Going back in time, we can see the early Earth. It is hot, volatile, and rough. Volcanoes are spewing out molten rock and at constant rate. If one were to stand on the surface of the early Earth, they would be incinerated almost instantly. It was not a nice place to be! On top of the scorching heat, there is a constant bombardment of rocky bodies from other parts of the Solar System. Every time an asteroid or comet hits the Earth, it impacts with a sound louder than a thousand jet airliners taking off!
Many millions of years later, and rain starts to fall. It keeps coming in a torrential downpour. The storm will not pass for millions upon millions of years. Oceans are created. The Earth we know of today is starting to take shape every so slightly.
As stated earlier, however, the setting is not limited to just the Earth. A giant ring of asteroids surrounds the inner Solar System (it lies between the planets of Mars and Jupiter). In this ring, asteroids moving at thousands of miles an hour crash and collide with one another in deafening silence. Every several million years or so, an asteroid will be ejected from the ring and hurtle towards the Earth. Comets are not so different.
Going even farther away from Earth, we reach an area of the Solar System known as the Kuiper Belt. Many comets that we know of today come from this region. Attracted by the suns gravity, they sometimes pass into the Solar System and start to eject their brilliant, wispy tails. Even more rarely than asteroids, a comet may sometimes collide with the Earth.
Reading this setting description, one might get the idea that the Solar System, and our planet, are very violent places. Truth be told, that's not an entirely wrong assumption!
NASA's Goddard Space Flight Center. NASA’s Hubble Telescope Finds Potential Kuiper Belt Targets for New Horizons Pluto Mission. October 16th, 2014 via Flickr. Attribution License.
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