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 stony-irons.
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.