Comets

Comets are small extraterrestrial bodies composed of a frozen mass that travels around the Sun in a highly elliptical orbit. Comets are visible and brightest, only when they are near the Sun especially at sunrise or sunset. This is because they reflect the light from the Sun, rather than having their own source of light, there by creating a visual of a Shooting Star or a Meteor. When close enough to the Sun, they also displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes a tail as well. As a Comet approaches the Sun, the radiation evaporates the ice. This causes gases and dust to drag from the nucleus - the coma. Radiation from the Sun blows the dust away from the head of the Comet, creating the Dust Tail. The solar wind drags ionized gas from the Comet, creating the Ion Tail. Comets are sometimes known as 'Dirty Snowballs' or 'Icy Mudballs' as and when far from the Sun, their nucleus is very cold and its material is frozen solid within the nucleus. In this state Comets give an appearance of  'Dirty Snowballs' or 'Icy Mudballs' since over half of their material is ice. A Comet keeps on loosing some of its ice and gases every time it goes past the Sun. After about 500 passes, most of the ice and gas is lost, and the Comet turns an asteroid type of object. Therefore, the amount of time for a Comet to orbit the Sun, determines how long the Comet will last. There are various theories regrading the importance of Comets, many astronomers consider that Comets are responsible for bringing large amounts of water to Earth. It is also considered that this water allowed life to begin on Earth. Many scientists also believe that a Comet caused the extinction of the dinosaurs and gave rise to the age of mammals 65 million years ago. As of May 2010 there are about 3,976 known Comets. However, there are estimated one trillion Comets still to be discovered. Comets are designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery, for example the fourth Comet discovered in the second half of February 2006, would be designated 2006 D4.

Prefixes are also added to indicate the nature of the Comet.

P/ indicates a Periodic Comet.
C/ indicates a Non-Periodic Comet.
X/ indicates a Comet for which no reliable orbit could be calculated.
D/ indicates a Comet which has broken up or been lost, also referred to as Dark Comet.
A/ indicates an object that is mistakenly identified as a Comet, but is actually a Minor Planet.

Classification and Types of Comets

Comets are often classified according to the length of their orbital periods

1.  Periodic Comets - Periodic Comets are those Comets that moves about the sun in an elliptic orbit and are seen at two of its approaches to the Sun.

There are 2 Types of Periodic Comets

• Short-Period Comets - Comets whose orbital periods are of less than 200 years are known as Short-Period Comets. They usually orbit more-or-less in the ecliptic plane in the same direction as the Planets. Their orbits typically take them out to the region of the outer Planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. At the shorter extreme, Encke's Comet has an orbit which never puts it farther away from the Sun than Jupiter. Short-period Comets are further divided into the Jupiter family (periods less than 20 years) and Halley family (periods between 20 and 200 years). Short-Period Comets are considered to have originated from the Centaurs and the Kuiper belt / scattered disk (a disk of objects in the transneptunian region). They are also known as Periodic Comets. There are abou155 Short-Period Comets. Halley's Comet is famous and orbits the Sun once every 76 years.

• Long-Period Comets - Comets which have orbital periods higher than 200 years are known as Long-Period Comets. These Comets have highly eccentric orbits and their orbital periods range from 200 years to thousands or even millions of year. Long-Period Comets are considered to have originated from spherical Oort cloud. There are about 655 Long Period Comets. Comet Kohoutek has a period of 75,000 years.

2.  Intermediate-Period Comets - Intermediate-Period Comets are those Comets which have orbit periods between 20 and 200 years. The Intermediate-Period Comets have on average a larger inclination of the ecliptic, and 5 of them turn around the Sun in a retrograde direction. The most famous of the latter is P/Halley with 30 appearances. 11 of the 20 Intermediate-Period Comets have been observed during a single appearance.

3.  Single-Apparition Comets - Comets which have parabolic or slightly hyperbolic (non-periodic) trajectories when in the inner Solar System are known as Single-Apparition Comets. As of a 2010 epoch, Comet C/1980 E1 has the largest eccentricity of any known hyperbolic Comet with an eccentricity of 1.057. They are also known as Non Periodic Comets.

4.  Main-Belt Comets - Comets which orbit within the main asteroid belt and which have shown Cometary activity during part of their orbit are known as Main-Belt Comets. These have been recently discovered. Main-belt Comets follow near-circular orbits within the Asteroid Belt that are undistinguishable from the orbits of many standard Asteroids.

Some of the Main-Belt Comets

133P/Elst-Pizarro.
176P/LINEAR.
P/2005 U1 (Read).
P/2008 R1 (Garradd).
P/2010 A2 (LINEAR).

Characteristics and Physical Features of Comets

Parts of a Comet - A Comet consists of:

• Nucleus - Nucleus of Comets are composed of rock, dust, water ice, and frozen gases such as carbon monoxide, carbon dioxide, methane and ammonia. Comet nuclei have irregular shapes as they have low mass which does not acquire a spherical shape under their own gravity. Comet nuclei may include methanol, hydrogen cyanide, formaldehyde, ethanol and ethane, and perhaps more complex molecules such as long-chain hydrocarbons and amino acids. Comet nuclei range from about 100 meters to more than 40 kilometres across. Cometary nuclei are among the least reflective objects found in our Solar System.

• Coma - When a Comet approaches the inner Solar System, solar radiation causes the volatile materials within the Comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the Comet which is known as the Coma. This dense atmosphere is a dense water cloud, made up of carbon dioxide, hydrogen and neutral gases, that comes off of the nucleus. The Coma is illuminated by the Sun and may become visible from Earth when a Comet passes through the Inner Solar System, the dust reflecting sunlight directly and the gases glowing from ionisation. The coma and the nucleus together constitute the head of the Comet.

