They are considered pristine samples of early solar system matter, although in many cases their properties have been modified by thermal metamorphism or icy alteration. Other meteorite types which have been geologically processed are irons and pallasites. Iron meteorites are classified into thirteen major groups and consist primarily of iron-nickel alloys with minor amounts of carbon, sulfur, and phosphorus. These meteorites formed when molten metal segregated from less dense silicate material and cooled, showing another type of melting behavior within meteorite parent bodies. Thus, meteorites contain evidence of changes that occurred on the parent bodies from which they were removed or broken off, presumably by impacts, to be placed in the first of many revolutions.
Pallasites are stony iron meteorites composed of olivine enclosed in metal.
Meteors often occur in showers and swarms . The showers often last for a couple weeks, with peaks of a couple days. The reason is shown below. These rocks are clustered together at first as a swarm, then later spread out along the old comet orbit. When the Earth passes through the old orbit it encounters a fraction of the meteors causing a shower or swarm.