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Comets are spectacular celestial bodies orbiting the Sun.



asteroid, meteor, comet, Halley-üstökös, Halley, astronomy, Solar System, Oort cloud, asteroid belt, astronomical object, Kuiper Belt, space research, orbital period around the Sun, outer space, physics, Sun, geography

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The Solar System

  • comet
  • Oort cloud - A spherical "cloud" that forms the outermost boundary of our Solar System. It is composed of billions of cometesimals.
  • Kuiper Belt - A belt of asteroids situated beyond the orbits of the giant planets of the Solar System.

From time to time, we can observe spectacular phenomena taking place in the Solar System. These include the appearance of comets. Comets are celestial bodies that are visible from Earth to the naked eye; however, they only appear in the sky at a specific time.


  • head - The coma and the nucleus together form the comet's head.
  • gas tail - The comet's straight, longer tail aligned along the Sun-comet line. It is made up of gases released from the nucleus.
  • dust tail - The comet's curved, shorter tail. It consists of particles of dust particles driven off the nucleus by the escaping gases.
  • solar wind

Comets consist of a nucleus, a coma and a tail. They are made up of the oldest substances of the Solar System. They were formed when some of the material of the Solar Nebula was expelled to the outer regions of the Solar System where the temperature was low enough for the gases to freeze and thereby cement together rock debris and dust particles. Therefore, the nuclei of comets, held together by water ice and various frozen carbon compounds, are irregular in shape and have a porous crust. This is the reason why comet nuclei are often compared to dirty snowballs.

When a comet passes near the Sun, some of the frozen gases in its nucleus sublime, that is, they enter the gas phase, taking the dust as they escape. This is how the coma, the atmosphere-like envelope of the cometary nucleus is formed. Each time the comet passes near to the Sun, it loses some of its material and eventually it will disappear.

Solar wind pushes gas and dust particles in the direction opposite the Sun, thereby forming the tail of the comet. Comets have two tails: the longer, straight tail is made up of gases, while the shorter, curved one is made up of dust particles. The gas particles in the gas tail are in ionised state, therefore they emit bluish light; while the dust particles in the dust tail (or antitail) reflect the Sun’s light, this is why the dust tail appears bright and yellowish.


  • Kuiper Belt - A belt of asteroids situated beyond the orbits of the giant planets of the Solar System.
  • short-period comet - The orbital planes of such comets are similar to the plane of symmetry of the Solar System. They have orbital periods of less than 200 years. They originate in the Kuiper belt.
  • long-period comet - Such comets have orbital periods longer than 200 years, up to several million years. They originate in the Oort cloud.
  • part of the orbit close to the Sun

Comets usually originate in the two farthest regions of the outer Solar System, the Kuiper belt or the Oort cloud. As these regions are unstable from the point of view of gravity, any kind of disturbance can result in a comet leaving its orbit and starting to move towards the inner Solar System.

According to their orbit, comets can be classified as short-period or long-period comets. Short-period comets originate in the Kuiper belt and their orbital period is less than 200 years, that is, they return to the inner Solar System relatively often. Long-period comets, however, originate in the Oort cloud and their orbital period varies between 200 and several millions of years.

Halley's Comet

  • Sun
  • Jupiter
  • Saturn
  • Uranus
  • Neptune
  • Halley's Comet - It has an orbital period of 75.3 years. It was last observed from Earth in 1986 and it will next appear in 2061. It is supposed to be the earliest observed comet.
  • part of the orbit close to the Sun

Halley’s Comet is the earliest comet to be observed by man. It is visible to the naked eye. The last time it was visible from Earthe was in 1986 and it will return in 2061; its orbital period is about 75 years. It is a short-period comet.

During its appearance in 1986, several space probes were studying the comet. According to the data then collected, its 15x15x8 kilometre nucleus is made up mainly of water ice, covered by a dark, thin crust.

Comet 67P/Churyumov–Gerasimenko

  • Sun
  • Earth
  • Mars
  • Jupiter
  • Saturn
  • Uranus
  • Neptune
  • Comet 67P/Churyumov–Gerasimenko - Currently, it has an orbital period of 6.5 years. It was discovered in 1969. It is the first comet to be orbited and landed upon by robots from Earth.

The Comet 67P/Churyumov-Gerasimenko probably originated in the Kuiper belt but its orbit was greatly modified by Jupiter’s gravity. It currently orbits in the inner Solar System and has an orbital period of 6.5 years.

The shape of its nucleus resembles that of a rubber duck, as it consists of two larger lumps connected by a narrow neck. A significant amount of water vapour escapes from its surface when the comet passes near the Sun. It was the first comet on which measuring instruments landed.

Jupiter's gravity

  • Comet Shoemaker–Levy - It was discovered in 1993. It probably started orbiting Jupiter in the 1970s, when the planet's gravity captured it. Jupiter's gravity pulled it apart into several pieces probably in 1992. The pieces of the comet collided with the southern hemisphere of Jupiter in July 1994.

Jupiter’s mass is large enough to modify the orbits of comets coming from the outermost regions of the Solar System or even to attract them into its own gravitational field, thereby protecting the inner planets from colliding with comets. One of the comets captured by Jupiter was the Shoemaker–Levy, discovered in 1993. It probably started orbiting Jupiter in the 1970s, when the planet’s gravity captured it. It consisted of several pieces, as it was probably pulled apart by Jupiter’s gravity in 1992. The pieces of then collided with the southern hemisphere of the planet in July 1994.

Comet, asteroid, meteoroid

  • comet - A celestial object consisting of dust and ice. When it passes close to the Sun it forms a tail.
  • asteroid - A celestial object with a diameter greater than 1 km, but smaller than a planet.
  • meteoroid - Objects in space smaller than asteroids. If they collide with the Earth's surface, they are called meteorites.
  • meteor - A light phenomenon produced by a meteoroid reaching the Earth's atmosphere. It is also known as a "shooting star".
  • meteorite - A piece of a meteoroid that has collided with the Earth's surface and survived the impact.

Many of us cannot tell the difference between comets, asteroids and meteoroids.

The tail that forms near the Sun is one of the characteristic features of comets. Asteroids, which are similar in size, do not have tails. Their material composition is different as well, due to their different place of formation. Asteroids were formed near the Sun, while comets were formed far away from it.

Meteoroids are smaller than comets and asteroids but larger than interplanetary dust particles. When they enter the Earth’s atmosphere, they interact with the particles of the air. The resulting light phenomenon is what we call a meteor, or 'shooting star'. If a meteoroid does not completely burn in the atmosphere it reaches the Earth’s surface as a meteorite.

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