The night sky shines with little, distant, bright objects called the stars. Sometimes, we may see a bright object with a tail moving fast in the night sky.
Is it a star? The simple answer is No.
So, what is that beautiful object?
It is a “comet.”
A comet is a small object made of dust and ice that orbits, or travels around, the sun and is known for its long, streaming tail. Comets are leftovers from the formation of the solar system, which was 4.6 billion years ago.
Comets are not stars, and they appear in the sky differently. A comet does not appear as points of light like stars but as a bright object with a long, narrow tail of light.
Compared to planets, comets are very small in size. The average diameter of a comet is usually between 750 meters and 20 kilometres long. Recently, astronomers have found larger distant comets with diameters of 300 kilometres or more. However, they are still small compared to planets.
The solar system has billions of comets, but the number of known comets currently is 3,743. However, most comets never pass close to Earth.
The best-known comet throughout human history is called Halley’s Comet. It was named after English astronomer Edmond Halley who observed it in 1531, 1607 and 1682. Halley’s comet is a periodic comet that can be seen from Earth about every 76 years. It was last seen in 1986, and it will return to the vicinity of Earth around 2061.
The central part at the heart of a comet is called the nucleus. It is a solid core made of frozen water, gases, dust, and rock. The nucleus is often a few miles across.
A comet often consists of the nucleus alone. However, it starts heating up when it gets close to the sun until it sheds gas and dust. The gas and dust create a huge, fuzzy cloud around the nucleus called the coma. The coma is often larger than Earth, and it makes the head of the comet look blurry.
As a comet moves in the solar system, it gets closer to the sun. When it does, the energy and streams of particles from the sun push some of the coma’s dust and gases out into bright, long streams called tails. The tail of a comet can stretch behind it for millions of miles. The tail and coma of a comet last only when it is close to the sun.
Astronomers have found that comets have two separate tails. The first looks white and is made of dust. The second is bluish and is made up of ions or electrically charged gas molecules. The dust tail traces a broad, curved path behind the comet, and the ion tail always points directly away from the sun.
Most comets are formed in the outer solar system in the Kuiper Belt or the Oort Cloud. The Kuiper Belt is a wide disk beyond the orbit of planet Neptune. Comets in the Kuiper Belt are called short-period comets, and they take less than 200 years to orbit the sun.
The Oort Cloud is the outer edge of the solar system, located about 50 times farther from the sun than the Kuiper Belt. Comets found in the Oort Cloud are called long-period comets because they take much longer to orbit the sun.
The orbit of an object in the solar system is the path it takes to travel around the sun. While most planets have circle orbits, the orbit of a comet is shaped like a long oval. A comet can take several to thousands of years to complete one orbit.
The answer is simple: Gravity. The gravity of a planet or a star can pull comets from the outer solar system and redirect them toward the sun.
While being pulled faster toward the sun, the comet swings around behind it until it heads back toward where it came from. However, some comets dive into the sun and never come back.
We can see comets in our skies only when they are in the inner solar system, either coming or going.
As Earth moves, it may pass through the orbit of a comet. When it does, bits of dust from the comet crash into the atmosphere of Earth. Upon this, they burn up in the sky, causing a meteor shower.
Comets have been known and admired for thousands of years. Still, a comet’s coma made it impossible to see a comet’s nucleus from Earth clearly.
In recent years, several spacecraft have studied comets up close. For example, NASA’s Stardust mission collected samples from Comet Wild 2 and brought them back to Earth. Besides, the European Space Agency’s Rosetta mission studied Comet 67P Churyumov-Gerasimenko. It orbited the comet for two years after dropping a lander on the nucleus.
Such missions and many others have helped us know more about comets and the types of chemicals found in them. It is even helpful in learning more about the formation of the solar system.
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