A comet is a small body, roughly the size of a small town, in the Solar System. It is made up of ice, rock, dust, carbon dioxide, ammonia, methane and more. Some researchers think comets might have originally brought some of the water and organic molecules to Earth that now make up life here. Comets become visible as they near the Sun. The tail of a comet is its most characteristic feature. Round and round a comet goes in its orbit and when it comes into the inner solar system.
Each time a comet gets closer to the Sun, the ice on the surface of the nucleus, measuring ten miles or less, begins turning into gas, forming a cloud known as the coma. Radiation from the sun pushes dust particles away from the coma, which can reach 1 million miles wide, forming a dust tail, while charged particles from the sun convert some of the comet’s gases into ions, forming a stream of volatile materials known as an ion tail. Some tails can reach 100 million miles long.
The tail of the comet always points away from the Sun, since they are shaped by sunlight and the solar wind, so that when the comet is receding from the Sun, its tail actually runs before it. Typical comet loses about one tenth of a percent of its mass every time it passes near the Sun. After one thousand passages or so, Comets lose all their ices, leaving behind only an orbiting stream of fragile, inactive objects, meteoric dust, ice, and pebbles, similar to an asteroids. When Earth, in its annual journey around the Sun, passes through one of these dusty tracks, we are treated to a meteor shower.
For centuries, scientists thought comets traveled in the Earth’s atmosphere, but in 1577, observations made by Danish astronomer Tycho Brahe revealed they actually traveled far beyond the moon. Isaac Newton later discovered that comets move in elliptical, oval-shaped orbits around the Sun, and correctly predicted that they could return again and again. Throughout history comets inspired both wonder and fear. They were known as “hairy stars” resembling fiery swords that appeared unpredictably in the sky. Often, comets seemed to be omens of doom.
The most ancient known mythology, the Babylonian “Epic of Gilgamesh,” described fire, brimstone, and flood with the arrival of a comet. (Goldman, 2009) Chinese astronomers kept extensive records on comets for centuries, including illustrations, observations and celestial positions of Halley’s Comet going back to at least 240 BC; historic archives that have proven valuable resources for later astronomers. A few dozen comets are discovered annually by astronomers with telescopes or on photographic plates. Comets are usually named after their discoverer, and/or has spacecraft missions incorporated into their name.
Most Comets are too faint or small to be seen without a telescope. Only every few years does a comet grow bright enough to be seen with the naked eye. A highly visible comet was Hale-Bopp, which came within 122 million miles of Earth in 1997. Its unusually large nucleus gave off a great deal of dust and gas, roughly 18 to 25 miles across, appeared bright to the naked eye. Our solar system is surrounded by a sphere, or cocoon, of cold, dark comets called the Oort Cloud, far beyond the orbit of Pluto, that reaches halfway to the nearest stars.
Most Comets are believed to inhabit The Oort Cloud, however it has never been observed directly; but it must exist to account for the comets that arrive in our vicinity from enormous distances. (Dickinson, 1999). The Kuiper Belt, was discovered in an attempt to locate the non-existent tenth planet, planet X. The Kuiper belt was named after Gerard Kuiper an astronomer who predicted its existence in 1951. The Kuiper belt is a belt of comets. The first of these comets was picked up in 1992 during a deliberate search by astronomers. It is a chunk of primordial ice about 200 kilometers in diameter orbiting the Sun at 1 ? imes Neptune’s distance. In the years since the discovery, more than 100 similar sized objects have been found in orbits outside Neptune’s path. This is known as Trans-Neptunian Objects, (TNO). These giant cosmic ice balls are left over from the formation of the solar system. There are millions of comets six miles in diameter and thousands measuring a few dozen kilometers across, that form the Kuiper Belt. They are short-period comets. They take less than 200 years to orbit the sun, and in many cases their appearance is predictable because they have passed by before. Short period comets come around with steady regularity.
Halley’s Comet is likely the most famous short period comet in the world; it becomes visible to the naked eye on its return, every seventy-six years. Its last visit near Earth was in 1986. At that time five spacecraft flew past it, attaining extraordinary information, coming close enough to study its nucleus which is normally concealed by the comet’s coma. The potato-shaped, nine mile long contains equal part ice and dust, with about 80 percent of the ice made of water, and about 15 percent of it consisting of frozen carbon monoxide. Researchers believe other comets are chemically similar to Halley’s Comet.
The Comet Shoemaker-Levy 9, discovered by Gene & Carolyn Shoemaker and David Levy. March 25, 1993 (Raymo, 2001). This short-periodic comet was the comet of the Great Comet Crash of 1994. The comet Shoemaker-Levy 9 collided spectacularly with Jupiter in 1994, with the giant planet’s gravitational pull ripping the comet apart for at least 21 visible impacts. The largest collision created a fireball that rose about 1,800 miles (3,000 km) above the Jovian cloud tops as well as a giant dark spot more than 7,460 miles (12,000 km) across, about the size of the Earth.
The Hubble Space Telescope, orbiting 375 miles above our planet’s surface, (Rosselli, 1998) has shown the effects of the comet smashing into the surface of Jupiter with an explosive power of 100 million megatons, which was the most violent event ever witnessed in the solar system. Hyakutake and Hale-Bopp are long-period comets. These Oort Cloud comets have orbital periods of thousands or millions of years. Their appearance is unpredictable. They follow long cigar shaped trajectories that take them far out beyond Pluto. They move like wave, up and down.
At the cold, black tops of their curves, far from the Sun they proceed with a laborious leisureliness, taking as long as 30 million years to complete one trip around the sun. They gather speed as they fall toward the inner solar system, moving at its fastest, it punches around the Sun, then they slow again as they climb back to the tops of their trajectory. A potentially bright Oort cloud might be discovered at any time, typically somewhere near the orbit of Jupiter on its way center stage in the inner solar system. The brightest apparitions occur when a comet passes near Earth on its inward or outward journey.
A comet is not like anything anyone has seen before. It is a starkly fascinating, and amazing visual experience, evoking passions of fear, anxiety, admiration, wonder, and bewilderment to the enlightened and unenlightened observer. Comets, especially those that are bright enough to be seen with the naked eye, continue to fascinate the Earth’s population. With 2013 being deemed “Year of The Comet,” (Barnett, 2013), astronomers, scientist, and watchers of all kinds can look forward to a rare treat; two visible comets, Pan- STARRS (3/12/13) & ISON (11/28/13), in one year!