Meteorites -- Samples of Asteroids
Origins of meteorites
The meteorites we study are only those that survive passage through the Earth's atmosphere. Meteorites hit the atmosphere at typical speeds of 15 kilometers per second. They are subject to tremendous pressure forces and heating. Most break apart, especially those which have a former comet as their parent body. However, a large number of meteorites have fallen to Earth and been identified as nonterrestrial bodies. Nickel-iron meteorites and stony meteorites predominate in this mix.
A large class of meteorites are nickel-iron blobs which come from parent bodies which were gravitationally differentiated. By scientifically analyzing the pure nickel-iron asteroids, we can tell how long ago their parent body was broken up, the magnitude of the impact that broke up the parent planetoid, and the rough size of the parent planetoid. (The time of collision is determined by cosmic ray nuclear dating methods -- how long fragents have been exposed to cosmic rays after their crystalline formation. The magnitude of the collision is shown in the crystalline strain patterns, as well as by jammed together incompatible mineral grains in stony meteorites. The diameters of the parent bodies is determined by the crystalline structure of the metal alloy which reveals temperature and cooling rates. There are other methods as well which reveal, for example, how long stony meteorites have been solid.)
Interestingly, about 40% of of the pure metal alloy meteorites came from only two collisions, one at approximately 650 million years ago, and the other at about 400 million years ago, as sorted by cosmic ray dating methods. (When sorted by collision pressures and other catastrophic processes, these samples again sorted pretty neatly into the same two piles, reflecting the differences in magnitude between the two collisions.)
All of the numerous 650 million year old metal fragments show shock induced pressures of more than two million pounds per square inch, i.e., 250,000 tons per square foot, an immense collision indeed. Several nonmetal meteorites, which were probably near the surface of this impact, show that they experienced shock pressures of seven and a half million pounds per square inch. Samples commonly show shock heating of more than 1000 C (1800 F).
Metal meteorites have generally indicated parent bodies at least several hundred kilometers wide.
Many stony meteorites have generally revealed that surfaces of their parent bodies solidified around 4.6 billion years ago, about the same age as the early solar system and the oldest rocks from the Moon. (The Earth's crust has gone through continuous metamorphosis and mixing due to plate tectonics and weathering, so that there are few rocks near that age.)
One of the most authoritative asteroid researchers, Tom Gehrels, noting several areas of evidence, writes: "Chemical studies show that most meteorites come from as few as 4 to 30 parent bodies. Properties of iron meteorites indicate that their parent bodies were at least several hundred kilometers across."
In the early solar system, material accreted from the gravitational attraction of tiny bodies to each other. Big bodies attracted the most material. However, if two big bodies struck each other at high enough velocity, they risked breaking each other up into smaller bodies.
Our catalogs of asteroids' orbits and light spectra show several "families" of asteroids which have similar orbits (indeed, some orbit each other, too) and similar compositions which deviate marketly fromother asteroids in that orbital zone around the Sun. For example, the populous and well defined Themis family consists of a "core" of large asteroids surrounded by a "cloud" of smaller objects, which could be concluded to be the remnants of a planetoid at least 300 km in diameter. The Eos family, composed of two objects of about 90 km diameter, plus a few of around 50 km and many smaller chunks, was larger, around 550 km diameter by current estimates.
The dynamics of an asteroid collision could produce a variety of results, ranging from complete dispersal to formation of mixed bodies. For example, simulations have shown that collisions of 5 km/sec between bodies of 50 km and 200 km diameter may produce a cloud of shattered material most of which eventually falls back in on itself through gravitational attraction and collisions over time to produce a couple of bodies of shattered material around 150 km in diameter.
However, most collisions between asteroids in the solar system today would result in dispersal of the vast majority of material, since escape velocities are so low for most asteroids.
Much more frequent than big asteroids hitting each other are small impacts which serve to chip away at the outside surface of a large asteroid, with the crater ejecta escaping the asteroid's feeble gravitational field. In fact, telescopic spectroscopy reveals a few large asteroids apparently with large quantities of free metal at the surface, possibly the result of the stony exterior being chipped away over the eons to expose the mantle and core.
Asteroids are likely to be crumbly material due to all the shock waves of impacts. Whatever material was not blown off is likely to be pulverized.
Photographs of Mars' two tiny moons, which are themselves captured asteroids between 8 km and 28 km wide, show pulverized surface material, cracks, and steam tubes from volatiles that vaporized from the energy of impacts. One of Mars' moons, Deimos, is U-shaped when viewed from one angle.
As one can see, there are a great variety of sources for asteroidal material.
Next, we look at the composition of meteorites that fell to Earth's surface, since this is largely what we have to go on when we think of designing space industrialization around asteroidal materials. However, we may visit the asteroid with a probe before designing a specific mission to go there for materials retrieval.