• Cometary Tail - A Comet generally has a tail. The force exerted on the Coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. They rarely appear beyond 1.5 or 2 AU (astronomical unit) but develop rapidly with shorter heliocentric (having the sun as the centre) distance. The tail is illuminated by the Sun and may become visible from Earth when a Comet passes through the Inner Solar System when the dust reflects sunlight directly and the gases glow from ionisation.

Cometary Tail can be divided into 3 Types

The Ion Tail (Type I) - The neutral gas in Cometary comae can be ionised by solar UV photons, as in . The ions are susceptible to a magnetic force due to the solar magnetic field carried by the solar wind. Consequently, the ions are swept out of the coma into a long, distinctive ion tail. Because the most common ion, CO+, scatters blue light better than red, the Ion Tail often appears to the human eye as blue. Also, the magnetic force is very strong and produces ropes, knots and streamers that distinguish the ion tail from the dust tail. The solar wind sweeps past the Comet at about 500 km/s, causing the ion tail to be swept almost exactly in the anti-solar direction. Ion tails have been observed to extend one astronomical unit (150 million km) or more. Ion Tail is also known as  Plasma tails as they contain molecular ions as well as electrons.

Dust Tail (Type II) - The dust tail consists of dust particles that have been pushed out of the coma by radiation pressure from the sun. Compared to the ION tail, the dust tail is morphologically diffuse, and appears white or slightly pink (because the dust grains reflect sunlight slightly better at longer wavelengths than at shorter wavelengths). The dust particles in the tail are individually in orbit about the sun, each with slightly less attraction to the sun than the nucleus because of the effect of radiation pressure. This causes the dust tail to be curved as the Comet swings around the sun.

Antitail (Type III)- Antitail is one of the three tails, all pointing in different directions, which may appear to emanate from a Comet as it passes close to the Sun. The antitail appears as as spike projecting from the Comet's coma towards the sun, and thus geometrically opposite to the other tails: the ion tail and the dust tail. The antitail is formed of larger dust particles, which are less affected by the sun's radiation pressure or solar wind, and tend to remain in the Comet's orbital plane and eventually form a disc. The Antitail is Normally visible for a brief interval only when the Earth passes through the Comet's orbital plane. Most Comets don't develop sufficiently for an antitail to become visible but notable Comet that displayed anti-tails include Comet Arend-Roland in 1957 and Comet Hale-Bopp in 1997.

• Meteor Shower -  Meteor Showers sometimes occur when the Earth passes through the orbit of a Comet. Outgassing causes Comets to leave a trail of solid debris. If the Comet's path crosses Earth's path, then at that point there will likely be meteor showers as Earth passes through the trail of debris. Some showers occur with great regularity: the Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes through the orbit of Comet Swift-Tuttle. Comet Halley is the source of the Orionid shower in October.

• Short Life Span - All Comets have a Short Life Span on a cosmological time scale. This is because each time it visits the Sun, it loses some of its volatiles. Upon every subsequent visit, it keeps loosing it's mass and eventually becomes a rocky mass in the Solar System.

• Elongated Elliptical Orbits - Most Comets have elongated elliptical orbits that take them close to the Sun for a part of their orbit, and then out into the further reaches of the Solar System for the remainder.

• Fragile - Generally, Comets are fragile. Their tensile strength (the stress they can take without being pulled apart) appears to be only about 1,000 dynes/cm^2 (about 2 lb./ft.^2). They are so fragile that one can take a big piece of cometary material and simply pull it in two with their bare hands, something like a poorly compacted snowball.

Size - Their average diameters usually range from 750 meters (2,460 feet) or less to about 20 kilometres (12 miles).

Some of the Major Comets

2P/Encke.
3D/Biela.
4P/Faye.
14P/Wolf.
Comet McNaught (C/2009 R1).
Comet Lulin (C/2007 N3).
Comet Linear.
Hale-Bopp Comet.
Halley's Comet.
Shoemaker-Levy 9.

Origin and Evolution of Comets

Comets are actually collections of materials left over from the formation of the Solar System. Astronomers believe that Comets are leftover debris from a collection of gas, ice, rocks, and dust that formed the outer Planets about 4.6 billion years ago. Most astronomers consider that the material that became Comets condensed in the outer Solar System around the orbits of Uranus and Neptune and beyond. Gravitational effects from these giant Planets flung some of the Comets outward to the Oort cloud, while the Comets in the Kuiper Belt might have remained there. Residing at the farthest reaches of the Sun's influence, Comets did not undergo the same heating as the rest of the objects in the Solar System, so they remained, largely unchanged, the original composition of Solar System materials. As the preserved building blocks of the Outer Solar System, Comets offer clues to the chemical mixture from which the Planets formed some 4.6 billion years ago.

The geologic record of the Planets shows that, about 3.9 billion years ago, a period of heavy Cometary and asteroidal bombardment tapered off. The earliest evidence of life on Earth dates from just after the end of this heavy bombardment. The constant barrage of debris had vaporized any water on Earth, leaving the Planet too hot for the survival of the fragile carbon-based molecules upon which life is based. Scientists concluded that Comets are abundant in both water and carbon-based molecules and they provided the basis for life to begin when they landed on the Earth's surface providing water or carbon-based molecules to support life on Earth. Comets are also considered to be partially responsible for the replenishment of Earth's oceans after the vaporization of an early ocean during the late heavy bombardment.