Children of the Asteroids

A Delta rocket is poised on a launch pad at Cape Kennedy to begin the first- ever dedicated study of an asteroid. If all goes well, on February 16th -- or a few days later -- the automobile-sized spacecraft called NEAR will begin its multi-year journey to encounter a city-sized mountain of rock (or gravel-pile, or even a chunk of metal?) named Eros. And that is just the point. After decades of peering at Eros and its siblings through telescopes, scientists still don't know just what minerals these little worlds are made of. It is the prime goal of the Near Earth Asteroid Rendezvous (NEAR) mission to find out.

Eros is not really far away, by interplanetary standards. It often approaches Earth more closely than Mars or Venus. That is why it is considered to be a "near Earth" asteroid. NEAR won't reach Eros until 1999, however, because of the long trajectory necessary for NEAR to precisely match Eros' orbital motion around the Sun. The only previous close-up studies of asteroids, accomplished a few years ago by the Galileo spacecraft enroute to Jupiter, were serendipitous fly-bys. They were zoom-bys, really, at several kilometers per second: most of the data had to be collected within 30 minutes. NEAR, however, by matching speeds with Eros, will hover above the oddly shaped, 35 by 16 kilometer object, slowly orbiting it and mapping it for a full year.

It has also been a long journey for asteroid researchers to get a mission underway. The first proposals for asteroid missions date back to the 1960's, but they were set aside when an influential scientist argued, over-optimistically, that asteroids could be understood by telescopic observation supplemented only by laboratory analysis of meteorites -- the presumed pieces of asteroids fallen to Earth. Missions weren't required, he said. Finally, in the 1980's the Comet Rendezvous Asteroid Flyby mission made it out of the starting gate only to be cancelled by NASA part-way through development. It has taken the burgeoning interest in meteorites and the larger projectiles that crash into our own world, plus the deepening mystery about where they come from, to spur the first dedicated asteroid mission.

NEAR may finally solve one of the major mysteries that has perplexed planetary scientists for more than half-a-century: What is the connection between the countless asteroids and the fragments of stone and metal that plummet blazingly through our night skies, the meteorites? Two centuries ago European scientists first began to appreciate that meteorites actually come from outer space, and are not "thunderbolts," volcanic missiles, or hallucinations, as had been supposed. Soon afterwards, astronomers discovered the first asteroids, and the plausible connection was made: the asteroids, circling the Sun between the orbits of Mars and Jupiter, are pieces of an exploded planet, and the meteorites are smaller bits from the explosion. Alas, this plausible idea is not right.

Scientists now believe that the asteroids are remnants of a planet that never formed, not a planet that blew up. But asteroids do occasionally crash into each other, yielding some debris that could well reach Earth. Eros was discovered nearly a century ago -- the 433rd asteroid to be discovered. It is one of the largest asteroids that have somehow gotten knocked partway out of the asteroid belt, and come perilously close to the Earth. In 1932, the first small asteroid was found that actually dips close enough toward the Sun to cross the orbit of the Earth. Eros and the smaller Earth-approaching asteroids evidently provide a link between the distant asteroid belt and objects that can strike the Earth. During the last two decades, telescopic search programs from Mt. Palomar and Kitt Peak in the southwestern United States have augmented the list of known Earth-approaching asteroids, including some house-sized objects, observed when they actually passed through the Earth-Moon system. The link to meteorite-producing projectiles seems ever stronger.

Yet some of these projectiles may be comets, not asteroids. They aren't the flashy kind of comet, with long tails of gas and dust stripped from their icy surfaces. But they could be pieces of dead comets, like the blackened crusts of old snow banks along a city's street at the end of winter. Such weak cometary remnants may well be too fragile to ever survive a fiery fall to Earth to be collected as meteorites.

All of the Earth-approaching objects are in chaotically unstable orbits. Within hundreds of millions of years, they will all either collide with the Earth, the Moon, Venus, Mars, or the Sun, or they will be deflected by the gravity of one or more planets into trajectories that will escape the solar system forever. The Earth- approachers live transitory existences inside the orbit of Mars, and they must be continually replenished from somewhere else -- the main asteroid belt, where 99.9% of asteroidal material resides, or the cometary reservoirs in the outer solar system. Most researchers believe that the stronger bits that actually survive atmospheric passage as meteorites must be asteroidal. But which asteroids in particular are they pieces of?

