Difference between Asteroid and Comet

Introduction

Difference between Asteroid and comet is an important concept of Astronomy. Celestial bodies like planets, asteroids, comets etc orbit the sun. Asteroids and comets are sometimes termed as “time capsule” since they hold information about the solar system. They both are made from the leftovers of the moon and planets and formed about 4.5 billion years ago. These two sometimes can have unusual orbits due to gravitational disturbances and drift nearer to any planet.

The main difference between asteroids and comets is their composition i.e., with what they are made of. Asteroids are metallic or rocky bodies without atmosphere whereas comets are composed of ice, dust or rock. Majority of asteroids are moving around the sun in a region between Mars and Jupiter. Most of the comets reside in the farthest regions of our solar system.

What is an Asteroid?

Asteroids are rocky or metallic celestial bodies that orbit the sun. Asteroids were formed billions of years ago from the leftover space debris.

History of Asteroids

About 4.5 billion years ago, when a big cloud of gas and dust collapsed, our solar system started to form. After the collision, most of the material fell into the centre of the cloud and formed the sun. Planets were formed with some condensing dust from the cloud. The asteroids never had an opportunity to integrate into a planet. These asteroids are witness to the birth of planets and their satellites.

Types of Asteroids

Asteroids are generally classified into three types based on their compositions −

• C-type: Carbonaceous

• M-type: Metallic

• S-type: Salicaceous

Characteristics and Size - Asteroid

• Asteroids are rocky or metallic objects in the solar system.

• Asteroids generally live between Mars and Jupiter and this region is known as the “asteroid belt”. Some asteroids stay in other places too. As for example, some of them are found in the orbital path of the planets.

• ‘Asteroid belt’ region is too warm for ice to remain solid. So, it is quite impossible to have ice for asteroids.

• These asteroids are much smaller compared to the planets. Even the total mass of all the asteroids doesn't exceed the mass of Earth’s Moon.

• Most of the asteroids of this ‘asteroid belt’ revolve in elliptical, stable orbits.

• Asteroids rotate around the sun in the same direction as that of Earth in a time range of three to six years. Although some of them have larger orbital periods.

• Asteroids are of different shapes. They have a jagged and irregular shape. Generally, two asteroids don’t have identical shapes.

• Size of the asteroids varies over a wide range. Some of them are like small stones or bullets where some of them have a diameter of hundreds of kilometres. Ceres, Vesta etc with a diameter of 500 km or above are examples of some large asteroids.

• Asteroids don’t have a tail like comets.

What is a Comet?

Comets are celestial bodies that are made of ice and dust and orbit the sun. They are usually identified by their streaming tails. Comets were also formed before the formation of planets from the leftover materials of our solar system.

Coma or tail of a Comet

When the Comets come close to the sun, they become warm and some portion of their ice melts. Due to the heat of the sun, when some part of ice melts, the other materials of its composition vaporise. In this process, visible atmosphere is produced and often we named it as a tail. These tails always point away from the sun. The ice and other materials produce a fuzzy cloud-like shell known as coma.

Types of Comets

Comets can be classified into two groups depending upon their orbital periods- Periodic comets and non-periodic comets.

• Periodic Comets − The comets orbiting the sun less than 200 years are known as Periodic comets. About 440 periodic comets have received official designation till date. Among them, the most popular is Halley’s Comet.

• Non-periodic comets − Comets comprise of orbital periods more than 200 years or passed the solar system once are known as Non-periodic comets. C/1980 E1, C/2000 U5 etc are examples of Non-periodic comets.

Kuiper belt and Oort cloud

Comets are mostly found in the outermost regions of the solar system. Some of them reside in a wide disk outside the orbit of the dwarf planet Pluto named as ‘Kuiper belt’. This belt consists of millions of icy comets. These Kuiper belt Comets are often called Short-period comets since they rotate the sun less than 200 years.

Other comets live in the sphere-shaped outer edge of the solar system named as Oort cloud. In this region, trillions of comets revolve the sun at huge distances up to 20 trillion kilometres. These Oort cloud comets are also known as Long-period Comets because they take more than 200 years to rotate the sun.

Characteristics and Size - Comet

• Comets are composed of ice, dust and hydrocarbons.

• When Comets approach closer to the sun, the icy part of comets starts to melt and the materials start to vaporise. This results in a glowing halo that extends outwards as it sails through space. This is known as coma and sometimes we call it the tail of a Comet.

• Comets orbit the sun in eccentric elliptical orbit.

• The orbital periods of Comets vary in a range from several years to several millions of years.

• Comets having orbital periods less than 200 years generally stay in the Kuiper belt. While the higher orbital period comets stay in the Oort cloud region.

• Compared to planets, comets are very small in size. The average diameter of comets generally ranges from 750 metres to about 20 km.

Difference between Asteroid and Comet

Asteroid	Comet
Made of metal and rocks.	Made of ice, dust and hydrocarbons.
They have slightly elliptical orbits.	They have highly eccentric elliptical orbits.
Do not produce tail or coma in the atmosphere.	Produce coma or tail in the atmosphere when it reaches closer to the sun.
Orbital period is small comparatively.	Orbital period is large comparatively.
Stays nearer to the sun, mostly between mars and Jupiter.	Stay in the outermost region of the solar system.

Table-1: Difference between Asteroid and Comet

Conclusion

Asteroids and comets revolve the sun in some elliptical orbits. From the leftover materials of the solar system asteroids and comets were formed much before the formation of planets. Along with some similarity they have a number of differences. Composition of asteroids and comets is completely different. Where tails are the unique identification of comets, asteroids don’t have tails. A great difference is also observed between the orbital period of these two. Asteroids stay comparatively closer to the sun than comets.

FAQs

1. Can an asteroid hit the sun?

Asteroids usually stay in the asteroid belt between Mars and Jupiter. No asteroids have been observed to hit the sun. But occasionally something escapes them out from the orbit and allows them to roam in the solar system.

2. How big was the asteroid that killed the dinosaurs?

We don't know its size exactly and where it came from. According to NASA, it was around 12 km wide.

3. What is the most famous comet?

Hally’s comet. It is a periodic comet.

4. What are comets called?

Comets are sometimes called dirty snowballs or snowy dirtballs, depending on whether they contain more ice materials or rock materials.

5. What are celestial bodies?

Celestial bodies are objects in space such as the sun, moon, planets, stars etc.