What are asteroids?

Scientists say asteroids may impact Earth:

Asteroids which are referred to as planetoids or minor planets by scientists are actually small bodies of rock and iron left over by the formation of planets about 4.5 million years ago. Made of rock and metal, they can also contain organic compounds. Billions of these small Solar System bodies are said to be orbiting the Sun .

Most asteroids orbit the Sun between Mars and Jupiter and this area is called the asteroid belt. Of the millions of asteroids orbiting the Sun,the biggest asteroid is Ceres. It was the first asteroid discovered It is about 600 miles (960 km) around.

Unlike most asteroids, Ceres is spherical in shape. Ceres was discovered by the Italian astronomer Giuseppe Piazzi in 1801 as he searched for a planet which was predicted to exist between Mars and Jupiter.

A second minor planet " Pallas " was found by Wilhelm Olbers in 1802. This was followed by Juno (1804) and Vesta (1807). Since 1847, not a year has passed without the discovery of at least one asteroid.

The word asteroid means 'star-like' because they could only be seen as small points through most telescopes.

The possibility of an asteroid colliding with Earth has received increased attention over recent years.

The Shoemaker-Levy comet that collided with Jupiter in 1994 were given widespread media coverage.

There are now many groups and organizations that use automated systems to discover near Earth asteroids. While many are discovered, they rarely have the potential to cross paths with Earth.

However,according to scientists a huge asteroid is expected to pass very close to Earth this November(see box).

Many scientists have also predicted the possibility of asteroids impacting Earth in the near future, even in 2012..

Giant asteroid heading close to Earth

A giant asteroid weighing 55 million tonnes will just miss the Earth later this year, NASA experts have predicted.

The rock, which is quarter of a mile across, will pass between our planet and the moon in November and will be visible with small telescopes.

Robin Scagell of the Society for Popular Astronomy said:"It's rarely we get the chance to see an asteroid up close."

If it were to hit the Earth, the asteroid, named YU55, would have an impact equivalent to 65,000 atom bombs and would leave a crater more than six miles wide and 2,000ft deep. Passing by at a distance of just 201,000 miles, the asteroid will be the largest object ever to approach the earth so close. NASA has officially labeled it a Potentially Hazardous Object, but have stressed there is no danger of impact while on its current course.

YU55 orbits the Sun every 14 years and last passed the Earth in April 2010 at a distance of 1.5 million years. There are currently 874 near Earth asteroids which are considered to be potentially dangerous.

They are closely monitored and the list is being constantly added to as astronomers discover new objects in near space.