A little known chunk of information that is floating around the intellectosphere is that since 2014, June 30th has been “Asteroid Day”. Why is this? Because just over a hundred years ago, in the northern forests of Siberia, an asteroid impact with the Earths atmosphere caused mile-wide devastation. The Tunguska event, as it is now commonly known, was the first encounter with space objects that humanity had seen in modern times. Completely unexpected, but luckily far away from civilisation, the explosion in the sky caused unprecedented damage to the local surroundings. Trees were flattened as far as could be seen, and anything alive in the blast zone would no-doubt have been instantly crushed and burned.
Since those early days of black and white photography, astronomers have learned fairly little about asteroids. They are thought to be the remains of the early solar system, made up of either the original rocks that went to form planets or the debris from collisions that shattered larger worlds in the long distant past. By inspecting the contents of the samples of meteorite that fall to Earth, it can be detected than some pieces seem to have undergone metamorphosis, and some have not. This would validate the theory as to the origins of these rocks in space.
Several teams of astronomers are searching the skies for objects including asteroids. The main purpose is to predict and detect if any are on a collision course with Earth, so far none of these have been found and published however, many objects have been discovered, catalogued, and tracked with computer models. Near Earth Objects are those asteroids and comets (slushy balls of ice and rock whose origins are from the outer reaches of the solar system in a place called the Kuiper Belt and the Oort Cloud) which are projected to orbit within close proximity to Earth and therefore pose a certain degree of risk.
Many ideas are around as to how to deter an object from colliding with the Earth in the future. In films it is often a bomb which blows the object into tiny bits, which harmlessly burn up in the atmosphere. We actually have never done anything like this in real life so it's difficult to predict exactly what would happen to an asteroid if hit with a bomb, but the risk is that it will simply produce a bigger threat of several equally as dangerous chunks which have unpredictable movement. The most likely way we will use according to current thinking is the placement of a large source of gravity in the path of the object that will cause it to move out of the way. This would be a giant ship filled with material that would have a large mass and therefore a significant gravity. Placing it in the right place would only require the object to move a few inches to one side for the path to swing way off our line of travel.
The moons of Mars, Phobos and Deimos, are believed to be captured asteroids. These would have been pulled into orbit around Mars from other orbits around the Sun. The gravity of Mars at close range would have been enough to attract and keep the irregularly shaped moons. Most asteroids can be found in the asteroid belt, which is a river of free floating debris that orbits the Sun in between Mars and Jupiter. The gravity of Jupiter is believed to be the reason these rocks didn't coalesce into a new planet. It is also the gravity of Jupiter that sometimes upsets the order of flow and sends rocks into the direction of the inner planets, including ours. It is believed however that because of this giant world on our cosmological doorstep, we have been shielded from many more impacts which could have halted life from forming.
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