by For thousands of years, Comets and their tails have fascinated astronomers and philosophers. Ancient people believed that comets were actually the signs of either good fortune or a bad omen. The formation of tails in the comets is due to the presence of carbon dioxide, carbon monoxide and other particles present in the comet's surface. It is believed that there are billions of comets outside of our solar system. So what are comets and why do they have tails?
What is a Comet?
Comets are born in the dark outer skirts of the solar system and they are lonely wanderers roaming around the star systems. Comets got their name from the Greek word 'Comet' which means 'wearing long hair' where the long hair refers to the long, bright tails of the comet. Comets are pretty similar to asteroids except they are made up of ice, dust and frozen gases whereas the asteroids are mostly rocky and made up of metals. Comets are also referred to as 'dirty snowballs'. These comets have an elliptical orbit and their orbital period might range from only a few years to several million years. Typically the speed of a comet is about 3200 kilometres per hour (2000 miles per hour) when they are outside the solar system, but could travel at up to 1,60,000 kmph (100,00 mph) when they are closer to the sun.
 |
| Comets, also known as the Dirty Snowballs |
These comets are observed to be coming from two regions outside our solar system. The Kuiper Belt is the hive of the comets that comes to our solar system and they might be similar to the asteroid belt present in our solar system. Most of the comets coming from this region have a shorter orbital period. The area just outside the solar system is the main origin of all the comets we have observed so far and this region is referred as the Oort cloud. These have longer orbital periods than the comets originating from the Kuiper belt. The orbits of these comets might also be affected by the other interstellar objects.
Some comets pass extremely close to our sun and they are termed as 'Sungrazing comets'. These comets are either evaporated into small fragments or completely destroyed when they reach close proximity to the sun. The comets which orbit the sun for less than 200 years are classified as periodic or short-period comets and the comets which have an orbital period in the range of 200 to 1000 years are called long-term comets. The comets which have an orbital period of more than 1000 years are termed near-parabolic comets, and there are some comets that pass through our solar system once, they are known as hyperbolic comets.
How Comet's tail is formed?
In the deep void of space, they are customary blocks of ice and rock floating on the outskirts of the solar system. We have observed over 3,000 known comets over several centuries and we are still continuing to explore the comets. The comets floating in the void of space are impossible to view from Earth due to their tiny size. But as they enter our inner solar system, the solar radiation heats up the icy layer of the comet and the ice sublimates into gases. These gases released from the comet surface forms an extremely delicate atmosphere which is called as the Coma of the comet.
 |
| The Comet's tail stretching across several million kilometres |
These gases surrounded the comet and they trail along them facing the opposite side of the sun as the clouds of dust particles are pushed by the solar winds. Typically a comet's tail might stretch as far as 10 million kilometres (6 million miles). Two different tails are produced and the solar radiation exerts a force on the dust clouds. Although most comets are too dim to be spotted without telescopes, some of the comets are bright enough to be seen in the night sky with our naked eye.
Why does a comets tail point away from the Sun?
Generally, the Comets have two tails: an Ion tail and a dust tail. These two tails usually trail along in different directions. The dust tail, also known as the plasma tail is blown away by the solar winds and they usually point away from the sun. The Plasma tail follows the same trajectory as the path of the solar wind and so it forms as a straight line behind the comet following the direction of the solar radiation. These tails even form a rope-like formation or even break up completely due to the changes in the flux of the sun's particles.
 |
| Comets with Ion and plasma tails |
Halley's comet
Halley's comet is undoubtedly the most well-known comet, it returns to Earth's proximity about every 75 years. This comet was first spotted by astronomer Edmond Halley, who observed the same comet in 1531, 1607 and 1682. He predicted that the same comet would reappear after 75 and as he predicted the comet returned to Earth's vicinity in 1758. The Halley comet was last seen in the year 1982 and it is expected to return in 2061. It was spotted by Chinese astronomers in 240 BC. This is believed to be originated from the regions of the Oort cloud.
 |
| Halley's comet. Source: NASA |
When Halley's comet is near the inner solar system, the carbon dioxide and carbon monoxide in the atmosphere sublimates and the atmosphere is blown away extending the tail of the comet up to 10 million km. During its last appearance in 1986, the surface of comets was observed by the Hale Telescope at Palomar Observatory in California. As it loses its atmosphere, it continues to shrink in size, and it is believed that this comet might lose mass over the next few millennia, and might completely get evaporated.
