Facts About Comet's Tails, Composition, Orbit And More

Comet Hyakutake  - Facts About Comets
Comet Hyakutake

Comets are the small icy objects with an irregular shape which eject gas and dust on their journey through the solar system. Numerous ancient cultures have been observing comets since antiquity and considered them as ill of omens of war and famine. The word "comet" is derived from the Latin word "Cometa" which means "long-haired" that relates to the tails of comets. Following a highly eccentric elliptical orbit, they can take up to several millions of years to complete an orbit. Comets are the remnants left after the formation of the sun, planets and moons. Its bright tail and highly inflated atmosphere distinguish it from the rocky asteroids.

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Quick Facts About Comets

  1. The nucleus of a comet can be small as few hundred metres but can be substantial as tens of kilometres.
  2. However, the length of its tail can stretch up to one astronomical unit (the distance between earth and sun).
  3. It is theorised that comets might have brought water and essential components for life on earth.
  4. Great comets are those comets which are incredibly bright and can be easily seen by the naked eye.
  5. There are 6,619 (as of July 2019) comets are known.
  6. Astronomers estimated that in the outer solar system, there could be one trillion comets.
  7. Comets are only visible when they are near to the sun.
  8. When Earth passes through the orbit of a comet, the debris left by the comet falls on earth which results in meteor showers.

Detailed Facts About Comets

  • Comets are often referred to as dirty snowballs of the space

Different Parts Of A Comet
Different Parts Of A Comet
© Wikimedia

The cometary nucleus is the solid rocky central part of a comet. The nucleus is made of rocks, dust and water ice along with frozen carbon dioxide, methane, and ammonia. They are often called "dirty snowballs" due to their composition. Researchers also found some complex molecules such as Amino acids and various organic compounds.

A research conducted in 2014, suggests that comets are like "deep-fried ice cream" as their outer surface is the composition of ice and organic compounds while the interior is cold and less dense.
  • The nucleus of a comet is darker than asphalt

Nucleus Of Comet Tempel 1 (9P/Tempel)
Nucleus Of Comet Tempel 1 (9P/Tempel)

The comets are among the darkest object in the solar system.
The exterior surface reflects very less sunlight, about 3% of the light that hits its surface. By comparison, asphalt reflects only about 7% of light. As it reaches close to the sun, the heat of the sun evaporates the lighter volatile compounds from the outer surface and leaves behind darker like crude oil, larger organic compounds.
  • The coma of a comet can be larger than the size of the sun

Coma of comet 17P/Holmes
Coma of comet 17P/Holmes
© Wikimedia

When a comet passes closer to the sun, the heat of the sun vaporizes the ice at the surface of the nucleus. As a result, a thin atmosphere around the comet is formed called "Coma". The coma of a comet is typically composed of ice and dust. The water comprises about 90% of the volatile materials that escaped from the nucleus when a comet is at around 3-4 AU  from the sun.

The coma can grow as large as the diameter of the Jupiter. The "Great Comet of 1811" had an incredibly large coma, about the size of the width of the sun. At the distance of about 1.5 AU from the sun (at the Mars orbit), the solar winds become too strong that it blows the gas and dust away from the coma and extend its tail.
  • There are two types of comet tails

Ion and dust tail of a comet during its orbit around the sun
Ion and dust tail of a comet during its orbit around the sun
© Wikimedia

Comets tail is the primary feature of the comets that also distinguishes a comet from asteroids. When a comet enters in the inner solar system, Solar wind and radiation from the sun exert pressure on the coma and pushes the dust particles from the coma, forming long comet tails. There are two tails: One that carries dust (called dust tail) and another carry gases (called gas tail or ion tail).

Due to their composition, the two tails point in a slightly different direction and reflect distinct colours. The gaseous tail interacts with the magnetic field of the solar wind and always points along the streamlines of the solar wind. In comparison, the dust tail does not influence by the solar wind. It follows the comet's orbital trajectory and is slightly curved, also called "anti-tail".
  • Comets can break apart or smash into another celestial body

Brown spots left after the imapact of Comet Shoemaker–Levy 9
Brown spots left after the impact of Comet Shoemaker-Levy 9

The nucleus of a comet may be fragile enough that it can splits or break apart whey they come incredibly close to the sun or planets. A comet named Shoemaker-Levy 9, when discovered, it was already turned into chunks and thought to have been orbiting around Jupiter for decades. In July 1994, for six days, Astronomers observed 21 distinct impacts on Jupiter caused by the fragments of the comets.

