Comets are icy leftovers of dust and other rocky objects during the formation of solar system. These astronomical objects are small solar system bodies that revolve around the center of solar system, The Sun in a periodic manner. When it passes close to the Sun, it warms up and begins to release gases. This process is known as outgassing. It produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of Solar radiations and Solar wind acting upon the nucleus of the comet.
Comets nuclei range from few hundred meters to tens of kilometers and are composed of collection of ice, dust particles and tiny rocky objects. The coma may be 15 times the Earth's diameter while, the tail may stretch beyond one astronomical unit. If sufficiently bright, comets can be viewed without the aid of telescopes.
- Physical Characteristics
The solid core structure which makes the comet is known as 'nucleus' which composed of amalgamation of ice, dust gases, rock particles and also frozen carbon dioxide, carbon monoxide, methane and ammonia. These comets are usually referred as 'dirty snowballs' or 'icy dirtballs'. Researches shows that comets are like 'deep fried ice-cream' in which the surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.
The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden inside the surface crust. The nuclei also contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, ethane, and perhaps even more complex molecules such as long-chain hydrocarbons and amino
acids. The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. Due to their low mass, comets do not become spherical under their own gravity thus remain irregular in shape.
The ejection of dust and gas released form a huge and extremely thin atmosphere around the comet called the 'coma'. The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous 'tail' to form pointing away from the Sun. Coma is made up of dust and water of which, water makes upto 90% which outflows from the nucleus when the comet is within 3 to 4 astronomical units of the Sun. The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization. Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure. The comet nucleus is generally less than 60 km
across, where as the coma stretches about millions of kilometers, sometimes even greater than the Sun. Both the coma and tail are usually illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust gets the sunlight reflected directly while the gases glow from ionization.
In the outer solar system, the comet remains frozen and thus is extremely difficult to detect them from earth due to their small size. As the comet passes through the inner solar system, due to solar radiation, the volatile materials of comet vaporizes leaving behind dust and gases which is carried along its path. The streams of dust and gas each form their own distinct tail, points in slightly different directions.
The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail. At the same time, the ion or type I tail which is made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than the dust which follows magnetic field lines rather than an orbital trajectory. The ion tail of a comet is formed as a result of the ionization by solar ultra-violet radiation of particles present in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an 'induced magnetosphere' around the comet region. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles.
- Orbital Characteristics
Short period comets are those which have a period of revolution less than 200 years. They usually orbit in the elliptical plane in the same direction of planets. Their orbit usually takes them away from outer solar system at aphelion. Sometimes even beyond the orbit of Neptune. Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.
Long period comets are those which have highly eccentric orbit and period ranging from 200 years or even thousand to millions of years. The long-period comets are gravitationally bound to the Sun, those comets that are ejected from the Solar System due to close, passes by major planets are no longer properly considered as having 'periods'. The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic.
Some breathtaking images of Comets
The Spacestellar Creations