Jasmine Grover Content Strategy Manager
Content Strategy Manager
Halley’s comet is notably the most famous comet out of all the known comets. For a long period of time, people believed that comets were a divine phenomenon. Comets were known to flash through our solar system and later they vanished into interstellar space. Comets are composed of frozen gases, dust, and rocks and they are in an orbit around the sun. When they come closer to the sun, the ice in the comets melt and release gases which therefore form a tail behind them. An English scientist, Edmond Halley predicted the orbit of a comet which was later named after him as “Halley’s Comet.” In this article, we will learn more about Halley’s comet.
Key Terms: Halley’s Comet, Comets, Galaxies, Solar System, Universe, Interstellar Space, Earth, Orbit, Sun, Kuiper Belt
What is Halley’s Comet?
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Halley’s Comet is a periodic comet that returns to Earth after every 76 years. It was last seen in the year 1986 and is expected to return again in the year 2061. It was discovered in 1758 and is referred to as 1P/Halley. Edmond Halley studied the reports of approaching comets for 1531, 1607, and 1682. He derived a conclusion that all of these were the same comet that was returning to Earth again and again.
Halley’s Comet
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Origin and Orbit of Halley’s Comet
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It is believed that Halley’s Comet originates from the Kuiper Belt. The matter that is left from the formation of the solar system around 4.6 billion years ago - which is mainly composed of ice and rocks is pulled deep into the solar system and therefore transforms into active comets. It orbits around the sun in a direction opposite to the motion of planets, therefore has a retrograde orbit.
It is a short-period comet that completes its orbit in 20 years or even less. The shape of the orbit of Halley’s comets around the sun is highly elliptical in shape. In contrast, the comets which are originated from the Oort Cloud are long-period comets that have orbits of around a thousand years or more. Due to the retrograde orbit of the comet, its velocity is the highest relative to the Earth.
Orbit of Halley’s Comet
The retrograde motion of Halley’s comet is unusual among other short-period comets. This is because the greatest distance of the comet from the Sun i.e. its aphelion lies beyond the orbit of the planet Neptune in the solar system. With each and every orbit around the sun, this comet loses about 1 to 3 meters or about 3 to 10 feet of the material that ejects from the surface of its nucleus.
Therefore, as the comet is aging, its appearance becomes dimmer. Eventually, the comet may also lose all the ice present in the nucleus. At this stage, the tail of the comet will be lost and the comet finally is transformed into a mass of dark rocky material or will eventually dissipate into dust. Halley’s comet has been in its orbit now for about 16,000 years, but still, it has not shown any signs of aging.
Read More: Motion Of Celestial Bodies In Space
Structure and Composition
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As Halley’s comet approaches the sun, jets of sublimating gases are expelled from its surface. This expulsion knocks the comet off its orbital path. The release of these gases leads to the formation of two kinds of tails which are:
- An ion tail, which is a bright tail of ionized gases.
- Another is a faint tail that is composed of multiple dust particles.
The tail of the comets is also referred to as a coma, which means a small atmosphere that spreads across over 100,000 km. The coma is made up of volatiles such as methane, water, ammonia, and carbon dioxide. It is known as a snowy dirtball rather than a snowball This is because it is mostly covered by a cloud of dark dust while only a small part of it is formed of ice. Only 10% of the surface of Halley’s comet is believed to be active.
Some of the notable features of Halley’s comet are:
- Despite having a coma that spreads across multiple kilometers, Halley’s comet is relatively small and has a 15 kilometers long nucleus that is about 8 km wide and around 8 km thick.
- It has a mass of about 242.5 billion tons.
- The average density of Halley’s comet is estimated to be 0.6 gm/cm3.
- Halley’s comet is among the least reflective and darkest objects of the solar system.
- It has an albedo of about 0.03. This means that the comet reflects only 3% of the total light that falls on it.
- The gases ejected out of the comet were observed to be mainly composed of 80% water with 17% carbon monoxide with traces of hydrocarbons, and about 3-4% carbon.