Scientists would dearly love to make the exact connection. The Apollo Moon program fostered development of sophisticated laboratory techniques for exquisite analysis of returned Moon rocks, and meteorites as well. Assays of minerals, chemical elements, and even isotopes can be made of minute samples of such rocks. Microscopic examination can reveal damage to mineral crystals by cosmic rays, by the intense shock of impacts, and by the prolonged heat of metamorphism. All these clues can be synthesized into a picture of the complex, changing environment on a sample's parent body during the early epochs of solar system history. But on which bodies were these processes happening? Do the temperatures, pressures, degrees of oxidation/reduction, etc. inferred from particular meteorites apply to a small asteroid, to a large one, to one in the inner belt, to a Trojan trapped in Jupiter's orbit -- or might they even apply to a comet, or to Mars, or to some other solar system or interstellar object, if our supposition that most meteorites come from the asteroids is faulty?

For the last quarter century, the combined talents of meteoriticists and asteroid astronomers have turned to the task of associating particular classes of meteorites (e.g. the irons, the stony-irons, the several kinds of primitive "chondrites", and the more evolved rocky meteorites like the basaltic achondrites) with various classes of asteroids. By the 1960's, it was clear that asteroids had different colors, and they even exhibited crude spectral signatures (e.g. absorption bands in the spectrum of reflected sunlight) of constituent minerals. There was optimism that spectral comparisons of asteroids and meteorites would soon establish the long-sought link.

The greatest puzzle of all concerns the so-called ordinary chondrites and the S- type asteroids. Many natural history museums have displays of meteorites (cut and polished to reveal their interiors). The most common specimens fall in the ordinary chondrite class -- rocks, which to the eye show the glints from numerous tiny flecks of metal. A magnifying glass reveals that, in addition to the metal grains, they are composed of countless millimeter-scale spherules, called "chondrules", made up of the silicate minerals olivine and orthopyroxene. Chemical analyses show that the abundances of the non-volatile chemical elements are in cosmic proportions -- that is, they have the same composition as the Sun (except, of course, for the gaseous and volatile chemicals), so they presumably represent the solid materials from which the Earth and other planets were made.

Planetary scientists would very much like to understand the "primitive" worlds from which ordinary chondrites come. Our own planet, the Moon, and the other planets have all undergone enormous evolution since they were formed, driven primarily by the heating (and attendant metamorphism, melting, volcanism, etc.) due to decay of radioactive elements deep within planetary interiors. To some degree, the violent impact processes during planetary accretion also deformed the primitive character of the original solar system materials, even before planetary heating continued the transformations.

The ordinary chondrites, however, prove that some bodies -- presumably some asteroids -- have evaded the violent heating processes that drives planetary chemistry, melts rocks, and destroys evidence from the time the solar system was forming. We would very much like to study these bodies -- the WHOLE bodies, not just the random hand-samples that happen to penetrate our atmosphere. There is another class of primitive chondrites, too, that are presumably even more abundant in interplanetary space than the ordinary chondrites. Called carbonaceous chondrites due to the several percent of carbon that gives them a black hue, they are rather fragile and only a small percent of them survive an atmospheric plunge.

In the early 1970's, Tom McCord (now of the University of Hawaii), myself, Torrence Johnson (Jet Propulsion Laboratory), and Michael Gaffey (Rensselaer Polytechnic Institute) began a spectral search for the meteorite parent bodies. Most of the brighter asteroids showed reflection spectra dominated by the signatures of olivine and orthopyroxene. They were termed silicaceous, or S-type, asteroids. S- types are the most abundant type of asteroid in the inner third of the asteroid belt, from which we believe most meteorites come.

There was something strange about the spectra of S-types, however. Instead of the strong, deep absorption bands of minerals (and meteorites) measured in the laboratory, the S-type absorption bands were weak and shallow. Furthermore S-type spectra are skewed, tilted to the right (see Figure) with red and infrared light being reflected better than the green, blue, and violet. Maybe S-types had just a bit of silicate rocks in them and were mostly made of something else that lacks absorption bands and has a reddish tint. This was the idea of McCord and Gaffey, who identified the dominant component as metal. Nickel-iron alloy is slightly reddish and exhibits no absorption bands. To explain the spectra, much more metal is required than the metallic flecks in ordinary chondrites. S-types are composed, McCord and Gaffey proposed, of at least as much metal as silicate, like the very rare stony-iron meteorites.

