How were asteroids formed?

There are different theories about how asteroids were formed. Most scientists agree that asteroids are material that was leftover from the formation of our Solar System some 4.6 billion years ago, and others say that asteroids are the remains of a planet that was shattered to pieces following a collision with another planet.

What are asteroids?

Asteroids are chunks of rocks of uneven shape, which can be very large or as small as a pebble. Most asteroids are found in the asteroid belt between the orbits of Mars and Jupiter.

They don’t have much gravity and they are grouped according to the way in which they reflect sunlight.

Fireballs over Russia

Early on the morning of Friday, February 15, 2013, Russians in the Chelyabinsk region were shaken and over a 1,000 were hurt when a meteor exploded in the air. Scientists say that the meteor was about 55 feet across (17 meters) and before entering the atmosphere, it weighed about 10,000 tons. It travelled in the air for 32.5 seconds before exploding and the blast is estimated to be equivalent of a 470-kiloton explosion!

Impact from the blast caused considerable damage in the region and many people were injured from breaking of windows, windshields and buildings.

Scientists say the Russian meteor explosion was the biggest and most devastating since another 130-foot-wide (40m) space rock exploded over Russia’s Siberian region in 1908 and flattened 825 square miles of forest, mostly uninhabited.

According to NASA’s Near-Earth Object Programme Office Paul Chodas, “We would expect an event of this magnitude to occur once every 100 years on average.”

According to scientists, there have been bigger impacts from space objects — fifty thousand years ago, a rock about 150 feet wide (46 meters) crashed into what is now Arizona, and a crater 0.7 mile in diameter (1.2 km) can still be seen today.

In 1994, the planet Jupiter was observed to have been hit by fragments from comet Shoemaker-Levy 9.

The largest and smallest asteroid

The largest asteroid in the Solar System is 4 Vesta, measuring 578km by 458km and has a mass of 2.67 x 1020kg. It can be easily observed with binoculars on a clear night. Images from it reveal ancient lava flows and a gigantic impact basin at its South Pole. Scientists conclude that Vesta is the parent body of many meteorites. Ceres is bigger but has been promoted to dwarf planet status.

One of the smallest asteroids discovered is 1991 BA, which is only about 20 feet (6 metres) across.

Discovery and exploration

On January 1, 1801, Giuseppe Piazzi, an Italian priest, mathematician and astronomer discovered an object that he first believed was a comet. But the orbit of this object made it clear that it was more like a small planet, which he named Ceres. The others to be discovered after this were Pallas, Vesta and Juno and now several hundred thousand asteroids have been discovered and given provisional designations. There are many that are so small that they can’t be seen from Earth. They are never visible with the naked eye but many can be see with the help of binoculars or a small telescope.

NASA’s spacecraft Galileo took the first close-up images of asteroids in 1991, and in 1994 it discovered the first moon to orbit an asteroid.

NASA’s NEAR spacecraft was the first to land on an asteroid in 2001, when it touched down on the asteroid Eros, after following and studying it closely for more than a year.

Japan’s spacecraft Hayabusa takes the credit for being the first to land on and take off from an asteroid, a small near-Earth asteroid named 25143 Itokawa, in 2006, and it carried samples of rock from it when it returned to Earth in June 2010.

Physical characteristics and orbit

Almost all asteroids are irregularly shaped, often with deep pits or craters. The average temperature on the surface of a typical asteroid is considered to be minus 100 degrees F (minus 73 degrees C).

Asteroids rotate and often tumble while they revolve around the Sun in elliptical orbits. Some asteroids have one or two small companion moons, and scientists have found some double and triple asteroid systems in which two or three asteroids of about the same size orbit each other.

Asteroids that have been captured by the gravitational pull of a planet become its moons, such as Phobos and Deimos that are moons of Mars and most of the distant outer moons of Jupiter, Saturn, Uranus and Neptune.

Types of asteroids

The main classification of asteroids is according to their chemical composition and spectra.

C-type, or carbonaceous, they are very dark, with clay and stony silicate rocks being, presumably, their main composition. These greyish coloured bodies make up 75 per cent of known asteroids.

S-type, or silicaceous asteroids, are greenish to reddish in colour because of their chemical composition of metallic nickel-iron mixed with iron- and magnesium-silicates. Seventeen per cent of known asteroids are of this type.