The collision of these fragments caused fireballs, and the released energy is equivalent 6,000,000 megatons of TNT. After the impacts, it left dark spots of thousands of kilometres wide that were eventually wiped out by the Jupiter's strong winds. There are some comets called Kreutz Sungrazers that broke apart when passing too close (within a few thousand kilometres) to the sun. They sometimes meet a more astonishing end by diving into the sun (though they do not really fall instead wipe around it).
  • Comets can take up to thousands of years to complete an orbit

Location of Kuiper Belt and Oort Cloud in the solar system
Location of Kuiper Belt and Oort Cloud in the solar system

Astronomers have categorised comets based on the time taken by them to complete their orbits. Comets are classified as follows:

Short Period Comets

Short-period comets are also known as periodic-comets because they have periodic orbits. A short period comet has an orbital period of less than or around equal to 200 years. Periodic comets can be seen once or twice in a person's life. They are thought to originate from Kuiper Belt, a disk-shaped region of icy objects beyond the Neptunian world. The gravitation pull of giant planets drags them inwards towards the sun, making them active. The periodic comets can be seen once or twice in a person's life.

The short-period comets are further divided into Halley Type (whose orbital period is over 20 years) and Jupiter Type (whose periods are under 20 years).

Long Period Comets

Long-period comets are the highly eccentric comets whose orbital period ranging from 200 years to thousands of years. They originate from the Oort cloud, a dark region even farther from the Kuiper Belt. At aphelion, they can get a trip of the outer solar system and take thousands of years to return. Single-apparition or non-periodic comets are similar to Long-period comets. Unlike Long-period comets, they are not bound to the sun and may escape the solar system.
  • Comets may have more than one name

Astronomers have been observing Comets for thousands of years, and through this time many naming conventions have been adopted.

Old Naming System

Great comet of 1882, named after its discovery year
The great comet of 1882, named after its discovery year

Before 1994, Comets name had the year of their discovery followed by a lowercase letter that indicates its order of discovery. For example, the name of "Comet 1969i" indicates that it was the 9th comet found in the year 1969. Earlier to this convention, people used to name comets by using the names of people associated with their discovery or the first detailed study. Like Comet Hale–Bopp named after its discoverer Alan Hale and Thomas Bopp and Halley's Comet named after Edmond Halley, who first predicted its orbit .

Present Naming System

Halley's Comet, named after Edmond Halley , now has the numerical designations 1P/Halley and 1P/1682
Halley's Comet, named after Edmond Halley, now has the numerical designations 1P/Halley and 1P/1682

In1994, IAU (International Astronomical Union) gave rise to a new method of naming. The new naming convention is followed by the following patterns:
  1. A prefix is used that indicates the comet type. The prefixes are:
    • P/ is used to indicate a periodic comet.
    • C/ is used to indicate a non-periodic comet.
    • X/ is used for those comets whose orbit cannot be determined.
    • D/ is used for a periodic comet that has disappeared, broken up, or been lost.
    • I/ is used to indicate an interstellar comet.
  2. The year on which the comet has been discovered.
  3. An uppercase letter (except I ) which indicates the half-month of the discovery (A for the first half of January, B for the second half and so on).
  4. A number that represents the order of discovery within that half month.

For example, Comet Hale–Bopp was discovered on the second half of July 1995, so it is renamed to C/1995 O1. If a periodic comet is observed after its second perihelion passage, an additional prefix number is added. The number indicates the order of their discovery. So Halley's Comet, the first observed periodic comet, has the systematic designation 1P/1682 Q1.

To complete the name, a comet is given the last names of the first two people who have discovered the comet, separated by hyphens. Exceptionally, a comet name may consist of three people. If the discoverer is a team of astronomers or observatory or agency, one word or acronym is used. Examples are C/1995 O1 (Hale-Bopp) and C/2012 S1 (ISON).

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