The rare metal-rich meteorites (irons and stony-irons) served as anvils and tools for ancient peoples. Today they are prominently displayed in museums because of their unearthly, pock-marked beauty. But they are rare, indeed, among meteorites. They originated, researchers believe, 4 1/2 billion years ago when a mysterious source of heat in the early solar system melted a few of the bodies aggregating in the asteroid region. Molten iron sank to their centers, which were in turn mantled by dense silicates, especially olivine. More buoyant magmas apparently flowed onto their surfaces, like basaltic lavas on Earth. These miniature worlds then cooled, with their spherically segregated layers -- core, mantle, crust. Later they collided with other asteroids, were smashed up, and the metal-rich core material was excavated -- eventually to reach Earth as iron-rich meteorites. Only one melted asteroid, Vesta, has survived the collision derby in the asteroid belt -- reflection spectroscopy reveals that Vesta still retains its basaltic crust.

If S-type asteroids are remnants of the cores of these once-melted bodies, then we must look elsewhere to find the parent bodies of primitive meteorites. Where are they? A decade ago, Jeffrey Bell (Univ. of Hawaii) proposed that ordinary chondrites would be found among the smaller asteroids, around 10 kilometers in diameter, a little too faint to be surveyed by the spectral studies of the 1970's and 1980's. During the last 5 years, using a spectrometer with high-tech CCD (charge-coupled device) detectors, Richard Binzel (M.I.T.) has risen to the challenge and surveyed hundreds of asteroids around 10 kilometers in diameter, chiefly in the inner belt. The result: none of them, with one possible exception, look like ordinary chondrites.

What's going on? For many years, I have had a nagging suspicion that reflection spectroscopy may be fooling us. Asteroids may not be what they seem to be. After all, telescopic spectra of the Moon look nothing like the spectra of Moon rocks returned to Earth by the astronauts. The Moon is covered with a layer of soil, in which the returned rocks were embedded, rather like the Scottish countryside or the desert. But instead of developing from decaying plants, the tunneling of worms, and the blowing sands, soils on the Moon were created by aeons of cratering and bombardment by micrometeorites and solar wind particles. On a microscopic scale, the pummeling has mangled its minerals, turned crystals to glass, and induced minor chemical and physical changes that influence the way photons are scattered and reflected. The result is that the spectrum of the Moon has been reddened and the absorption bands of its silicate minerals are weakened or erased altogether. The effect is called "space weathering".

Researchers have long doubted that space weathering could be effective on asteroid surfaces. Micrometeorites strike at slower velocities in the asteroid belt, the solar wind is diminished, and asteroidal soils must be short-lived compared with the ancient lunar soil layer, due to the minimal gravity of asteroids. Yet, we can hardly expect that abundant meteorite types come from extremely rare asteroids. Asteroids still tinier than Binzel has surveyed are collisionally smashed up in just tens or hundreds of millions of years and must be re-supplied from break-up of larger, longer- lived bodies. Where are such storehouses of primitive chondritic material, asteroids large enough to survive for 4.5 billion years? Maybe some large ordinary chondrite parent bodies are masquerading as something else. Maybe, despite our doubts about how it would work on asteroid surfaces, we MUST invoke some limited space weathering to explain the apparent absence of ordinary chondrite spectra among the main belt asteroids.

Some experiments recently conducted by a team of Russian scientists, led by Lyuba Moroz of the Vernadsky Institute in Moscow, may hold the answer. They crushed an ordinary chondritic meteorite and then zapped the powder repeatedly with a high-power laser, simulating micrometeorite bombardment. Indeed, the reflection spectrum of the powdered meteorite gradually changed to look like the spectrum of an S-type asteroid! So maybe some of the S-type asteroids, instead of being metal- rich, are space-weathered ordinary chondrites.

