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asteroid Itokawa
Asteroid Itokawa (an Apollo and Mars-crossing asteroid) imaged by the Japanese spacecraft Hayabasa. Image courtesy of JAXA.

Asteroids are metallic, rocky bodies without atmospheres that orbit the Sun but are too small to be classified as planets. Known as "minor planets," tens of thousands of asteroids congregate in the so-called main asteroid belt; a vast, doughnut-shaped ring located between the orbits of Mars and Jupiter from approximately 2 to 4 AU (186 million to 370 million miles/300 million to 600 million kilometers). Asteroids are thought to be primordial material prevented by Jupiter's strong gravity from accreting into a planet-sized body when the Solar System was born 4.6 billion years ago.

Asteroids with orbits that bring them within 1.3 AU (121 million miles/195 million kilometers) of the Sun are known as Earth-approaching or near-Earth asteroids (NEAs). It is believed that most NEAs are fragments jarred from the main belt by a combination of asteroid collisions and the gravitational influence of Jupiter. Some NEAs may be the nuclei of dead, short-period comets. The NEA population appears to be representative of most or all asteroid types found in the main belt. Since 1995 the number of identified NEA’s with estimated size less than 1 km has risen from ~150 to over 2300 and greater than 1 km has risen from ~200 to over 700.

Asteroids can be considered as potential sources of commodities in the same sense that a terrestrial geological terrain may have mineral potential. In principle, aspects of the three-part method used by the USGS for mineral resource assessment can be applied to assessment of asteroids for a variety of commodities that would be needed for extraterrestrial human survival. For instance, we believe we can improve current information on the viability of platinum extraction. Currently more than 3000 NEAs have been remotely identified and characterized by their size and individual type from reflectance spectroscopy studies. In addition, all types of asteroid appear to be represented by meteorites classes so that estimates can be made of asteroid chemistry. All of these factors suggest that grade-tonnage models for specific classes of asteroids can be developed, especially as meteorite studies provide descriptive information that can be extended to asteroids (essentially a descriptive asteroid deposit model by class). It is also possible to evaluate crudely the economic factors involved in asteroid commodity extraction. These calculations allow estimates of economic and uneconomic asteroids based on meteorites classes (i.e. chemistry) and orbital parameters (i.e. transportation costs) making it possible to identify asteroids that have the greatest mineral potential for a specific commodity.

Asteroids, regardless of their orbit, are classified into different types according to their albedo, composition information derived from spectral features in their reflected sunlight, inferred similarities to known meteorite types, and general physical characteristics. The majority of asteroids fall into the following three categories:

  • C-type (carbonaceous): Includes more than 75 percent of known asteroids. Very dark with an albedo of 0.03-0.09. Their composition is thought to be similar to the Sun, but depleted in hydrogen, helium, and other volatiles. C-type asteroids inhabit the main belt's outer regions
  • S-type (silicate): Accounts for about 17 percent of known asteroids. Relatively bright with an albedo of 0.10-0.22. Their composition is metallic iron mixed with iron- and magnesium-silicates. S-type asteroids dominate the inner asteroid belt.
  • M-type (metallic): Includes many of the rest of the known asteroids. Relatively bright with an albedo of 0.10-0.18. Their composition is apparently dominated by metallic nickel-iron. M-type asteroids are most common in the main belt's middle region.

Determining the chemical composition of asteroids relies on the combination of remote observations from telescopes and satellites with studies of meteorites collected at the Earth’s surface; meteorites being dominantly fragments of asteroids. The geochemistry of meteorites has been studied in great detail, with two main classes having been recognized: iron meteorites and stony meteorites. Iron meteorites are made up of pure nickel- iron metal with some impurities such as graphite and the mineral troilite (FeS) and it is generally accepted that they originated from metallic asteroids that were the cores of larger “parent body” asteroids. Stony meteorites are subdivided into chondrites and achondrites. Chondrites are the most common type of meteorite. These are meteorites which have not undergone significant pre-terrestrial alteration or changes since they were formed, and are considered to preserve the raw materials for planet formation. They are made up of circular mineral blobs called chondrules that formed in space 4.5 billion years ago and accreted together shortly after. A sub-category of chondritic meteorites, called carbonaceous chondrites, contain some of the most primitive material in the Solar System including organic compounds and water. Achondrites are meteorites that contain minerals which have been melted, changed and altered since they were formed, which makes them different to chondrites.

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