Beyond the Limits of the Earth
The existence of asteroids was confirmed at the dawn of the nineteenth century when, in 1801, the minor planet Ceres was observed for the first time. Scientists would soon come to distinguish asteroids from meteorites, the former having a diameter greater than one metre, the latter less than one metre. With comets, the diference is qualitative: while asteroids mainly consist of mineral and rock, they are composed of dust and ice.
Like the planets, asteroids orbit the sun, although few of them are purely spherical. The ones that are, such as Ceres, are often referred to as ‘dwarf planets’ as they are so large that their own gravitational mass has compressed them into a sphere. More generous estimates believe there may be 200 dwarf planets in the Kuiper belt of the outer solar system, as well as more than a million asteroids larger than a kilometre in diameter.
In terms of medium-term prospecting, however, there is a more interesting group of objects that reside far closer to home. At present we know of more than 16,000 near-Earth asteroids (NEAs) ranging in size from one metre to more than thirty-two kilometres. The number of NEAs more than a kilometre in diameter is estimated to be around 1,000, while the number of NEAs wider than 140 metres is around 8,000. Upper estimates speculate there are more than 1 million NEAs measuring forty metres in diameter or less, of which around 1 per cent have been discovered.
Whether it’s Moon Express prospecting the Earth’s only moon before moving on, or Planetary Resources sizing up NEAs, the potential abundance of of-world mineral wealth almost escapes comprehension. One estimate claims that a platinum-rich asteroid measuring 500 metres wide could contain nearly 175 times the annual global output of the metal, 1.5 times known world reserves. Even a smaller asteroid measuring the size of a football field could contain as much as \$50 billion worth of platinum.
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