We explain what comets are, their classification, their component parts and other characteristics. Also, Halley’s comet.
What are comets?
In astronomy, it is known as comets to certain types of mobile astronomical objects , members of the Solar System , that travel orbits of different trajectory and duration around the Sun. For the most part, comets are trans-Neptunian objects from clusters of icy objects known as the Kuiper Belt or, further still, the Oort Cloud.
The only one that can be looked at regularly from the surface of our planet is the famous Halley’s Comet. However, the study of comets, especially after the invention of the telescope, has been a concern of astronomers since ancient times.
In some cases, the recurring appearance has been interpreted as a symptom of omens, source of revelations or sign of the end of one era and the beginning of another. Myths like that of the Star of Bethlehem in the Bible may have been mystical interpretations of these astral travelers.
Comets can be classified based on two criteria, the first being the distance traveled in their orbits and the type of orbit they present. Thus, we can talk about:
- Short or medium period comets : Those generally from the Kuiper Belt, located 50 Astronomical Units (AU) from the Sun
- Long-period comets : Those that come from the Oort Cloud, almost a hundred times farther, in the very limits of the Solar System.
Similarly, we can distinguish between periodic and non-periodic comets , being the first ones whose orbit takes 200 years or less to complete; and the second ones whose orbit takes from 200 years onwards. Similarly, their orbits can be elliptical, parabolic or hyperbolic .
Finally, comets are classified according to their size, in the following categories:
- Dwarf comets , between 0 and 1.5 kilometers in diameter.
- Small kites , between 1.5 and 3 kilometers in diameter.
- Medium kites , between 3 and 6 kilometers in diameter.
- Large kites , between 6 and 10 kilometers in diameter.
- Giant comets , between 10 and 50 kilometers in diameter.
- Kites “Goliath” , above 50 kilometers in diameter.
Parts of a comet
Comets are made up of two clearly recognizable parts:
A nucleus , composed of the solid mass of the comet, where its composing materials are found (usually ice and inorganic compounds , although with usual traces of hydrocarbons), and which is usually basically a rock in motion.
A comma , also called hair, which is a wake several kilometers long, composed of gases ejected from the comet during its warming by the Sun, or stellar dust and fragments that it leaves behind in its path. In many cases two different commas can be observed:
- The gas coma , composed of water vapor that is ejected from the comet and that holds the direction opposite to the sun’s rays.
- The dust coma , composed of solid remains of the comet that remain in suspension in space, and that when entering the Earth ‘s atmosphere , when our planet crosses some orbit of a comet, triggers meteor showers.
Characteristics of a comet
Comets have diverse shapes, usually irregular , that can range from a few kilometers to several tens of diameter. Its composition is one of the most usual enigmas of astronomy , partially solved by the close observation of Halley’s Comet on its last pass in 1986.
It is known today that comets contain an important presence of frozen water, dry ice, ammonia, methane, iron, magnesium, sodium and silicates . Such a composition suggests that comets could have been part of the contributors of organic matter that allowed the emergence of life on Earth.
In the same way, it is thought that they could be material witnesses of the formation of the Solar System itself, and have physical secrets inside them about the origin of the planets and the Sun itself.
Some of the best known comets are:
- Halley’s Comet , with a period of approximately 76 years, the only one visible from the Earth’s surface.
- Comet Hale-Bop , one of the most observed of the twentieth century, unleashed numerous rumors in its passage near the Earth during 1997, given its enormous brilliance.
- Comet Borrelly , named by its discoverer, the Frenchman Alphonse Borrelly, was visited in 2001 by the North American space probe Deep Space 1.
- Comet Coggia , a giant non-periodic specimen that could be observed with the naked eye from Earth in 1874. He visited our planet twice more before disintegrating in 1882.
- Comet Shoemaker-Levy 9 , famous for having crashed into Jupiter in 1994, allowing us to witness the first documented extraterrestrial impact in history.
- Comet Hyakutake , discovered in January 1996, the year it passed very close to Earth: the closest distance a comet had passed in 200 years. He was seen from all over the world and emitted significant amounts of X-rays. Its approximate period is 72,000 years.
It is the most famous of the comets and visits our planet approximately every 75 years . It is named after Edmund Halley (1656-1742), the first scientist to describe its orbit and predict its appearance. However, it is known and recorded in history since ancient times.
This comet was initially of a long period, born in the distant Oort Cloud, but was caught in the gravity of the planets of the Solar System, especially Venus. Therefore, it is currently short-orbit . In 1986 he was the first comet to be visited by numerous space probes, which since then became known as Halley’s army .