A meteorite is any fragment of rock from the solar system that passes through the atmosphere and survives its impact with Earth. About 3 percent of meteorites found on Earth's surface are made almost entirely of the metals iron and nickel, and are called iron meteorites. When the parent body was partially molten, the heavy iron and nickel metals segregated from the less dense silicate minerals and concentrated in the core, just as metals sank to Earth's core while our planet was forming. Scientists believe that these dense metallic fragments may have come from the cores of certain minor planets, called M-type asteroids, when they were blasted apart by a collision with other bodies during the early days of the solar system, more than 4 billion years ago.

The mineral composition of meteorites ranges from nearly solid iron and other metals to nearly solid silicates. In general, iron meteorites are much denser than stony meteorites. If two meteorites of the same size enter Earth's atmosphere, the stony meteorite is more likely to break into pieces and burn up completely than the iron meteorite. In fact, some carbonaceous chondrites, which contain organic compounds and water in addition to silicates, can crumble in a human's hand. Other stony meteorites are hard to the touch and may resemble terrestrial rocks. They contain silicate mineral grains, with some metal grains mixed in. These stones came from smaller rocks and dust that accreted (stuck to each other) during the earliest days of the solar system, 4.5 billion years ago.

Chondrules, small spherical inclusions inside a stony meteorite, are usually less than 1 cm (0.4 inch) in size, but they provide huge clues to the formation of our solar system. The silicate minerals olivine and pyroxene are the most common ingredients of chondrules, though their outer layers may consist of feldspars or other silicates. Scientists think that chondrules started out as aggregates, or clumps, of small, primitive dust particles, which were quickly heated by some cosmic event - perhaps some outburst from the young Sun, or simply the friction from small bodies ("planetesimals") crashing into each other. Some chondrules contain even smaller grains of silicates that may be some of the oldest materials in the solar system. Scientists measure the amounts of trace radioactive isotopes in chondrules and meteorites in order to estimate their ages.