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Halley’s Comet and Meteor Showers
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With the return of Halley each time in the inner solar system, there is a release of gases from its nucleus in the form of rock and ice into space. This stream of debris in the interstellar space every year leads to two weak meteor showers: the Orionids shower in October and the Eta Aquarids in the month of May.
Halley’s Comet and Associated Meteor Showers
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Halley’s Comet: Key Points
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Halley’s comet can be therefore summarized as:
| Halley’s Comet | |
|---|---|
| Discoverer | Edmond Halley discovered the periodical nature of Halley’s comet. |
| Date of Discovery | The first predicted perihelion of the comet is 1758. |
| Recent Perihelion | February 9, 1986 |
| Next Visit of Halley’s Comet | 2061 |
| Orbital Period | 75.32 years |
| Associated Meteor Showers | Orionid and Eta Aquarids Meteor Showers |
| Dimensions of Nucleus | 15 km x 8 km |
| Density | 0.6 g/cm3 |
| Mass | 2.2 x 1014 kg |
| Period of Rotation | 2.2 Earth Days |
| Shape | Peanut Shaped |
| Shape of the Orbit | Elliptical Orbit with an inclination of 18O to the elliptical plane |
Things to Remember
- Halley’s Comet is a periodic comet that returns to Earth after every 76 years.
- Halley’s comet can be easily observed through the naked eye and may appear twice in a human lifetime.
- It can be usually seen during nighttime in the sky.
- The orbit of the comet is at an angle of 18o to the ecliptic plane.
- The rotation of the comet takes place every 2.2 days.
- As the comet comes closer to the sun, in the inner solar system, it warms up.
- The ice, dust, and gases from the nucleus of the comet are ejected from its surface and form a tail which is also known as a coma.
- Due to the release of the matter from the nucleus of the comet, its orbital path is altered.
- Halley’s comet is currently on its way back from the sun. It will reach its aphelion i.e. the furthest point from the sun in approximately November or December of the year 2023. Then, it will begin its 37.6 year-long journey back to the sun.
Sample Questions
Ques. Explain the composition of Halley’s comet. (5 Marks)
Ans. When Halley approaches the sun in the inner solar system, a coma or a comet tail is formed behind it. The tail of the comet is mainly composed of lighter elements such as carbon dioxide, water, carbon monoxide, and other ice and dust. It is ejected from the surface of the nucleus as the comet is warmed due to the sun. Halley’s coma is typically spanned across 1,00,000 km i.e. 62,137 miles. As the comet returns back to its farthest position to the sun i.e. its aphelion, the gases present in its nucleus again freeze. The comet then appears like a chunk of rock.
Despite having a widespread coma, the nucleus of the comet is rather small having a length of about 15 km, a width of 8 km, and a thickness of approximately 8 km. It resembles the shape of a peanut. When viewed from Earth, we can only look at the tail and coma of the comet.
Ques. What is the orbital path of Halley’s comet? (3 Marks)
Ans. The orbit of Halley’s comet is elliptical having an eccentricity of 0.967. It orbits around the sun in a backward direction and is 18o inclined to the plane. At its perihelion, the comet comes within 55 million miles i.e. 88 million km of the sun. At its aphelion, the distance is about 3.2 billion miles i.e. 5.2 billion km which is somewhere in the Kuiper Belt even beyond Pluto. The comet is visible to us on Earth during its approach into perihelion.
Ques. Describe the main parts of a comet. (5 Marks)
Ans. The five main parts of a comet are as follows:
- Nucleus: The nucleus is referred to as the core of a comet. Typically, it is solid and is formed mostly of ice and gases with traces of dust and some other solid materials such as rock. It is mainly a chunk of rock that is surrounded by frozen water and gases.
- Hydrogen Cloud: A huge cloud around the comet that typically hides its interior. When viewing a comet from the Earth, the hydrogen cloud is what makes the comet visible and easily identifiable.
- Coma: A thick cloud of gases and water that is emitted by the nucleus of a comet is known as a coma. This is the primary cloud layer of any comet and appears as fog.
- Dust Tail: The tiny particles of dust form a tail behind the comet. This tail of the comet can stretch for miles beyond and consists of bits of ice and rocks along with the dust particles.