Recently, two asteroids were observed close-up by the Galileo spacecraft, as an accidental by-product of that mission's 6-year spiralling journey out to Jupiter. During brief encounters with Gaspra in October 1991 and with Ida in August 1993, Galileo's camera and spectrometer obtained intriguing data that whets our appetites for the more thorough studies that can be done from orbit. Gaspra is a little smaller than Eros, Ida somewhat bigger. Both happen to be S-types.

The results, especially for the better-observed Ida, were exciting. Ida turned out to have a moonlet, named Dactyl, just a mile across. From watching Dactyl move through part of an orbit around Ida, Galileo camera team leader Michael Belton (National Optical Astronomy Observatory) was able to employ Kepler's Third Law and deduce a mass for Ida. The pictures themselves were used to determine the volume of Ida's misshapen figure. Together, the mass and volume yield an unexpectedly low bulk density for Ida: about 2.5 grams per cubic centimeter. If Ida were the stripped metallic core of a melted and differentiated body, its density should be in the range of 5 to 8 g/cc. Even if collisions had broken Ida apart and it had reassembled into a rubble pile structure with internal void spaces, it is unlikely that an inherently metal-rich body could have such a low density. I have suggested that Ida may, instead, be of ordinary chondritic composition. Ordinary chondrites have inherent densities of about 3.5 g/cc and, with a rubble pile structure, Ida's density of 2.5 g/cc would be reasonable.

Galileo's cameras have revealed clear evidence of spectral alteration with time (space weathering) on both Gaspra and Ida. On Ida, the freshest, youngest craters have less-red spectra, with deeper absorption bands than most of Ida -- perhaps those impacts excavated pristine asteroidal bed-rock and there has not yet been enough time for space weathering to redden their spectra and weaken their absorption bands. The youngest of the larger craters on Ida -- perhaps less than 100 million years old -- is named Azzurra. Paul Geissler (Univ. of Arizona) has used a computer to simulate how the pulverized bedrock ejected from the crater would be distributed around the irregularly shaped asteroid. Azzurra's ejecta should form an irregular pattern, which matches almost exactly the distribution of terrain that shows deeper absorption bands and less-red spectra than most of Ida. These regions have spectra intermediate between ordinary chondritic material and the older, redder terrains that give Ida its typically S-type spectrum.

Space weathering also operates on Gaspra, according to the Galileo data. However, Gaspra cannot be an ordinary chondrite. No extrapolation backwards of space-weathering trends can change the fact that Gaspra is simply too rich in olivine, and too poor in orthopyroxene, to match any known ordinary chondrite. Gaspra may well be a fragment of the broken-up core of a once-melted asteroid.

Galileo, with its limited spatial resolution during its hurried fly-bys, was unable to find the Holy Grail of the S-type conundrum. Neither the Azzurra ejecta blanket, the small (100 meter diameter) youthful craters on Ida, nor Ida's comparatively youthful moonlet Dactyl show a pure ordinary chondritic spectrum. I think it is plausible that they represent intermediate stages of evolution between ordinary chondritic bedrock and a space-weathered S-type appearance. But we needed to study Ida in closer detail to find out if the hypothesized chondritic end-member actually exists.

Galileo zoomed past Gaspra and Ida at many kilometers per second, affording only a few minutes of good observing time. Indeed, for fear of running into space debris near Gaspra and Ida, which would have prematurely doomed the Jupiter mission, Galileo's trajectory was aimed thousands of kilometers away from these serendipitous targets. So even the best pictures could resolve surface features no smaller than a very large building.

In contrast, the NEAR spacecraft will hover close to its target, 433 Eros, and will slowly orbit it from ever-closer distances during a full year. Eros is also an S-type asteroid, intermediate in size between Gaspra and Ida. NEAR's camera will snap images of Eros, perhaps down to a few meters resolution, or even less toward the end of the mission, through seven spectral filters carefully designed to explore mineralogical variations indicated by Eros' 1 micron absorption band. A complemen- tary instrument, the Near Infrared Spectrograph (NIS), lacks the super spatial resolution of NEAR's camera, but it can make regional maps of mineralogical variations across Eros. It observes at 64 separate wavelengths, which extend much farther than the camera's filters into the infrared, where minerals have additional absorption bands. Therefore, NIS can sort out the different minerals on Eros much more accurately than the camera.

