Earth's Visitors from SpaceThis page probably isn't as exciting as you thought it was going to be! Instead of talking about alien visitors from outer space, I'll be talking about visits to Earth by rocks. Thrilling!! For all of the Earth's existence, it has been under constant attack by rocks flying around space. These rocks are meteoroids. Most burn up in Earth's atmosphere, but some get through and hit the surface and form craters.
Meteors, Meteorites and MeteoroidsAlthough a meteor, meteorite and meteoroid is pretty much the same things, its name changes slightly depending on what its status is. It is known as a meteoroid while it is flying around space. A meteoroid is usually a piece of rock that has broken off another object, like an asteroid, comet, moon or another planet. If the meteoroid enters Earth's atmosphere, it forms a fiery tail as it burns up as it plummets to the surface. Its proper name when it is travelling through the atmosphere of Earth is a meteor, but it may also be down as a falling star and shooting star. If the meteor hasn't completely burned up during its journey through Earth's atmosphere, it will hit Earth's surface and cause a crater. When it does, it will be known as a meteorite.
Above is the Barringer Crater in Arizona, USA. It was formed by a meteorite which hit Earth between 25,000 to 50,000 years ago and is 175 metres deep and 1265 metres wide.
This is the Wolfe Creek crater from Australia. It is 875 metres and 50 metres deep (although it has a "wall" which is 25 metres high above the surface). It is believed that the meteorite that caused this crater hit the surface of Earth 300,000 years ago.
Earth is the most heavily bombarded inner planet by rocks from space, simply because it is the largest of them (Mercury, Venus and Mars are the other Inner Planets). However, the vast majority of meteoroids heading for Earth burn up in the planet's atmosphere and don't survive long enough to hit the surface. Also, the craters of those that do reach the surface are erased over time due to Earth's constant geological, tectonic (the movement of Earth's plates) and weathering processes. There are only about 120 craters recognised on Earth and these are all relatively new when you consider the planet is 4.6 billion years old. On the Moon and on planets like Mars and Mercury, there are no geological processes so these worlds are covered by meteorite craters which have been there for billions of years.