What is the tail of a comet?
You cannot see the nucleus of a comet with the naked eye, but you can sometimes see its tail. It appears as a smear of light that moves very gradually across the sky. As a comet moves closer to the Sun, the ice and other frozen gases in its nucleus begin to boil off, producing a long tail of gas and dust. The tail always points away from the Sun because light and other forms of radiation from the Sun push against the minute particles that are present within the tail.
What is an asteroid?
Asteroids are small rocky or icy bodies that orbit the Sun. They are sometimes called minor planets. Most asteroids are found in an orbit between Mars and Jupiter, and more than 7,000 of them have been identified.
Asteroids are smaller than any of the planets, and only a few have a diameter of over 30 km. The term asteroid is usually applied to objects larger than 1.6 km in diameter. One asteroid, called Ida, has a tiny moon of its own; this is the smallest known satellite in the Solar System. Asteroids were probably formed at the same time as the planets.
Space bombardment
Many asteroids have struck the Earth already, and many scientists believe that such an impact resulted in the extinction of the dinosaurs about 65 million years ago. At that time an asteroid or huge meteorite about 10 km in diameter struck the Yucatan region of Mexico. It gouged out a huge crater and hurled so much dust into the Earth’s atmosphere that the world’s climate changed drastically causing the death of the dinosaurs.