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Dubbed as Comet of the Century, ISON was in its full glory before going very close to the Sun. At least a part of it appears to have survived a close encounter with the Sun. It looks like the nucleus disintegrated and what you see now is basically the... remains.
It is not all bad news when a piece of 4.5-billion-year-old comet broke up into pieces, because scientists say they were able to study it and learn more about comets. One reason Comet ISON was thought to have a chance of survival was because a previous sun-grazing comet called C/2011 W3 (Lovejoy) did withstand an even closer pass by the sun in 2011. By comparing observations of both comets, scientists have theorized about what Lovejoy had that ISON lacked. Comet Lovejoy was not an Oort Cloud comet. It had been past the sun at least a couple times. It had perhaps built up a thick skin. Maybe that’s a factor. ISON's outer layer was raw and possibly volatile whereas Lovejoy had been burnt by the sun and could have had a hardier crust to withstand the solar heat and pull, according to the scientists who studied it. Another factor might be ISON's size. Determining the width of its nucleus is difficult, but the best estimates come from observations made by NASA's Mars Reconnaissance Orbiter (MRO) when ISON made its closest pass by Mars in October. Based on these observations, researchers think ISON's nucleus was probably smaller than 600 meters in diameter, making it relatively compact, so its breakup and evaporation is not surprising.
Since the beginning of history people have seen comets appear in the night sky, rising and setting with the stars. The ancient Greeks called them "hairy stars", and the name "comet" comes from the Greek word for hair. This is because the tail of a comet was thought to look like long hair streaming from a woman's head. The comets get their tails by going nearer the sun and the icy hard rock starts to melt. To be more specific, the tail of a comet is formed when the core of dust and ice heats up (by the sun) and sheds molten material.
Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central atmosphere immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. The discovery of main-belt comets and active centaurs has blurred the distinction between asteroids and comets ( for more detailed explanation please see the comments section below this article) .
Space probes sent to examine comets during the 1980s have provided much information to help us form a picture of what a comet is. Most astronomers now believe it to be a fairly small object, up to 75 kilometers (46 miles) across, that travels around the Sun along a very eccentric (that is, elongated) path. Comets are composed of a mixture of solid, rocky material (apparently mainly carbon), dust, and ice, and frozen gases. A widely accepted theory says that comets are the remains of materials from which the planets Uranus and Neptune were formed. The gravitational force of these large planets pushed the comets into their present extended paths, forming a huge spherical cloud on the outer fringes of the Solar System. Scientists call this the Oort Cloud, after the Dutch astronomer who invented the theory. Occasionally the gravitational force of a nearby star disturbs a comet in the Oort Cloud and sends it into the inner Solar System. Scientists think that there are billions of comets in the Oort Cloud, but only a few approach the Sun and form tails that can be seen from Earth.
In 1680 Sir Isaac Newton, who had discovered the law of gravity, watched a great comet that appeared that year and from his observations made an important discovery. He realized that comets are attracted to the Sun by the force of gravity and that they revolve round it as the planets do. Two years later another comet appeared and was observed by the astronomer Edmond Halley, a friend of Newton. Halley studied the accounts of 24 comets that had been seen from time to time since 1337 and calculated their orbits. He found that the comets of 1531, 1607, and 1682 moved in almost the same paths. Thus he came to the conclusion that they were all appearances of the same comet and that it would return in about 1758 or 1759. His forecast was correct, for the comet did appear again, on Christmas Day, 1758, although Halley himself did not see it as he had died in 1742. For the first time scientists realized that comets can be regular visitors, and the great comet of 1758-59 was named after Halley. It has since appeared regularly every 76 years or so, in 1834-35, 1910 and 1985-86, when scientists sent a series of spacecraft to intercept it and investigate its nature.
ISON was discovered by a pair of amateur Russian astronomers and named after the telescope array (the International Scientific Optical Network) that they used.
Comets like ISON are the remains of the formation of the solar system billions of years ago and are comprised of ice, rock and frozen gases like carbon monoxide. "Sungrazing" comets like ISON are rare, with astronomers reporting that no "dynamically new" comets (those that have been untouched since their formation) have been seen in at least 200 years. The comet was two-thirds of a mile wide as it got within 1 million miles (1.6 million kilometers) of the sun, which in space terms basically means grazing it. Made up of loosely packed ice and dirt, it was essentially a dirty snowball from the Oort cloud, an area of comets and debris on the fringes of the solar system. Two years ago, a smaller comet, Lovejoy, grazed the sun and survived, but fell apart a couple of days later. That's why it is expected that maybe this one would make it because it was 10 times the size (1).
