14. Comets

    • The sudden apparition, changing shapes, and unpredictable movements of comets have puzzled humanity for centuries. To ancient cultures they were harbingers of disaster and portents of great events.

    • Comet Halley has returned to fascinate and frighten the world for more than 2000 years.

    • Long-period comets, with orbital periods greater than 200 years, have been tossed into the planetary realm from a remote, spherical shell, named the Oort cloud, located about a quarter of the way to the nearest star.

    • A million million, or 1012, invisible comets have been hibernating in the deep freeze of the Oort comet cloud since the formation of the solar system 4.6 billion years ago.

    • Many Jupiter family comets, with orbital periods of less than 20 years, probably came from the Kuiper belt, which lies in the outer disk of the planetary system beyond the orbit of Neptune and may contain more than a billion unseen comets.

    • Comets light up and become visible for just a few weeks or months, when their orbits bring them near the Sun. The solar heat then vaporizes some of the comet water ices, permitting the comets to grow large enough to be seen. The water ice sublimates, or turns directly from solid ice to water vapor.

    • The solid comet nucleus is just a gigantic ball of water ice, other ices, dust and rock. Some of the comet nuclei are about the size of Paris or Manhattan and roughly one billionth the mass of the Earth. Others comets are much smaller.

    • No two comets ever look identical, and every comet changes shape and form as it whips around the Sun, but they all develop a glowing spherical cloud of gas and dust, known as the coma, when moving close enough to the Sun.

    • The comet coma can be larger than the Earth and as big as the Sun, and around the coma there is an even larger envelope of atomic hydrogen, known as the hydrogen cloud, that shines in ultraviolet light.

    • Some comets develop tails that flow away from the Sun, briefly attaining lengths as large as the distance between the Earth and the Sun, but other comets have no tail at all.

    • Comets can have two kinds of tails, the long, straight ion tails, that re-emit sunlight with a faint blue fluorescence, and a shorter, curved dust tail that shines by reflecting yellow sunlight.

    • The Giotto, Deep Space 1, Stardust, and Deep Impact spacecraft have respectively peered into the icy heart of four comets, Halley, Borrelly, Wild 2 and Tempel 1, showing that their nuclei are blacker than coal and reflect just a few percent of the incident sunlight.

    • Gas and dust jet out from the sunlit side of the nucleus of comet Halley, from fissures in its dark crust, but nearly 90 percent of the surface of its nucleus was inactive at the time of the Giotto encounter.

    • Comet Borrelly is covered with a dark, unreflective carbon-rich material, and contains surface features that are most likely supported by solid water ice.

    • Comet Wild 2 has a relatively young surface, exposed just a few times to the Sunís intense heat, and it has a dark, pockmarked surface with pits, craters and jets of gas and dust.

    • The Stardust spacecraft gathered dust from the coma of comet Wild 2 in January 2004, returning the dust in a capsule that was parachuted to Earth two years later. The returned comet dust contains a mix of minerals formed at both cold and high temperatures, two types of nitrogen-rich organic molecules, and the amino acid glycine.

    • The Deep Impact spacecraft collided with comet Tempel 1, on 4 July 2005; spectroscopic examination of the ejected cloud of dust revealed fine porous material, water vapor, water ice, carbon dioxide, hydrocarbons, and silicates or sand.

    • When a bright comet nears the Sun, it turns on its celestial fountains, spurting out about a million tons of water each day.

    • The recoil effect of jets of matter ejected from a cometís spinning, icy nucleus can push a comet along in its orbit or oppose its motion, causing the comet to arrive closest to the Sun earlier or later than expected.

    • Most of the comets seen during recorded history will vanish from sight in less than a million years, either vaporizing into nothing or leaving a black, invisible rock behind.

    • About 40 thousand tons of small, cosmic dust particles fall onto the Earth in a typical year, wafting gently through the atmosphere to the ground.

    • Visible comets are in their death throes, but they may carry the residues of creation in their ice and dust.

    • Meteor showers, commonly known as shooting stars, are produced when sand-sized or pebble-sized pieces of an icy comet burn up in the atmosphere, never reaching the ground.

    • Comets strew particles along their orbital path as they loop around the Sun, and when the Earth passes through one of these meteoric streams a meteor shower occurs, recurring at the same time every year.

Copyright 2010, Professor Kenneth R. Lang, Tufts University