Catch a falling star

What on Earth are comets? And why do they behave whimsically like they do?

IN DAYS of yore, the heavens were a vast expanse of terror. Unusual movements in the sky evoked a sense of wonder and fear. Solar and lunar eclipses were thought to be caused by monsters devouring the sun and the moon. Comets -- which seemed to appear from nowhere, zipped across the sky and then vanished -- were considered portents of doom.

The ancients' sense of wonder and fear may not have been entirely misplaced. Although comets did not portend famine, flood or death of kings, there is that outside chance that one of them might just drop out of the sky and crash into our planet. A cometary crash 65 million years ago is believed to have enveloped the earth with a thick cloud of dust that blocked out the sun, wiping out the dinosaurs.

However, the indications are that comets, being delicate bodies, would burn to cinders high up in the atmosphere. On the other hand, should a comet of 17 km diametre hit Earth, it could destroy life over an area as large as North America. The recent collision with Jupiter of the broken to pieces Shoemaker-Levy comet is only another reminder -- or warning -- that Earth could one day be a similar target.

Hairy ones
Earlier, in the absence of city lights and pollution, comets could be seen clearly by everyone. The word comet is derived from either the Greek kometes, meaning hairy one, or the Latin coma, which means hairy -- both aptly describe the bright comets spotted in those days.

Aristotle believed the heavens were perfect and unalterable. He thought comets emanated from the earth, lit up and were pushed to the upper atmosphere by heat. His views endured right until the 14th century, although in the first century AD, Roman philosopher Seneca had suggested that comets were "heavenly bodies".

Only after the 16th century -- and peaking in the 20th -- were there major breakthroughs in the study of these nebulous, cosmic streaks. Danish nobleman Tycho Brahe related the movement of the 1577 comet to the stars and calculated that the comet was located at least 4 times farther away than the moon, proving for the first time that comets were indeed heavenly bodies.

Back to earth,periodically
Then Isaac Newton found that the path of the Great Comet of 1680 was near-parabolic because its movement was governed by the sun's gravity. Newton's friend Edmond Halley went a step beyond. Halley studied the progress of 24 comets and found that the parabolic paths of comets were not open-ended, but part of an elongated ellipse that periodically brought them back to earth.

Sensational meteor showers associated with 3 comet flights between 1866 and 1885 led to the belief that comets consisted of dust or sand grains held together loosely. This concept, which became known as the "sandbank" model, held that the dust or grains were scattered along the orbit of the comet.

Early this century, however, spectroscopy revealed the presence of various chemicals. And much later, in the '70s, light gases were detected in cometary tails.

In 1950, Fred L Whipple proposed that comets were actually "dirty snowballs". When Comet Halley passed by in 1986, the European Space Agency spacecraft Giotto took the first close-up photos of a cometary nucleus and confirmed Whipple's idea. The Giotto photos showed up the very black colour of the nucleus, probably due to the presence of vast amounts of sootlike substances. The gases from Halley's comet were found to be mainly water vapour (more than 80 per cent), carbon monoxide (about 10 per cent), carbon dioxide (5 per cent) and small amounts of methane, ammonia and carbon disulphide.

For most of their lifetime, comets are essentially asteroid-like, eccentric bodies barging through the solar system. The nucleus, normally not more than a few km in diametre and covered by a dark crust, is its only permanent feature.

As the comet moves out of the frigid recesses of the solar system, the frozen dust and chemicals within the nucleus vapourise and escape from vents facing the sun. The vapour jets combine as they flow out, forming a fuzzy, bright coma -- a cloud of dust and gas. Although the coma contains only a fraction of the matter in the nucleus, its volume can be several times larger. Typically, comas can be 100,000 km or more in diametre, but comas with diametres of more than 800,000 km have been observed.

As the comet speeds closer to the sun -- at about the orbit of Mars -- the action of various forces like electromagnetic radiation and the solar wind (continuous emission of charged particles from the sun) drives the particles in the coma into tails, which almost always are away from the sun. The dust in the coma gets blown away by solar radiation and forms a wide, yellowish tail. A second tail, consisting of ionised gases dragged away by the solar wind, develops in a slightly different direction. It is usually narrow and bluish in appearance. Like the coma, cometary tails are enormous and can extend up to 1,000 million km. The coma contracts when the comet draws still closer to the sun, its tails becoming longer and brighter.

Mysterious origins
Back in 1932, Estonian-born astronomer Ernest J Opik proposed the existence of a cloud of unobservable comets somewhere around the solar system. In 1950, Dutch astronomer Jan Hendrik Oort established some evidence for just such a cloud. Later, Brian G Marsden and his colleagues confirmed the existence of this large, spherical cloud, which came to be known as Oort's cloud.

Oort showed that a few new comets are observed every year, passing for the first time through the solar system. He found that most of new comets originated in a vast region at the edge of the solar system. Oort hypothesised that about 100 billion comets moved around in a sort of "deep freeze" in this region until the gravitational action of a passing star stirred them out of the cloud and gradually, into the inner solar system.

The Oort cloud, bound by gravity to the solar system, is believed to have formed about 4,500 million years ago. Soviet astronomer V S Safronov theorised in 1972 that the planets originated from a ring of dust around the sun and cometary nuclei are nothing but the building blocks of planets.

Later, American astronomer A G W Cameron developed a similar model of cometary nuclei in which the planets formed at a distance much greater than the present solar system.

As comets travel repeatedly to the sun, the gravitational behaviour of the planets, especially Jupiter, alters their orbits gradually. Some are thrown out beyond the solar system into interstellar space, others go back to the Oort cloud and some are trapped in permanent orbits until they disintegrate.

Astronomers find the range of physical and chemical processes of comets intriguing. They are good tools for studying the solar wind. Many believe comets contain the oldest and best-preserved material in the solar system. If comets and planets formed at the same time, the structures of comets could throw some light on the early history of the solar system.

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