In ancient times comets symbolized all sorts of disasters such as wars, diseases, or punishments from God.
It's no wonder that they cause such alarm when they appear unexpectedly with such fierce intensity.
Scientists originally thought they were a phenomena in the Earth's atmosphere,
yet when Tycho Brahe calculated the distance in the 17th century, it became clear that they are very far away and not in our atmosphere.
Orbits of comets look like long stretched-out ovals, or ellipses.
Since the Sun is located near one end of this long ellipse, a comet spends most of the time moving slowly in the
outer solar system before whizzing past the Sun. These cold and icy objects become very active when they approach the
Sun and their remarkable tails form.
Astronomers often compare a comet nucleus
with a dirty snowball.
The small rocky core of a comet is covered with an icy layer mixed with dust.
Scientists believe that comet nuclei have the same composition as the primordial material that formed the Solar System.
It was quite surprising to find out that in addition to rocks and ice, comets contain some complex organic molecules that
could be building blocks for life. Generally, a comet only becomes visible when it approaches the Sun and begins to melt.
The frozen ice and dirt burn into a cloud of gas and dust which surrounds the nucleus. The dust and gas cloud are
called the comet's coma
The coma is so thick near the center that it is impossible to actually see the nucleus without a spacecraft.
As a comet moves closer to the Sun, the solar wind blows the coma out behind the comet into a tail.
The tail always points away from the sun. It has two parts: a yellowish dust tail and a bluish ion
The dust tail is the part we can see in the sky. Dust particles that escape from the nucleus can trail after
the comet for up to 10 million kilometers. The ion tail is formed from the solar wind and is slightly curved
following the lines of Sun's magnetic field. Even though comets shine brightly in the sky, the density
of the molecules of a typical tail is a thousand times less than Earth's atmosphere.
The Oort Cloud
is a big sphere surrounding the outer part of the Solar System.
It begins at 50,000 AU and spreads up to one light year from the Sun.
The Oort cloud contains a bunch of cold icy bodies which were left over from when the Solar System formed.
Sometimes these objects get sent into the inner Solar System and become comets due to gravitational
forces from the planets, nearby stars, or due to collisions. Some of the comets pass around the Sun only
once and then leave the Solar System, others simply burn up when they get close to the Sun.
If the comet becomes gravitationally trapped it will start to orbit in highly elongated pattern.
Such comets are called long periodic comets and have orbital period longer than 200 years.
With telescopic observation astronomers have successfully determined the orbit and chemical composition
of comet tails. However, the only way to learn more about the nuclei of comets is to use spacecraft.
The spacecraft Giotto
explored Halley's Comet
mission was the first project that succeeded in returning samples to Earth.
Pluto is no longer classified as a planet. Ever since Pluto was discovered by Clyde Tombaugh
people debated about whether it was a planet or not.
What makes an object a planet?
They are large, composed of rock or gas, and orbit the Sun. The inner four planets Mercury, Venus,
Earth, and Mars, are all pretty small and rocky. The outer four planets Jupiter, Saturn, Uranus,
and Neptune are all much bigger and mostly made of gas. Pluto is a ball of rock and ice,
but is very small, even smaller than our Moon. Pluto's orbit is very stretched out and tipped when
compared to the rest of the solar system. In fact, Pluto's orbit
is so stretched out that sometimes
it is closer to the sun than Neptune! Even though Pluto has its own moon, Charon
it is more like the Kuiper Belt
objects than any of the planets in the Solar System.
That's way the members of the International Astronomical Union (IAU) decided in 2006 to remove Pluto
from the list of the major planets in the Solar System and make it member of the small planets family,
where comets and asteroids belong. In order give Pluto recognition,
people suggested calling Pluto the "King of the Kuiper Belt."