How to Colonize an Asteroid
In the beginning, our solar system consisted of a very large rotating cloud of gas and dust, the remnants of a very large star which went nova, or exploded. This cloud of gas and dust is called a "nebula". The gravitational attraction of all the gas molecules and dust particles in this cloud caused it to begin contracting. Large amounts of matter began collecting at the center of this cloud, forming the beginnings of our present star, the Sun. The rotation of the cloud caused it to flatten out into a disk or pancake shape. Clumps of matter began to collect here and there throughout the cloud, forming blobs of material which in turn collected together, (through the action of gravity), to form larger blobs, (up to a kilometer in diameter), called "protoplanets".
The composition of the protoplanets depended upon their distance from the sun, and consequently the temperature at which they condensed. Higher temperatures close to the sun, near the orbit of Mercury, allowed only heavier elements to condense, such as nickle and iron. Farther out, where our own earth formed, the metallic elements still condensed, but the cooler temperatures allowed lighter elements, like silicon, aluminum, and oxygen, to join them.
The planets Venus, Earth, and Mars show this mixture to varying degrees. As expected from this theory, Venus has a higher percentage of metallic materials (closer to the sun, hotter condensation temperatures), and Mars has more oxidized metals and carbon or nitrogen compounds ( farther from the sun, cooler temperatures).
Beyond the orbit of Mars, the so-called "gas giants", Jupiter and Saturn, are composed mostly of very light materials, such as hydrogen and helium. Although Jupiter is composed of some of the lightest materials in the solar system, it is the heaviest planet because of its sheer size.