1. Comets

2. A comet is a small, icy celestial body that orbits around the Sun.
Made up of a nucleus (solid, frozen ice, gas and dust),
A gaseous coma (water vapor, CO2, and other gases)
And a long tail (made of dust and ionized gases).

4. Comet’s Tail Develops only when the comet is near the Sun.
Its long ion tail of always points away from the Sun, due to the force of the solar wind.
The tail can be up to 250 million km long, and is most of what we see.
Comets are only visible when they're near the Sun in their highly eccentric orbits.

8. Famous Comets #1 - Comet Halley
Most famous comet till date. Named after British astronomer Edmund Halley, this comet was seen in 1531 and 1607.
According to his observations, every time it approaches the Sun, its 15-km nucleus drops about 6 m of ice and rock into space.
This results in the formation of an orbiting trail, which, when it falls on Earth, is called Orionids meteor shower.
It is anticipated that Comet Halley will return to the inner Solar System in 2061.

10. Famous Comets #2 - Comet Hyakutake
Comet Hyakutake was discovered by an Japanese astronomer - Yuji Hyakutake. The scientist observed it on January 30, 1996, with the help of a pair of binoculars.
The comet seemed to be small and bright, with the longest tail ever observed. According to the calculations done by astronomers, the orbit of Comet Hyakutake will not bring it near the sun again for next years.

12. Famous Comets #3 - Comet Swift-Tuttle
American astronomers Lewis Swift and Horace Tuttle discovered Comet Swift-Tuttle in July 1862.
According to their observations, as this comet moves nearer to the Sun in every 120 years, a trail of dust debris is left behind, which provides the ingredients for a spectacular fireworks display seen in July and August.
Scientists predict that this comet will collide with Earth one day, because the two orbits closely intercept each other.

14. Famous Comets # 4 - Comet Hale Bopp
Alan Hale of New Mexico and Thomas Bopp of Arizona, discovered it on July 23, 1995 just outside Jupiter's orbit.
It was carefully analyzed using Hubble Space Telescope. Hale-Bopp comet is famous for holding a record for the longest period of naked-eye visibility.
It is predicted that this comet will not appear in another 2400 years.

16. Ion Tail:
A tail of charged gases (ions) always faces away from the sun because the solar wind (ions streaming from the sun at high velocities) pushes it away (it is also called the plasma tail).
When the comet is approaching the Sun, the ion tail trails the comet: when the comet is leaving of the Sun, the ion tail leads.
The tail fades as the comet moves far from the Sun. The ion tail can be well over 100 million km long.

17. Dust Tail:
The dust tail is a long, wide tail composed of microscopic dust particles that are buffeted by photons emitted from the Sun
This tail curves slightly due to the comet's motion. The tail fades as the comet moves far from the Sun.

18. Nucleus: The nucleus is the frozen center of a comet's head.
It is composed of ice, gas, and dust.
The nucleus contains most of the comet's mass but is very small (about 1 to 10 km across - or more).

19. Coma:
The coma is the roughly spherical blob of gas that surrounds the nucleus of a comet
It is about a million km across.
The coma is made of water vapor, carbon dioxide gas, ammonia, dust, and neutral gases that have sublimed from the solid nucleus.
The coma and the nucleus form the head of a comet.

20. Hydrogen Envelope:
Hydrogen gas surrounds the coma of the comet and trails along for millions of miles (it is usually between the ion tail and the dust tail).
The hydrogen envelope is about 10 million km across at the nucleus of the comet and about 100 million km long. It is bigger when the comet is near the Sun.