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By Root 1202 0

not at all like the almost perfectly circular orbits of Earth or Venus. Some asteroids have their far points from the Sun beyond the orbit of Saturn; some have their near points to the Sun close to the orbit of Mercury; some, like 1685 Toro, live out their days between the orbits of Earth and Venus. Since there are so many asteroids on very elliptical orbits, collisions are inevitable over the lifetime of the solar system. Most collisions will be of the overtaking variety, one asteroid nudging up to another, making a soft splintering crash. Since the asteroids are so small, their gravity is low and the collision fragments will be splayed out into space into slightly different orbits from those of the parent asteroids. It can be calculated that such collisions will produce, on occasion, fragments that by accident intercept the Earth, fall through its atmosphere, survive the ablation of entry, and land at the feet of a quite properly astonished itinerant tribesman.

The few meteorites that have been tracked as they enter the Earth’s atmosphere originated back in the main asteroid belt, between Mars and Jupiter. Laboratory studies of the physical properties of some meteorites show them to have originated where the temperatures are those of the main asteroid belt. The evidence is clear: the meteorites ensconced in our museums are fragments of asteroids. We have on our shelves pieces of cosmic objects!

But which meteorites come from which asteroids? Until the last few years, answering this question was beyond the powers of planetary scientists. Recently, however, it has become possible to perform spectrophotometry of asteroids in visible and near-infrared radiation; to examine the polarization of sunlight reflected off asteroids as the geometry of the asteroid, the Sun and Earth changes; and to examine the middle-infrared emission of the asteroids. These asteroid observations, and comparable studies of meteorites and other minerals in the laboratory, have provided the first fascinating hints on the correlation between specific asteroids and specific meteorites. More than 90 percent of the asteroids studied fall into one of two composition groups: stony-iron or carbonaceous. Only a few percent of the meteorites on Earth are carbonaceous, but carbonaceous meteorites are very friable and rapidly weather to powder under typical terrestrial conditions. They probably also fragment more readily upon entry into the Earth’s atmosphere. Since stony-iron meteorites are much hardier, they are disproportionately represented in our museum collections of meteorites. The carbonaceous meteorites are rich in organic compounds, including amino acids (the building blocks of proteins), and may be representative of the materials from which the solar system was formed some 4.6 billion years ago.

Among the asteroids which appear to be carbonaceous are 1 Ceres, 2 Pallas, 19 Fortuna, 324 Bamberga and 654 Zelinda. If asteroids that are carbonaceous on the outside are also carbonaceous on the inside, then most of the asteroidal material is carbonaceous. They are generally dark objects, reflecting only a small percent of the light shining on them. Recent evidence suggests that Phobos and Deimos, the two moons of Mars, may also be carbonaceous, and are perhaps carbonaceous asteroids that have been captured by Martian gravity.

Typical asteroids showing properties of stony-iron meteorites are 3 Juno, 8 Flora, 12 Victoria, 89 Julia and 433 Eros. Several asteroids fit into some other category: 4 Vesta resembles a kind of meteorite called a basaltic achondrite, while 16 Psyche and 22 Kalliope appear to be largely iron.

The iron asteroids are interesting because geophysicists believe that the parent body of an object greatly enriched in iron must have been molten so as to differentiate, to separate out the iron from the silicates in the initial chaotic jumble of the elements in primordial times. On the other hand, for the organic molecules in carbonaceous meteorites to have survived at all they must never have been raised to temperatures hot enough to melt