Impact craters
Barringer crater dimensions
1,300-meter (0.8 mile) diameter, 174-meter (570-feet) deep
How are impact craters formed
1. Compression Stage: During this stage, the impactor punches a (relatively) small hole in the target, and a shock wave begins to pass through the target. This is when the impactor's energy is converted into heat and kinetic energy in the target, as the pressure generated by the impact is so great that even solid material can act somewhat fluid, and flow away from the impact site. There is very little material ejected up and out of the forming crater during this stage, although a plume of impact-generated vapor rapidly expands above the crater. This stage is very quick, lasting an amount of time on the order of the impactor's diameter divided by its speed at impact (D/v). For Deep Impact, this stage will last only around (1 m / 10200 m/s) = 0.0001 seconds (100 microseconds). Christiansen, E.H., Exploring the Planets, 2/E, �1995. Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey. 2. Excavation Stage: During this stage, the shock wave begun in the compression stage continues outwards through the material. A very interesting part of this, however, is the fact that this wave spreads out from a point below the surface of the target. As a result, the wave actually spreads upwards from the impactor, and sends some of the target material up and out from the impact site. This material is referred to as the "ejecta." Initially the ejecta forms a plume of hot vapor, melt droplets and fine debris. Then a cone-shaped "curtain" of material spreads upwards from the impact site. Some or all of this ejecta will land in the area surrounding the crater, forming an ejecta "blanket." The crater itself grows very large very quickly during this stage, and material at the lip of the crater folds over creating a rim. Fractures often spread down into the target from the crater site as well. This stage is longer than the compression stage, lasting an amount of time roughly equal to the square root of the diameter of the impactor divided by the acceleration due to gravity from the target. For Deep Impact, this stage will last around 300 seconds. 3. Modification Stage: During this stage, loose debris from the impact will tend to slide down the steep crater walls. Some loosened material may slip in sheets, forming terraces along the crater sides. In some craters, a central peak may form as some of the target material splashes back upwards at the initial point of impact. This stage lasts about the same amount of time as the excavation stage, although of course the crater can be further modified by erosion, later impacts, lava flows or tectonic activity for millions of years afterwards depending upon conditions on the target. For Deep Impact, this stage is not very important, since the low gravity on the comet will probably only cause some small amount of collapse near the rim. There will not be the uplift that can be seen in larger craters, so there will be no central peak.
What determines the size of an impact crater
The size, mass, speed, and angle of the falling object determine the size, shape, and complexity of the resulting crater. Small, slow-moving objects have low impact energy and cause small craters. Large, fast-moving objects release a lot of energy and form large, complex craters. Very large impacts can even cause secondary craters, as ejected material falls back to the ground, forming new, smaller craters, or a series of craters.
Meteorite
a piece of rock or metal that has fallen to the earth's surface from outer space as a meteor. Over 90 per cent of meteorites are of rock while the remainder consist wholly or partly of iron and nickel.
Asteroid
a small rocky body orbiting the sun. Large numbers of these, ranging enormously in size, are found between the orbits of Mars and Jupiter, though some have more eccentric orbits.
