even with the most powerful telescopes. This is not only because the solid portion of a comet is so small, but also because the highly reflective nature of the coma's material obscures₂ the view to the nucleus.

The brightness of a comet depends primarily on two factors: its distance from the Sun and its distance from the Earth. When comets are at their closest approach to the Sun, called perihelion,₃ evaporation of the icy material occurs at a greater rate and volume, and the solar forces that scatter the gas and dust are stronger. However, when comets are far from the sun, they become less active and are often undetectable. Because comets come from the farthest reaches of the solar system, most take over 200 years to orbit the sun, and most of the time they are so far away that the solar influence does not create a coma or tail, causing the comets to become invisible₄.

The stronger determinant of a comet's brightness is its distance to Earth, especially in relation to its perihelion. If a comet passes its nearest point to Earth after the comet's perihelion, it will be much brighter than if it reaches its closest point to Earth while it is still relatively cold and solid, before the Sun evaporates much of the comet's matter. This explains why Halley's Comet, which was very bright during its first observed pass near Earth in 1910, was so disappointing to astronomers when it returned, this time much further from the Earth,₅ in 1986. The distance from the Earth also determines a comet's speed as observed by astronomers—the closer a comet comes to the Earth, the more quickly it moves across the sky. Typically, comets move about one or two degrees per day—much too slow to be perceived by the naked eye—₆and can remain visible for months. However, when comet IRAS-Araki-Alcock, the closest comet to pass the Earth in modern times, appeared in 1983, it looked both very bright and very fast. This comet moved so quickly that observers compared its motion to that of the minute hand on a clock, and it had twice the apparent diameter of the Moon.

Even dedicated sky watchers and professional astronomers are more likely to discover a comet by chance than by exacting calculations, because comets are only detectable for such a short portion of their orbits, and because it is so infrequent that the comets pass near enough to the Earth to be observed. However, astronomers' interest in comets lies in characteristics beyond the novelty of these comets. Comets are believed to be remnants of the original disc of chemical material that formed the solar system about four billion years ago. Because comets spend most of their time in the very cold areas barely within the Sun's gravitation, they are believed to have remained relatively unchanged during that time, and can thereby serve as a sort of "fossil record" of the solar system.

For this reason, planetary scientists have a great deal of interest in studying comets directly, rather than merely through telescopic observation. By studying the specific chemical composition of comets, scientists hope to learn more about the chemical origins of the solar system. Explorations of comets can provide glimpses into this past. For example, a recent collection of tiny dust particles left in the Earth's stratosphere by the passage of comet 26P/Grigg-Skjellerup has led to the discovery of a previously unknown mineral that had not been predicted by scientists to have been formed in the solar nebula. This highly unusual substance generates strong scientific interest because it, along with other new materials that may be found in comets, may cause scientists to reconsider models of how the solar system₇ formed. Future missions are planned to retrieve material directly from comets. Some scientists, who hypothesize that water and some organic compounds may have been delivered to Earth by collisions between comets and our planet in its earliest days, hope that comet material may reveal information about the origins of life on Earth₈.