BOSTON — Meteors start out as bits and pieces of passing comets, asteroids, and even our moon and planets. When they get knocked off their parent object, they continue to orbit the sun. In space, they are known as meteoroids.
Earth plows through batches of meteoroids all the time. In fact, Earth encounters several hundred tons of these leftovers every day. They range in size from microscopic to about the size of a baseball. Most are about the size of a grain of sand. When meteoroids are about 50 to 68 miles from Earth, friction heats them as they race into our atmosphere at 20,000 to 90,000 miles an hour. They become so hot that they leave a thin trail of glowing air behind them. That's when they become known as meteors.
Most meteors vaporize before they hit the ground. But some don't. If they are big enough, friction slows them to the point where they stop glowing. When they land, they are called meteorites. Scientists have found meteorites dating back 4.5 billion years, to the early days of the solar system. They also have found meteorites they think may have been knocked off of Mars.
On very rare occasions, meteors are unwelcome visitors. In October 1992, a woman in Peekskill, N.Y., heard a crash outside her home. When she went to look, a rock the size of a football lay near her car. The car's trunk was crushed. The rock was very heavy and still warm. It landed shortly after people in the Midwest had reported seeing a long-lasting "fireball" that sped through the sky from the southwest to the northeast.
The Perseid meteor shower is not the only one you can view. In January, the Quadrantids appear. In April, it's the Lyrids. In October, the Orionids take to the sky. And in mid-November, you can watch for the Leonids. Many showers are named for the constellation - Perseus, Lyra, Orion, and Leo, for example - nearest the showers' apparent point of origin.
While these showers last from several days to several weeks, they all have a peak, or maximum. This year, for example, the Perseids have been visible since July 17 and will be until Aug. 24. But the maximum will be the night of Aug. 11-12.