- Ion Tail: This tail of the comet is laced with streamers and rays when it interacts with the solar wind. It can be understood as the “wake of air” left behind due to the motion of the comet.
Ques. How is Halley’s comet associated with meteor showers? (3 Marks)
Ans. In its one single orbit, Halley’s comet passes twice by Earth: one during its motion towards the sun and second on its return. The Earth, therefore, also passes twice through the trail of debris left behind by the comet. The first is in the form of a meteor shower known as Orionids each year in late October.
Another meteor shower, Eta Aquarid, is caused due to Halley’s comet in May. Although it is uncertain yet that whether the debris from Halley’s comet leads to these showers or the comet just disturbs the debris already present in space.
Ques. What is Halley's comet’s last seen date? (2 Marks)
Ans. Halley’s comet was last seen in the sky from Earth in the year 1986 when it appeared in the inner regions of the solar system. It is a short-period comet that appears every 75-76 years. The comet will again be visible in 2061 on its journey around the sun in space.
Ques. What is Halley's Comets made of? (2 Marks)
Ans. When it comes close to the sun, the nucleus of the comet ejects gases and dust. The ejected gases of Halley’s comets are observed to be made of 80% of water, 17% of carbon monoxide, and 3-4% of carbon dioxide along with traces of other hydrocarbons.
Ques. Write down 10 facts about Halley’s comet. (5 Marks)
Ans. Halley’s comet is a type of short-period comet that is observed from Earth every 75 years or so. It has a highly elliptical orbit that extends it even beyond Pluto. Some of the most notable features of Halley’s comet are:
- Halley’s comet has a long history. The first recorded appearance of this comet dates back to 240 AD.
- An English astronomer, Edmond Halley, after whom the comet is named, identified the periodicity of the comet in 1705.
- The Halley’s comet of 1835 was seen in the sky for a long period of nearly a month.
- The nucleus of Halley’s comet is irregularly shaped and resembles a peanut shell having a dark color.
- In the earlier times, Halley’s comet was often associated with bad luck and doom. It was often related to the death of kings and bad times.
- The orbit of Halley’s comet is retrograde. This means that the comet moves around the sun in a direction opposite to the planets.
- Though the coma of the comet is extended widely, the nucleus is relatively small when the vaporized gases do not surround it.
- Halley’s comet is made up of volatile substances that sublime easily when it approaches the sun.
- Two of the meteor showers: Orionids and Eta Aquarids are associated with Halley’s comet. The Eta Aquarids meteor shower is observed in May while Orionids is observed in October every year.
- It was observed that Halley’s comet is more of a snowy dirtball as compared to the regular comets that are like snowballs.
Ques. What is a comet? (3 Marks)
Ans. Comets are like dirty snowballs in space. These are small celestial bodies that are mainly made up of ice, rock, and dust. They orbit around the sun in the same way as the planets. When they appear closer to the sun, the ice and the gases present inside its nucleus are melted due to the heat of the sun.
The ejection from the surface of the nucleus surrounds the comet in the form of an atmosphere known as a coma. The tail of the comet extends up to millions of miles beyond the comet. They are mainly found to be existing in either the regions of the Kuiper Belt or the Oort Cloud.
Ques. Why is Halley’s perihelion delayed at each orbit? (3 Marks)
Ans. The analysis of the orbit of Halley’s comet has revealed that its perihelion, which is the point up to which the comet comes closest to the sun, is being delayed on each orbit by about four days. While it is generally accepted that the influences of gravity play an important role in delaying the orbit.
However, the key driver of this delay seems to be the huge sublimation effect. The release of dust and gases from the surface of the nucleus is blowing the comet off its orbit. This effect has led to a delay in the orbital period of the comet by about 5 years since it was observed in 240 BC.
The aphelion (which is the point farthest to the sun) and perihelion, (which is the point closest to the sun), of Halley’s comet, is 35 AU (Astronomical Units) and 0.57 AU respectively. This means that the closest Halley’s comet can come towards the sun is near Venus and the farthest it can go in the solar system is up to Pluto.
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