Impacts on Earth
We are very lucky today that the majority of debris, which enters Earth from space, is small in size. However, a small fraction of the pieces do survive the journey. These are meteorites, as we have discussed in the previous section. Meteorites usually cause not too severe damage than a hole in a house roof or a small crater in the ground.
On the other hand, things were not always this fortunate. During the past, Earth has been hit by much larger objects, which have left scars on the surface. For example, some scientists believe that the disappearance of the Dinosaurs about 65 million years ago was caused by the impact of a large asteroid in an area now covered by the Indian Ocean. Some also believe that a huge body (possibly the size of a planet) caused a large piece to fracture off our planet and form the Moon, which we see today. Here are several impact sites on Earth that we have been able to identify.
Here are some famous craters from around the world:
Barringer, Arizona, USA.
Barringer is 1.2 km across, and is about 49,000 years old. It is possibly the most famous terrestrial impact crater,
Wolfe Creek, Western Australia.
This crater is less than 0.3 million years old and has a diameter of 0.9 km.
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Pieces of material which cause meteors to enter Earth's upper atmosphere at very high velocity: around 260000km/h. The flash of light that we see happens when the object is around 100 km above the surface of the Earth. If the object were to be the size of a grape, than it would create a brilliant flash that could even cast shadows on the ground. Such bright meteors are called FIREBALLS. The image here shows a fireball.
As a comet orbits near the Sun, it loses materials, such as dust and rocks, which are given out by the tail. These materials cause a continuous trail to form behind the comet as it orbits. When the Earth crosses the path which a trail once was, the particles of materials that they contain are taken up by Earth's atmosphere. These appear in the sky as a shower of meteors.
The dates of these meteor showers are known. For example: The Eta Aquarid shower (around May 4th) and the Orionid shower (October 20th) are both cause by the trail left by Halley's comet.
This meteorite fell on Earth on August 14th, 1992 in Uganda. The meteorite was made of stone, which broke down along its fall causing debris to be distributed over an area of 3 x 7 km. At least 48 fragments have been found, and it is thought that the original object might have been 1000 kg. The largest piece recovered is shown in the picture.
Even though most meteorites come from asteroids, there are some that are thought to have come from comets, and other that have a composition, which is relatively close to the rocks found on the Moon. At least 7 other meteorites are of a type that match closely to the materials found on Mars, which suggests that there was probably an impact on the surface of Mars. A Martian meteorite is shown here.