What is an asteroid? NASA explains

Asteroids are small rocky objects left over from the formation of our solar system they range from the size of a small rock to the size of asteroid Ceres which is more than 600  miles across. Ceres is so large it is often called a dwarf planet like Pluto, rather than an asteroid. 

Old astronomers studied the night sky, mapping everything they saw. Giuseppe Piazzi found the first and largest asteroid in 1801 and named it "Ceres".

Most asteroids reside in the asteroid belt between the orbits of Mars and Jupiter. Probably they are formed inside the orbit of Jupiter.

Their diameters range from the size of small rocks to more than 600 miles. They are composed of rock and metals. Their surface is solid and stable, showing craters where other objects have crashed into it. The surface does not boil off and here are no trails.

Asteroid orbit is stable and fairly circular they have left craters on earth and may have caused mass extinctions such as those of dinosaurs.

To ensure that no sizeable asteroid crashes on Earth, space agencies like NASA track them. All those deemed to be potentially hazardous are tracked on a daily basis. 

NASA even carried out its DART mission, which was aimed at deflecting an asteroid from its path by crashing a small spacecraft into it. 

The mission was successful and may be used if any asteroid threatens Earth in the future.