There's another consequence of this theory: the smaller asteroids are, the less weathered they are. Our instruments still aren't sensitive enough to get good near-IR spectra of little asteroids out in the Main Belt, which would explain why we haven't detected ordinary-chondrite asteroids.

But over the past decade we've finally begun getting good near-IR mineral spectra of the little asteroids that wander into the inner Solar System -- and, sure enough, for the first time we're finding large numbers of asteroids whose near-IR spectra really do match those of ordinary-chondrite meteorites. But while the space-weathering explanation of the Ordinary Chondrite Mystery is very popular among planetary geologists at this point, by no means does everyone agree with it.

Michael J. Gaffey said at the Lunar and Planetary Science Conference (LPSC) last March that even if space weathering by micrometeoroids does occur, it can't explain ALL the spectral differences between big S asteroids and ordinary-chondrite rock.

He thinks that S asteroids, from the very start, really have been made of a different kind of rock than ordinary-chondrite meteorites, richer in flecks of separate iron-nickel.

If he's right, then we're thrown back to our original puzzle: why do meteorites consist so disproportionately of ordinary chondrites? Gaffey thinks that the explanation is that most of the meteorites that reach Earth don't come from small asteroids in general, but from just a few asteroids (including the big asteroid 6 Hebe) that happen to orbit near a few narrow "chaotic resonance zones" in the Asteroid Belt.

Resonance zones are areas in the Asteroid Belt where any asteroid has an orbital period which is a fairly simple fraction of the orbital period of Mars, Saturn or (especially) Jupiter -- so that those planets exert repeated slight gravitational tugs on the asteroid at the same few points over and over in each of its orbits, gradually stretching its orbit into a more elliptical one.

These have been known since 1867, and they seem to explain several mostly-empty gaps in the Asteroid Belt where any asteroids are pulled into orbits crossing those of their neighbors and eventually knocked into new orbits a short distance outside the Zones by collisions.

Most of the original asteroids were cleared out of these zones immediately during the Solar System's formation, but collisions still knock a small stream of new strays from nearby areas into them.

But only in the past 20 years, thanks to more powerful computers, has it been mathematically discovered that there are a few such resonance zones that produce "chaotic" effects on asteroids' orbits, far more dramatically stretching their orbits in just a few million years' time so that they quickly start flying all the way into the inner Solar System.

Most of them, in fact, soon have their orbits stretched to such a degree that they either crash into the Sun or are catapulted completely out of the Solar System --but a few actually crash into the inner planets, and others make close flybys of them that modify their orbits so that they escape from the original resonance effects. This explains the long-mysterious origin of the near-Earth asteroids.

So, according to Gaffey, almost all meteorites come from the few asteroids near the borders of these chaotic resonance zones -- which, by sheer chance, happen to have compositions different from the vast majority of asteroids.

Jeffrey Bell of the University of Hawaii thinks this is seriously stretching coincidence, and has come up with a third theory.

He agrees with Gaffney that the larger S asteroids really aren't made of OC rock -- but he also doesn't think that Earth's meteorites mostly just happen to come from a rare freak population of unusual-composition asteroids that just happen to have been located near the chaotic resonance zones. Instead, he thinks they come from that vast majority of asteroid "parent bodies" during the Solar System's early days which were small.

It's universally believed now that the Asteroid Belt formed because these rocky parent bodies in that region -- which were kept from coalescing into a larger rocky planet by nearby Jupiter's constant gravitational stirrings -- then began colliding with and shattering each other.

And their fragments did the same thing, eventually producing the vast cloud of objects -- from balls of rock hundreds of kilometers across to microscopic dust particles -- that populates the Asteroid Belt today.

Before that happened, however, the larger asteroid parent bodies would tend to have their rocks partially melted by heat from traces of the short-lived and very intense radioisotope aluminum-26, which is known to have infused the rocky material of the forming Solar System (maybe due to radiation from the same nearby supernova whose shock wave may have triggered the condensation of the System out of an interstellar dust cloud in the first place).

And this melting would tend to free some of the iron and nickel from the OC material of the asteroids to form separate flecks, before the radioisotopes decayed and the rock cooled down again in the Solar System's early days.

But the bigger an object is, the smaller its surface area is compared to its volume, and so its interior stores up accumulating heat better -- and so only the bigger asteroid parent bodies heated up enough inside to undergo this partial internal melting.

In fact, the biggest parent bodies -- several hundred km across and more -- got so hot inside that the melted iron settled to their cores, leaving them with solid metal cores which were occasionally later exposed by collisions to explain that small fraction of asteroids and meteorites that are made out of iron-nickel metal.

The smallest asteroid parent bodies, less than about 50 km across, remained ordinary unmelted OC rock -- and, according to Bell, they're so plentiful that they actually make up most of the material in the inner Asteroid Belt, and so naturally provide most of the meteorites broken off asteroids (especially since rock fragments are also blasted off small asteroids more easily than larger asteroids, because of their lower gravity).

Why haven't we detected them in the Asteroid Belt? Simply because -- as I said -- Earth-based telescopes weren't powerful enough to get good near-IR spectra of small asteroids, and so we've been analyzing a very seriously biased sample of big, non-OC, partially melted asteroids.

The trouble is that the much bigger share of OC rock that we see among the IR spectra of those tiny near-Earth asteroids that we're finally starting to analyze could be explained by any of these three theories. So the hope was that NEAR's close up analysis of Eros could provide important new data to help settle this interminable wrangle.