LONDON, November 14, 2013 — Is there a collective noun for comets? A single comet is a rare enough sight to have been regarded as an evil omen in ancient times. Our northern skies are graced with not one, but four comets this month. Comet C/2012 X1 (LINEAR) has recently brightened and spends most of this month in the constellation Boötes. Comet C/2013 R1 (Lovejoy) is travelling across Leo while Comet 2P/Encke is crossing Virgo, which also houses the much anticipated Comet C/2012 S1 (ISON).
Comets are thought to be material left over from the formation of the outer planets, although another theory is that many formed outside our solar system. There are currently almost 5,000 known comets, though there may be as many as one trillion in the outer Solar System.
A comet is a small nucleus of ice, rock and dust measuring between a few hundred meters to tens of kilometers across. They have orbits with periods ranging from several years to several millions of years. Encke has the shortest period of any known comet; it orbits the Sun every three years. Short period comets, with orbits lasting 200 years or less, originate in the Kuiper belt beyond Neptune while long period comets come from the distant Oort cloud.
Given a gravitational nudge, comets are pulled in from the outer solar system toward the Sun. Unlike asteroids, comets have an unbound atmosphere, called a coma, surrounding the nucleus. As they travel inwards, the sun’s light pressure, or solar wind, blows dust and gas out from the coma, forming a tail.
Often two tails can be seen: the gas, or ion tail always points directly away from the sun, while the dust tail is often curved. When earth’s orbit passes through the debris left in the wake of a comet we are treated to a meteor shower as the particles disintegrate in our atmosphere. This month’s Leonid meteor shower, for example, is courtesy of Comet 55P/Temple-Tuttle, while late October’s Orionids and the Eta Aquarids in early May are both due to Halley’s comet.
Repeated journeys in toward the sun and back out again can cause a comet to eventually lose all its volatile gases and dust and become an “extinct comet” that resembles a small asteroid. The two should not be confused — asteroids form inside the orbit of Jupiter — though the distinction has become blurred in recent times with the discovery of comets within the asteroid main belt, and with active asteroids such as P/2013 P5 that was recently spotted by the Hubble Space Telescope pretending to be a comet with no fewer than six tails caused by a rapid increase in the asteroid’s rotation.
Some comets pass extremely close to the sun, becoming “sungrazers”. Small sungrazers can be completely destroyed in the process, but larger comets, like C/2011 W3 (Lovejoy) which passed through the sun’s corona in December 2011, emerge intact and continue on back to the outer solar system.
Great comets occur about once every ten years. Halley’s comet (1P/Halley), probably the most famous of them all, can be seen every 75 to 76 years. It was the first comet to be observed in detail by the “Halley Armada,” which included the Giotto and Vega missions during its 1985/86 perihelion.
The most recent of the great comets, Comet C/2006 P1 (McNaught), became visible to naked eye observers in January 2007 and was the brightest in over 40 years.
We have a lot to thank comets for. Not only is it possible that they delivered water to the earth, it now seems that cometary impacts may have been responsible for kick starting life here. Scientists from the U.K.’s Imperial College and the University of Kent took water samples with chemical signatures similar to those found in various comets, they heat sterilised them to 932º F (600º C) and then froze them and fired steel projectiles at the samples from a gas gun to mimic the shock of a cometary impact. One sample containing carbon, hydrogen, and oxygen compounds produced not just the precursors, but amino acids themselves.
Predicting comets is notoriously difficult, with many taking astronomers by surprise, either by unexpectedly brightening, as we have seen recently with Comet C/2012 X1 (LINEAR) or failing to live up to expectations. There has been much hype in the media about Comet C/2012 S1 (ISON) being “The Comet of the Century” and becoming “as bright as the Full Moon,” but it could fizzle out at any moment. This is the first time ISON has made the trip around the sun, so who knows what may happen between now and November 28 when ISON reaches perihelion. However, with four comets in the sky there is no better time to give thanks for these rare celestial visitors.
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