Astro-QA 10 : What is a comet? Where do they come form? (Interesting Facts About Astronomy)

We all must have seen the celestial objects with a long “tail” behind - known as Comets.
 
But what are they?
 
Where do they come from?
 
Comets are small celestial bodies that orbit the Sun. Their orbits are highly elliptical which brings them very close to the sun from very outer edge of the solar system. Like asteroids, comets are thought to be remnants of Solar System formation. But while asteroids are generally comprised of rock and metal, comets are primarily made of dust and ice. They are composed of frozen gases such as carbon dioxide, methane, and ammonia, as well as water ice, dust particles, and rocky materials.
 
As a comet approaches the Sun, solar radiation "melts" the surface, vaporizes molecules of gas and dust. It creates two tails. The gases in the comet make the first tail, called the plasma or the ion tail. This tail is thin and tends to point directly away from the Sun. The dust in the comet is responsible for the second tail. This tail, also called the dust tail, tends to be broad and curved and most of the time trial behind the Comet. The two tails usually point in two different directions.
 
Comets spend most of their lives far away from the Sun in distant parts of the solar system. Based on their orbital periods there are two different types of Comets.  Short period comets, which orbit the Sun in 200 years or less, while long-period comets that take hundreds or thousands of years to orbit the Sun. So, based on that Scientists thought that there must be two different regions in the solar system from where the comets are originated. The search finally shows that there are in need two different regions. The first region is known as The Kuiper Belt. It is a disk composed mainly of icy bodies that stretches from Neptune's orbit, about 30 AU out to 50 AU from Sun. By the way, 1 AU is about 149.6 million km. The second region is known as the Oort Cloud. It is at the edges of the Sun's gravitational influence (about 50,000 to 200,000 AU) and this Oorts cloud divided into two regions: the inner, disc-like Hills cloud, and the outer spherical cloud, both composed of icy bodies.
 

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