NEAR's multispectral camera and NIS will do, except MUCH better, for Eros what Galileo's camera and Near Infrared Mapping Spectrometer did for Gaspra and Ida. But that is not all. NEAR also has an X-ray fluorescence spectrometer and a gamma-ray spectrometer, crucial detectors not included on the Galileo orbiter's payload. Packaged in an instrument with the acronym XGRS, they are sensitive to the amounts of specific chemical elements, regardless of what mineralogical structure the elements may comprise. The XGRS will sense gamma rays spontaneously emitted from Eros, for example from the decay of radioactive elements like potassium, uranium, and thorium. (Those are the elements that, within the Earth, cause the heating that keeps our planet geophysically active but which are far less effective at heating small asteroids.) XGRS will also sense X-rays, often stimulated by the Sun, from the major mineral-forming elements, like magnesium, silicon, calcium, and iron. Measurements of their abundances can resolve ambiguities in the mineralogical assays of the camera and NIS. The degree of XGRS's sensitivity to some elements will depend on the chance occurrence of a solar flare during NEAR's mission. Other elements are certain to be measured well, no matter what the Sun happens to do.

The complementarity of the CHEMISTRY of Eros determined by the XGRS, the MINERALOGY best determined by the NIS, and the fine-scale MAPPING of mineralogical differences by the camera will provide an unprecedented attack on the S-type conundrum. The groundbased spectrum of Eros looks more like that of Ida than of Gaspra. So Eros is certainly a candidate for being an ordinary chondrite. But its spectrum also seems to vary, at least slightly, from one side to the other. Perhaps the variations on Eros are great enough to indicate ancient differentiation, the beginnings of mineral segregation that happen when a body begins to melt and alter its original homogeneous, primitive mineralogy.

Whatever Eros is, NEAR will do practically everything a spacecraft could do, short of performing in situ laboratory analyses on a sample of the asteroid, to figure out its composition. NEAR should certainly provide a clear answer to the question of whether Eros is an ordinary chondrite, a stony-iron, or something completely different. The insights from Eros, when combined with the suggestive results from Gaspra and Ida and from continuing telescopic surveillance of the broader population of asteroids, may -- or may not -- solve the question of the S-types generally. But NEAR's study of Eros will certainly provide a benchmark of certainty in what has been an arena of confusion for several decades.

Needless to say, NEAR's scientific mission at Eros is much broader than simply solving the S-type mystery. The year-long orbit will yield definitive information on Eros's gravity field, including -- for the first time ever for an asteroid -- a precise measurement of its bulk density. The geology of Eros will be studied in excruciating detail. Perhaps Eros, like Gaspra, Ida, and the moons of Mars will have "grooves" -- linear chasms or channels that have puzzled planetary scientists since a few were first seen on the Moon. If Eros has such features, or other topographic features unique to small bodies, the sharp detail of the camera's images, taken from many aspects, should provide the clearest picture ever of the structure of nearly gravity-free bodies.

At the moment, Eros remains but a small point-of-light in telescopes, and we have no knowledge about what its geology may be like, whether it has a moon or moons, or what other surprises it may hold in store for us. Calculations last year by Paolo Farinella (Univ. of Pisa) and his colleagues at the Nice Observatory in France suggest that Eros has about a fifty-fifty chance of ending its existence by colliding with the Earth sometime in the future. There is zero chance that such a collision will happen anytime within our lifetimes -- tens or hundreds of millions of years in the future is most likely. Yet, by chance, Farinella's first simulation just happened to show Eros colliding with Earth less than 2 million years from now, a long time in comparison with human history but tomorrow morning so far as our planet is concerned.

If Eros does eventually perform a kami-kaze attack on our planet, it would dwarf even the K/T impact 65 million years ago that ended the reign of the dinosaurs and let the evolution of mammals -- and ourselves -- flourish. As our descendants, millions of years from now, decide how to deal with this unprecedented assault from the heavens, they may remember back to the beginning of the third millennium, when a small NASA spacecraft named NEAR told us everything we needed to know about the attacker. The solution of the nagging S-type/ordinary chondrite mystery will undoubtedly be but a small part of the story.

Available Abstracts, Preprints, Articles.

Clark R. Chapman's Publications.

Clark R. Chapman's Home Page.

SwRI Boulder Office (GAP) Home Page.

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