The Asteroid Belt
Between the orbit of mars and jupiter lies a belt of rocky and metallic objects, known as asteroids. Most of them have an irregular shape. The belt is centered about 2.7 AU from the sun (400 million kilometres away from the sun).
Among the largest we know is Ceres (also among the first discovered), about 1000 km in diameter and spherical in shape. Other commonly known asteroids are Pallas, Vesta, Juno, Eros and Ida (Ida has an asteroid orbiting it, called Dactyl!). Though the asteroids are great in number, they arent as many as sci-fi movies might give us the impression that they are.
The asteroids are remnants from the creation of the solar system. Because of this, scientists are very interested in their composition. For some reason the asteroid belt didnt form to a planet. Perhaps it was because of a giant collission in the past, perhaps because of the nearby planet jupiter's gravitation?
If all the asteroids would have been combined into one single mass, it would be a sphere of a diameter less than 1 300 km. The relatively small size is explained by the influence of jupiter's massive gravitational field.
Astronomers have classified asteroids depending on albedo (how much light it reflects), colour and spectral types. These properties are believed to correspond to their composition:
C-type: Carbonaceous asteroids, about 75% of the known.
S-type: Silicaceous, about 15% of the known.
M-type: Metallic, the rest of the asteroids.
However, astronomers have now refined this system into many more groups.
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As mentioned earlier, most asteroids orbit between the orbits of mars and jupiter at an average distance of 2.7 AU. Though, there are other, smaller groups of asteroids that have orbits that take them elsewhere.
Trojans and Pallas asteroids are examples of groups that orbit in the vicinity of Jupiter.
Aten, Apollo and Amor are groups that orbit between earth and mars, and some of these group members also cross the orbit of earth. There are also asteroid groups that cross the orbit of mars (making a collission possible), such as the martian trojans.
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Objects that orbit close to or cross the orbit of earth are called NEO (Near Earth Object). More and more objects are discovered each year that could pose a threat to the earth if they came too close. Some of those objects are doomed to hit the earth sooner or later.
Though most of them are small, and should they come too close, they will most likely burn up in the atmosphere or hit the surface in a size much smaller than they had before entering the atmosphere.
The ones that are really large and could cause a great damage if they would hit earth are rare. NASA estimates that asteroids big enough (about one kilometre in size) to cause catastrophic destruction could theoretically hit Earth every million years, or at longer intervals.
Many scientists believe that such a large asteroid (about 10 km in size) caused the 180 km wide crater on the Yucatan peninsula in south america 65 million years ago. It wiped out more than 2/3rd of all living creatures, including the dinosaurs. An extreme shockwave travelled the whole planet, followed by enormous quakes and tidal waves. Forests were annihilated beyond mexico’s borders. Tons, and tons of dust arose above the atmosphere and blocked the incoming light, turning the sky pitch black.
Deprived of light, and unable to complete the photosynthesis circle, vegetation was knocked out rapidly, making vegetarian animals starve.
The dust fell back to the surface, as toxic elements, along with bittering cold, hovering above the freezing point.
This disastrous event was the end for the dinosaurs and opened a window for smaller creatures to evolve and gradually dominate the earth.
Before 1994, when the broken up comet Shoemaker-Levy (21 pieces) hit Jupiter, few scientists took the threat from asteroids/comets seriously. When the scientists witnessed the great impact the comets had on jupiter, they started to think seriously about it
and now there are astronomers who dedicate their time to catalogue and discover objects that could pose a threa to the earth.
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Space Art 1: A hypothetic view of the asteroid belt. The asteroids are actually very spread out in the belt and the amount of asteroids has been exaggerated in this imagery.
Illustration 1: View the orbits of the asteroid groups as well as the main belt.