Asteroids

There is a big gulf between the orbits of Mars and Jupiter. This vast void of more than 550 million km is the domain of numerous tiny dwarfs, known as Asteroids or Planetoids. Also known as minor planets, asteroids are actually cosmic fragments. They are the rocky debris left uncoordinated since the early days of the Solar System about 4.6 billion years ago. It is possible that these debris could not complete the planet-forming process. Early in the history of the Solar System, the gravitational pull of the newly formed Jupiter did not allow them to form planetary bodies in this region and caused the small bodies to collide with one another, thereby fragmenting them into millions of asteroids. Though they revolve around the Sun and have their own elliptical orbits, they are too small to be called planets.

Asteroids are mostly irregular in shape, while a few of the largest are nearly spherical. As they revolve around the Sun, they also rotate and sometimes tumble quite erratically. They have very little mass, the mass of all the asteroids taken together is less than that of Earth's moon. Ceres, the largest among the Asteroids, is 940 km across, while the smallest asteroid, which was observed when it made a close flyby of Earth in October 2015, is only two meters wide. Some of the Asteroids are cratered and squashed.

‘Vesta’ has a giant crater some 460 km in diameter. It is considered that, more than 150 Asteroids have a small following moon, while some have two. Sometimes, two rocky bodies of roughly equal size, orbit each other. They are known as Binary Asteroids. Likewise, there are triple asteroid systems as well. The average temperature of the surface of a typical asteroid is - 73º C.

Asteroids have three domains in the solar system. The main asteroid belt, between the orbits of Mars and Jupiter, is larger than 100 km in diameter and it holds more than 200 asteroids. However, everything in the main belt is not an Asteroid. Ceres, once considered as an asteroid, is now also considered a dwarf planet.

There are many asteroids that lie outside the main belt. Some Asteroids, known as Trojans, lie in the same orbit as Jupiter. They have stable orbits, 60 degrees ahead or behind the position of the planet, which are called the Lagrange points of Jupiter's orbit. Jupiter Trojans are the most numerous in number, nearly as high as the main asteroid belt. Neptune, Mars and Earth also have Trojan asteroids. Near-Earth asteroids circle closer to Earth than the sun and they actually cross the orbital path of the Earth. Among them, 1,409 Asteroids have been classified as potentially hazardous which could pose a threat to the Earth.

Based on their composition, Asteroids are classified into three categories. The C-type or Carbonaceous Asteroids are most common, probably consist of clay and silicate rocks, are grayish in colour. The S-type or Silicaceous Asteroids dominate the inner asteroid belt. They are composed of silicate materials and nickel-iron and look greenish to reddish in colour. The M-type or Metallic Asteroids seem to be made up of nickel-iron. They dwell in the middle region of the main belt and are reddish in colour.

Since the formation of the Earth, asteroids and comets routinely slammed into the planet. When an asteroid, or a part of it, crashes into Earth, it's called a meteorite. An asteroid can create global disaster. However, NASA assures us that, the most dangerous asteroids are extremely rare.

An asteroid slammed into the atmosphere over the Russian city of Chelyabinsk on 15 February, 2013. It created a tremendous shock wave that injured 1,200 people. The said space rock is thought to have measured about 65 feet (20 m) wide when it entered Earth's atmosphere.