Meteorites are space rocks that crash land on the Earth's surface. The majority of these objects are shards of smashed asteroids (huge rocks that orbit the sun). Iron is abundant in rocks derived from asteroids' cores. Each year, only approximately 500 meteorites larger than a football strike Earth, and the vast majority of them end up in the water. A very few large stones are found on land (such as the dinosaurs' extinction event 65 million years ago).
Asteroids fall to Earth for two reasons: they're either destroyed when they pass between the Earth and the Sun or else they reach Earth intact but are then consumed by the planet's gravity.
You may have heard that there is life on other planets. This is called "extraterrestrial life" and it exists mainly in two forms: bacteria and viruses. Bacteria are easy to grow in laboratory conditions and viruses are very difficult to grow even under controlled conditions. However, scientists have discovered both forms of extraterrestrial life inside meteorites!
In 1997, scientists analyzed three different types of meteorite and found that all of them contained small amounts of organic material that had been burned by heat generated during the meteor's flight through the atmosphere. They also found evidence for previous exposure to light at some time before they landed on Earth.
The most famous meteorite story involves the discovery of gold coins from the mid-18th century on Antarctica.
The largest space objects, comets and asteroids, hardly never collide with the Earth. The tiniest meteors disintegrate in the atmosphere. However, medium-sized space objects often collide with the Earth's surface. These are known as meteorites. Every year, around 500 meteorites impact the Earth. Of these, about 100 may be recovered by scientists.
Asteroids larger than approximately 10 meters (33 feet) and comets make up most of the remaining population. They tend to follow elliptical orbits around the Sun, but some become significantly more eccentric over time. When an asteroid or comet passes close to the Earth, it can result in a destructive event: a collision that generates heat energy enough to melt rock at the surface; a collision that produces a large air blast; or exposure to cosmic radiation that affects organisms on the ground.
The number of potentially hazardous objects increases each year due to our better understanding of the solar system. Currently, the best way to identify potential threats is via computer models that estimate the likelihood of impacts for all near-Earth objects greater than 1 kilometer in diameter. Scientists also use observations from NASA spacecraft and aircraft to learn more about the properties of these objects.
Potentially hazardous objects include those that come within 0.05 AU of the Earth (five million miles) or that have an estimated impact velocity greater than 5 km/s. Objects less than 1 kilometer across cannot be detected by current technologies.
A meteoroid is a space rock that is drifting around in space. Micrometeoroids are extremely tiny space pebbles (space dust). When a meteoroid hits the Earth's atmosphere, friction causes it to heat up and burn. The fire flashes into gas, which expands rapidly, causing the meteorite to break up into many pieces--sometimes even including super-hot rocks that fall out of the sky without warning or notice.
Where did this thing come from? Meteorites are the leftovers of asteroid impacts. As gravity pulls them in, they slowly spin round until they tumble onto a planet's surface.
How big was it? The size of the object depends on how far away it is when it enters the Earth's atmosphere. Objects close by appear bigger because more of their volume is compressed into a smaller area as the meteoroid rushes toward the ground at over 30,000 km/h (19,800 mph). Objects that are further away look smaller because they're traveling so fast that they dissipate much of their energy before reaching the ground.
Where will it go? If the meteoroid is not too heavy, then it will land somewhere on the planet's surface. But if it's very light, it will keep going around the Earth and come back again! This is called "ballistic motion."