Why mine asteroids?
Simply put, there is an incredible amount of wealth to be found in the asteroid belt. Many thousands of times the Earth's entire GDP could be gained from mining the asteroid belt. By mining in space we will also allow ourselves to expand the Earth's economy exponentially. To a point where we have moved to a post-scarcity economy with Rare-Earth minerals and Platinum group metals, even Gold being abundant and resultingly, cheaper. By comparing 3 different types of asteroids we can gain a much better understanding of exactly what each contains and the benefits of their size and composition.
There are 3 main types of asteroid, classified by composition:
C-type Carbonaceous asteroids are mainly composed of Carbon alongside mineral and ore deposits, though it is on these asteroids that sizeable water ice deposits are expected to be found.
S-type Silicaceous asteroids are known as 'stony' asteroids and are the most 'Earth-like' asteroid containing mostly iron-oxides and magnesium-silicates and they could contain hydrated silicate clays within their interior potentially containing amino acids: the building blocks for life. Both C and S-type asteroids are the most abundant within the Solar System, though it is the final asteroid classification that is of most interest to the asteroid miner.
M-type Metallic asteroids are mainly composed of Nickel-Iron, though they can contain sizeable quantities of Platinum group metals and Gold, among the other Rare Earth elements which can be found on all asteroids. As an added bonus, M-type asteroids are monolithic globules of material, meaning that unlike Earth the heavier elements are more evenly distributed on the edges rather than being concentrated at the centre of mass in the asteroids core.
On the lower achievable targets is to capture small asteroids in their totality, utilising all the material recovered. An asteroid 25 metres across could contain 30 tonnes of Platinum or around £750 million.
Using 3554 Amun or the catchily named (6178) 1986 DA as an example: both asteroids are of approximately similar size 2.5-3km in diameter and both asteroids are M-type Metallic asteroids. If they follow a similar composition to recovered meteoric samples then they contain at least 10,000 tonnes of Gold, worth approximately £300 billion and also contain 100,000 tonnes of Platinum at least worth an approximate £2.5 trillion. This is not even taking into account the value of the other materials present of which there are at least 10,000,000 tonnes of Iron and Nickel which would be used as a construction material in Space. These asteroids are no larger than some open-cast mines on Earth so it is foreseeable that these will be mined in their totality by the end of the century, giving human civilisation access to previously unthinkable quantities of precious resources.