ISON has made its way to our solar system from the Oort Cloud (left); a spherical cloud of icy bodies thought to lie a light year (10 trillion kilometres) from the Sun. The outer borders of the Oort Cloud define the limits of our solar system, beyond which the gravitational pull of the Sun is negligible. ISON has likely between travelling towards the inner reaches of our solar system for more than 10,000 years. Scientists have analysed ISON's flight-path and deduced that it has never entered our solar system before. ISON is thought to be fairly small for a comet, with various estimates suggesting that it is between 0.12 miles and 3 miles across. This makes it an average or smaller-than-average size.
In 11 months' time, Comet Siding Spring will breeze past Mars at a distance of little more than 100,000km. And then in November 2014, Esa's Rosetta mission will attempt to place a probe on the nucleus of Comet 67P/ Churyumoy-Gerasimenko(2). It is very interesting and fun to watch these celestial guests, study them and try to understand the universe.
A brilliant comet with a bright head and a long tail reaching halfway across the sky is a rare and glorious sight. Many amateur astronomers are so fascinated by comets that they devote all their spare time to observing and studying them. Telescopes all over the world as well as 13 space observatories and hundreds of amateur astronomers trained their sights on ISON over the past several months. I too saw some of these comets, including comet Halley in 1986, when I was a young science student, with a small telescope and a pair of binoculars. They look so cute. I even painted a picture of them titled "Visitors from space"
("Visitors from space" from http://www.kkartfromscience.com )
That view is from the world of science.
Now for a view from ancient and today's laymens' world.
Some superstitious beliefs are that comets are a sign of bad omens, as demonstrated in the battle of 1066 when Halley's Comet passed (once every 76 years).
Comets are generally seen as bad signs in most cultures all around the world, and the arrival of comets are linked to all kinds of misfortunate events. An ancient Chinese book from 300 B.C. collects (3) the various disasters casued by comets, such as: whales die, war, conspiration against the emperor, fish, salt and riice become expensive, etc. It is believed by historians that the aztec rules did not fight Cortes in 1519, because he saw a comet, which he believed to be the sign of the end of his realm. Especially the comet Halley is seen as a bad omen: it is believed to have caused the plague. of 141 A.D., the conquest of England by William the Emperor, the attack of Genghis Kahn of the Western world, etc. Even in 1909 people got panicked, and anti-comet sickness pills were sold on the streets.
Some people still think that comets sign the doomsday, and some priests still call people to convert before that happens. People also associate the death of prominent political leaders with comets!
As there is always something bad happening in the world, it is very easy to link a coincidence to the appearance of a comet. Wouldn't earth quakes, cyclones, tsunamies, death of leaders happen all the time? What has these things got to do with comets? Isn't it just a superstition originated in the pre-scientific age when people feared everything they came across as they had no clue what they were? Now we know enough to say they are welcome to visit us to give us more knowledge about the universe. Unless they come very close to earth on a suicidal path, they don't cause any harm to us.
So there is nothing to worry about comets and asteroids. Enjoy the free celestial shows and celebrate life for all these wonderful things happening around us.
Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid. The term "asteroid" is ill-defined. It never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union from 1853 on. In 2006, the term "small Solar System body" was introduced to cover both most minor planets and comets. Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English for larger minor planets such as the dwarf planets. The word "planetesimal" has a similar meaning, but refers specifically to the small building blocks of the planets that existed when the Solar System was forming. The term "planetule" was coined by the geologist William Daniel Conybeare to describe minor planets, but is not in common use. The three largest objects in the asteroid belt, Ceres, 2 Pallas, and 4 Vesta, grew to the stage of protoplanets. Ceres is a dwarf planet, the only one in the inner Solar System.
When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroids. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets.
For almost two centuries, from the discovery of Ceres in 1801 until the discovery of the first centaur, 2060 Chiron, in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as 944 Hidalgo ventured far beyond Jupiter for part of their orbit. When astronomers started finding more small bodies that permanently resided further out than Jupiter, now called centaurs, they numbered them among the traditional asteroids, though there was debate over whether they should be considered as asteroids or as a new type of object. Then, when the first trans-Neptunian object, 1992 QB1, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: Kuiper-belt object, trans-Neptunian object, scattered-disc object, and so on. These inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets and not asteroids.
The innermost of these are the Kuiper-belt objects, called "objects" partly to avoid the need to classify them as asteroids or comets. They are believed to be predominantly comet-like in composition, though some may be more akin to asteroids. Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional comet nuclei. (The much more distant Oort cloud is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the Stardust probe, are increasingly blurring the distinction between comets and asteroids, suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.
The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations. However, it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System.Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the asteroid belt, Jupiter trojans, and near-Earth objects.
When the IAU introduced the class small Solar System bodies in 2006 to include most objects previously classified as minor planets and comets, they created the class of dwarf planets for the largest minor planets—those that have enough mass to have become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally the term 'Small Solar System Body' will be preferred." Currently only the largest object in the asteroid belt, Ceres, at about 950 km (590 mi) across, has been placed in the dwarf planet category, although there are several large asteroids (Vesta, Pallas, and Hygiea) that may be classified as dwarf planets when their shapes are better known.
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