Comets, Meteors and Asteroids
Comets, meteors and asteroids come from space. Comets appear as shining lights and cannot be seen without a telescope. Meteors look like thin streaks of light in the night sky. They can be seen by the naked eye. Asteroids are chunks of rock that circle the Sun. They measure about 3 miles across.
A comet in the outer parts of the Solar System is too small to be seen from Earth. It looks like a large, dirty snowball. It has an icy core, covered by a layer of black dust. The nucleus is mainly water and gases, frozen and mixed together with bits of rock and metal.
The ice melts when the comet gets closer to the Sun. It changes into a gas. Dust particles spread out around the nucleus in a cloud called a coma. The Sun causes them to glow. The coma of an average comet is sixty thousand miles across, but very thin. Radiation and the solar wind drive the gases of the coma away. They form a straight tail. The tail can grow to ninety million miles long.
The orbits of most comets are oval-shaped. Short-period comets take less than two hundred years to travel around the sun. Halley's Comet returns every 76 years. The comet with the shortest period is Encke's comet. It orbits the Sun every 3.3 years. Most comets get brighter when they get closer to the Sun. Bright comets are visible in the sky only once or twice in a century, but for a long period of time.
Meteors flash in the sky every night. They are sometimes called falling or shooting stars. Meteors begin as meteoroids pushing through the Earth's atmosphere. Friction made by rubbing against air particles makes them look red hot. Then they are called meteors. They last for only a few seconds. Meteors come much closer to the Earth than comets.
Earth might pass through an old comet orbit and bump into particles from its nucleus. This event is called a meteor shower. Perhaps more than a dozen meteors might be visible in an hour. If many more meteors are together, this event is called a meteor storm.
Asteroids are the largest of the space rocks. Most of them go around the Sun between the orbits of Mars and Jupiter. The region they are located is called the Asteroid Zone. There are more than three thousand known asteroids. Most are only a few miles across. Ceres is the largest. It is six hundred miles across.
Sometimes asteroids spin out of their zone to follow different orbits. More than twenty-five asteroids share the same orbit as Jupiter. They are called Trojan asteroids. Apollo asteroids come across Earth's orbit and approach the Sun like a comet.
In summary, comets, meteors, and asteroids are three types of space travelers which can be seen by humans. Comets are shiny patches in the sky from time to time. Asteroids are chunks of rock which circle the Sun. Both can be viewed only by using a telescope. Meteors are thin streaks of light every night. These can be seen without a telescope.