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Asteroids Comets and Meteoroids – Explanation and Properties

Grade 8
Jun 10, 2023

Asteroids Comets and Meteoroids


Our planet Earth is a part of the solar system, which has 8 planets revolving around a star named the Sun. Additionally, there are natural satellites revolving around most planets in the solar system. Other celestial bodies, asteroids, meteors, and comets are all part of the solar system. In this session, we will look at asteroids, comets and meteoroids.


Besides the planets, the Sun and the Moons in the solar system, other objects revolve around the Sun. These are the asteroids, meteoroids, and comets. Let us look at them one by one.


Asteroids are rocky objects, smaller than a planet in size, and are found between the orbits of Mars and Jupiter. Their elliptical orbits around the Sun can sometimes be disturbed, which can cause them to hit the Earth and other planets.


There are hundreds and thousands of asteroids. They were the leftover from the solar system’s formation about 4.6 billion years ago. They do not have an atmosphere, which is a result of weak gravity. Due to this, they have craters on their surfaces as a result of impacts on other objects.



A comet is a small, icy body in orbit around the Sun. Some of them have a highly elliptical orbit, bringing them close to the Sun and taking them well beyond the orbit of Pluto. They may be the leftover from the formation of the solar system, just like asteroids.


Structure of a Comet:

  • A comet has a solid part called the nucleus, which is a loosely packed lump of icy material that is often only a few kilometres across. Apart from water ice, the nucleus contains frozen gases, dust, and bits of rock and a small, rocky core.
  • The heat vaporizes the frozen gases when the nucleus is close to the Sun. As a result, an atmosphere called the coma is produced when gases and dust are released through the vents from the nucleus. This atmosphere can extend tens of thousands of kilometres beyond the nucleus.

A tail extends from the coma and forms only when a comet is near the Sun. Solar radiation causes the gases in the coma to glow, allowing us to see a comet from the Earth.

Comet – A Probe:

Deep Impact spacecraft launched a small probe on July 4, 2005, on a collision course with a comet named Temple- 1. The probe recorded extraordinarily clear images of the comet before it collided with Temple- 1 at a speed of 10 km/s. This showed that the comet’s surface is covered in craters, similar to that of the Moon and Mercury. As a result of the impact, the material inside Temple- 1 was ejected, and the scientists were able to obtain a great deal of data on the composition of the comets.

Deep Impact

1. Short Period Comets:
  • Short-period comets are those whose period of revolution around the Sun is less than 200 Earth years. Some comets sweep very close to the Earth and are hence seen often. E.g., Halley’s Comet, in 1986. It has a period of 76 years. Most short-period comets come from a region of icy bodies beyond the orbit of Neptune. These icy bodies are called Kuiper belt
  • The Kuiper belt is an area of the solar system that extends about 50 AU from the Sun toward the orbit of Neptune. For many years it was assumed that most short-period comets originated from the Kuiper belt. However, these days many astronomers suggest that some of them originate from a region of space that is more distant than the Kuiper belt.

Kuiper belt

2. Long Period Comets:
  • Long-period comets have an orbital period longer than 200 years. In fact, some of them have orbital periods of millions of years. Some scientists have proposed that these comets originated from a spherical, shell-like swarm of comets called the Oort cloud, which surrounds the solar system.
  • It is estimated that the outer edge of the Oort could be as far as 100000 AU from the Sun, which is about half of the distance to Proxima Centauri, the star nearest to the Sun.


Most meteorites found on the Earth are rocky in composition similar to the Earth’s mantle, suggesting a similar origin. That’s why some astronomers believe that meteorites have come from a small planet that broke apart during the formation of the solar system, which would have had a small iron-nickel core and a large, rocky mantle.


Our Earth, just like other objects in the solar system, is continually bombarded by meteoroids. Most of them burn up in the atmosphere itself before reaching the Earth’s surface. Some are large enough to reach the Earth’s surface. These produce impact craters on its surface. It is estimated that the meteoritic material that falls to Earth each day is somewhere between 1000 to 10000 kg.

The picture shows the Barringer Crater from Arizona, which was formed over 50000 years ago when a large meteoroid struck the Earth. Most craters get filled in over time due to erosion from wind and water. However, the Barringer Crater is located in a dry area of the US, hence is still visible.

However, most moons in our solar system do not have a significant atmosphere, which would prevent them from these meteoroids. This gives rise to craters on them. As there is no erosion due to the absence of air or water, the craters are not filled in.

Impact crater

Questions and Answers:

Match the following



i—e, ii—c, iii—a, iv—f, v—b, vi—d

Asteroids Comets and Meteoroids


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