Comets – Concept, classification, parts and characteristics

We explain what comets are, their classification, their component parts and other characteristics. Also, Halley’s Comet.

Comets are astronomical objects that move in orbits around the Sun.

What are comets?

In astronomy, comets are known as certain types of moving astronomical objects, members of the Solar System, that travel orbits of different trajectories and durations around the Sun. Most comets are trans-Neptunian objects from conglomerates of icy objects known as the Kuiper Belt or, further afield, the Oort Cloud.

Comets trace highly concentric orbits around the Sun on their way, many of them returning after hundreds and even thousands of years. Its typical image is that of a bright, oval body, which leaves behind a trail or coma composed of glowing gases.

The only one that can be seen 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 recurrent appearance has been interpreted as a symptom of omens, a source of revelations or a sign of the end of one era and the beginning of another. Myths like the one about the Star of Bethlehem in the Bible may have been mystical interpretations of these astral travelers.

Kites of kites

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-period or mid-period kites. Those generally coming from the Kuiper Belt, located 50 Astronomical Units (AU) from the Sun
  • Long period kites. Those that come from the Oort Cloud, almost a hundred times farther, at the very edge of the Solar System.

Similarly, we can distinguish between periodic and non-periodic kites, the first being those whose orbit takes 200 years or less to complete; and the latter those whose orbit takes 200 years onwards. In the same way, their orbits can be elliptical, parabolic or hyperbolic.

Finally, comets are classified according to their size, in the following categories:

  • Dwarf kites. 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.
  • Big kites. Between 6 and 10 kilometers in diameter.
  • Giant kites. Between 10 and 50 kilometers in diameter.
  • Comets “Goliath”. Above 50 kilometers in diameter.

Parts of a comet

comet meteor shower
A comet’s dust coma can produce a meteor shower.

Comets are made up of two clearly recognizable parts:

  • A core. Composed of the solid mass of the comet, where the materials that compose it 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 trail of several kilometers long, composed of gases ejected from the comet during its heating by the action of the Sun, or stardust and fragments that it leaves behind on its way. In many cases, two different commas can be seen:
    • The gaseous coma. Composed of water vapor that is ejected from the comet and that sustains the opposite direction to the Sun’s rays.
    • The dust coma. Composed of solid remains of the comet that are suspended in space, and that upon entering the Earth’s atmosphere, when our planet passes through an orbit of a comet, triggers meteor showers.

Characteristics of a comet

Comets have various shapes, generally irregular, which can range from a few kilometers to several dozen in diameter. Its composition is one of the most common riddles in astronomy, partially solved by close-up observation of Halley’s Comet on its last pass in 1986.

It is known today that comets contain a significant 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 organic matter contributors that allowed life to emerge on Earth.

In the same way, it is thought that they could be material witnesses to the very formation of the Solar System, and have within them physical secrets about the origin of the planets and the Sun itself.

Examples of comets

kite example Hyakutake
Comet Hyakutake was discovered and photographed in 1996.

Some of the best known comets are:

  • Comet Halley. With a period of approximately 76 years, the only one visible from the surface of the Earth.
  • Comet Hale-Bop. One of the most observed of the 20th century, it sparked numerous rumors as it passed near Earth during 1997, given its enormous brilliance.
  • Comet Borrelly. Named after its discoverer, the French Alphonse Borrelly, it was visited in 2001 by the North American space probe Deep Space 1.
  • Comet Coggia. A non-periodic giant specimen that could be seen with the naked eye from Earth in 1874. It visited our planet two more times 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. It could be seen from all over the world and emitted significant amounts of X-rays. Its approximate period is 72,000 years.

Halley’s Comet

Halley comet
Halley’s Comet is visible from Earth every 75 years or so.

It is the most famous of comets and visit our planet every 75 years approximately. Named after Edmund Halley (1656-1742), the first scientist to describe its orbit and predict its appearance. However, it has been known and recorded in history since ancient times.

This comet was initially long-period, born in the distant Oort Cloud, but was caught in the gravity of the planets of the Solar System, especially that of Venus. Because, is currently short orbit. In 1986 it was the first comet to be visited by numerous space probes, which have since been called Halley’s navy (Halley’s Navy).