A meteor occurs when a meteoroid enters the Earth's atmosphere and vaporizes, heating itself and atmospheric gases so that they glow. Most meteoroids are no more than 1 cm in diameter.
High meteor rates occur during meteor showers, when the Earth runs into a swarm of meteoroids. Showers take place on or close to the same date each year, when the Earth crosses the common orbit of the meteoroids.
The number of recovered meteorites has risen dramatically with the discovery that Antarctic ice fields collect and preserve meteorites for millions of years.
Meteorites are classed as stones, irons, and stonyirons. Stones resemble Earth rocks and are the most common meteorites. Carbonaceous chondrites are a type of stony meteorite and may represent unaltered material from early in the history of the solar system. Irons are alloys of iron and nickel and stony-irons are mixtures of stone and metal.
The radioactive elements in meteorites show that most of them solidified at almost the same time as the oldest Moon rocks, about 4.6 billion years ago.
Many meteorites appear to have been kept at high temperatures for a long period of time or to have cooled very slowly. To cool slowly, they must have been part of a body at least 100 km in diameter. The orbits of meteorites show that the bodies from which they came had orbits that carried them into the region between Mars and Jupiter.
Most of the known asteroids orbit in a belt located between the orbits of Mars and Jupiter. These asteroids are very widely spread out.
Many asteroids are not found in the asteroid belt. The Trojan asteroids, for example, either trail or precede Jupiter in its orbit around the Sun. In addition, the asteroids Hidalgo and Chiron have orbits larger than Jupiter's.
Many asteroids have orbits that carry them inside the orbit of the Earth. Within tens of millions of years, these asteroids are likely to be destroyed by striking the Earth.
The reflectance spectra of many asteroids resemble those of various kinds of meteorites. However, asteroids that resemble ordinary chondrites, the most common kind of meteorite, are very scarce.
The nucleus of a comet is a low-density chunk of ice and dust. Upon coming within 3 AU of the Sun, however, the nucleus is warmed enough by sunlight to release gas and dust. These flow away from the nucleus to produce the coma and the dust and plasma tails.
Comets with orbital periods shorter than 200 years are called short-period comets. Comets with orbital periods longer than 200 years are called longperiod comets.
The Sun is surrounded by the Oort cloud, a swarm of comets extending as far as 100,000 AU from the Sun. There may be as many as 1 trillion comets in the Oort cloud. Passing stars alter the orbits of Oort cloud comets, causing some of them to enter the planetary system and become visible as new comets. Short-period comets are thought to come from the Kuiper belt, a disk of comets just beyond the orbit of Neptune.
A comet loses icy material each time it passes the Sun. Eventually, the ice is entirely eroded. The dust particles left behind form meteoroid swarms that produce meteor showers. Some comet nuclei may have rock cores that become asteroids once the surrounding ice is gone.
Comets may have formed in a region near the orbit of Neptune. The bodies recently found beyond the orbit of Neptune may be among the largest comets that still remain in that region.
If a large meteoroid or comet struck the Earth, there would be serious local and global consequences. The global consequences might include darkness for weeks or months, very acidic rain, and temporary heating of the atmosphere.
An excess of the element iridium, discovered in rocks formed at the end of the Cretaceous period 65 million years ago, suggests that an asteroid struck the Earth at that time. The consequences of the impact may have played a role in the Cretaceous extinctions. However, the way that the impact affected life on the Earth and the relationship between extinctions and impacts is not yet understood.
