NEXT week, a spacecraft should set off on a three-year journey to a lump of flying rock the size of a city. The Near Earth Asteroid Rendezvous (NEAR), scheduled for launch by NASA on 16 February, will be the first mission to study an asteroid.
When it reaches its final destination, NEAR will spiral round the asteroid Eros for a year and reveal exactly what that little world is made of. It could help solve a mystery that has baffled scientists for more than half a century: what is the connection between asteroids and meteorites, the fragments of stone and metal that burn through our night skies and plummet to the Earth's surface?
Researchers have suspected for some time that meteorites are spawned from asteroids, but no one has managed to find a match. The problem is that the few clues researchers have found seem to rule out the likeliest asteroid candidates - they seem to be made of the wrong rocks. Now researchers are hoping that NEAR's study of Eros will resolve the conundrum once and for all.
Most asteroids live in the asteroid belt, a region between Mars and Jupiter that contains many thousands of minor planets between ten and a thousand kilometres across, and countless smaller ones. The best guess is that they are the remnants of innumerable small bodies that condensed when the Solar System formed but which never managed to merge into a single large planet.
But to be the true parents of meteorites that reach the Earth, these fragments of asteroids would have to escape from the asteroid belt and head close enough to the Sun to cross the Earth's orbit. The first evidence that this might happen came back in 1932, when telescopes revealed a lump of rock whose orbit periodically crosses that of the Earth. Over the past two decades, telescopes at Mount Palomar and Kitt Peak in the southwest of the US have shown that asteroids approaching Earth is not an unusual phenomenon - objects the size of a house have been seen passing between the Earth and the Moon.
The supply of asteroids must be continually replenished, say researchers, because their orbits are unstable. They are likely to collide with planets, fall into the Sun or fly out of the Solar System altogether within hundreds of millions of years - which is much less than the age of the Solar System. So they probably come from the main asteroid belt because that is where 99.9 per cent of asteroid material resides.
But how do they escape from the asteroid belt? Scientists believe that within the asteroid belt, bodies sometimes crash into each other, scattering asteroid fragments into chaotic orbits where they are perturbed by the gravity of Jupiter and other planets. From these escape hatches, they can be propelled out of the asteroid belt, and end up crossing Earth's orbit. Sometimes they even fall into the atmosphere, where they plummet towards the ground as burning meteors.
The small fragments of icy, dead comets that cause meteor showers are very weak and burn up easily in Earth's atmosphere. But asteroid fragments are made of stony material, at least part of which may survive the fiery plunge and hit the Earth. These fragments are the meteorites that sit, cut and polished, in museum displays.
By looking at these meteorites, scientists are trying to find out about the strange asteroid worlds that spawn them. By far the most common meteorites fall into the ordinary chondrite class - rocks with tiny flecks of metal. These meteorites also contain countless spheres about a millimetre in size. These so-called chondrules are made up of the silicate minerals olivine and orthopyroxene. A similar group of meteorites called carbonaceous chondrites contain a small amount of carbon, which gives them a black hue. Chondrites are made from the same stuff as the Sun - apart from the gaseous and volatile elements, which evaporated long ago as the asteroids were forming. So the chondritic meteorites presumably are just like the solid materials from which the Earth and other terrestrial planets formed.
The Earth, the Moon and the other planets have all evolved dramatically during their lives. In fact, these worlds have changed beyond recognition as they have acquired - and sometimes lost - atmospheres and oceans, melted and metamorphosed, and undergone volcanism and tectonic action driven by the decay of radioactive elements deep within their interiors.
But the chondrites suggest that some asteroids have mostly evaded these processes. So for the past quarter of a century, scientists have searched for the parent worlds of these chondrites. By the 1960s, it had become clear that asteroids had different colours, and that they even exhibited crude spectral fingerprints - absorption bands of their constituent minerals - in the sunlight that they reflect. Many scientists were optimistic that simply by looking at the asteroid spectra, and comparing them with the make-up of meteorites, they would soon find the chondrite asteroids.
In the early 1970s, Tom McCord, now of the University of Hawaii, Torrence Johnson of NASA's Jet Propulsion Laboratory, Michael Gaffey of Rensselaer polytechnic Institute in New York state and I began a systematic spectral search for the meteorites' parents. Most of the brighter asteroids had reflection spectra showing the fingerprints of the same silicates, olivine and orthopyroxene, that dominate chondrites. Because of this, they became known as silicaceous, or S-type, asteroids. Promisingly, they were the most abundant type of asteroid in the inner third of the asteroid belt, which is where the chief escape hatches to Earth are found.
But then things began to go wrong. We soon realised that there was something strange about the spectra of S-types. Instead of the strong infrared absorption bands that chondrites show in the lab, the S-type bands were always weak. Unlike the ordinary chondrites, S-type asteroids seem to reflect red and infrared light much better than green, blue and violet
While this was nothing like chondrites, it did match the composition of very rare meteorites called stony irons. Many researchers believe that these stony iron meteorites come from bodies within the asteroid region that -mysteriously - melted while the Solar System was forming. When one of these bodies melted, molten iron sank to the centre and dense silicates formed a layer immediately above, while the most buoyant magmas must have flowed onto the surface, rather like basaltic lavas on Earth. These miniature worlds must then have cooled, with their layered structure preserved - core, mantle and crust. Later, they broke up when they collided with other asteroids, uncovering the metal-rich cores, pieces of which could reach Earth as iron-rich meteorites.
So perhaps S-type asteroids are the remnants of the cores of these once-melted bodies. If this is true, we would have to look elsewhere for the parents of primitive meteorites. A decade ago, Jeffrey Bell of the University of Hawaii proposed that the parents of ordinary chondrites were too dim to show up in the spectral surveys of the 1970s and 1980s. He suggested that the chondrites come from smaller asteroids, around 10 kilometres in diameter, which would have been too faint to study even with the technology available in the 1980s.
Over the the past five years, Richard Binzel of the Massachusetts Institute of Technology has risen to the challenge. Using a spectrometer and sensitive electronic detectors, he has studied hundreds of asteroids measuring around 10 kilometres in diameter, lying chiefly in the inner belt. The bottom line is that none of them, with one possible exception, looks like an ordinary chondrite.
The lack of such asteroids is baffling. It could mean that ordinary chondrite meteorites come from asteroids so rare that we have seen only a couple of examples. It seems implausible, though, that the most common meteorites should come from very rare asteroids. So perhaps meteorites come from comets after all, or from somewhere unexpected - from other planets or even from distant solar systems.
Before resorting to such outlandish explanations, there is another, more likely possibility. For many years, I have had a suspicion that some S-type asteroids may be ordinary chondrites in disguise. The idea is that some S-type asteroids may be made of ordinary chondrite material, but that the effects of the space environment have weathered the asteroids, making them seem redder and more metallic-looking than they really are. There is plenty of evidence that such processes have taken place on other bodies in the Solar System.
Take the Moon, for example. Spectra of the Moon from Earth-based telescopes look nothing like the spectra of Moon rocks brought back by astronauts. Most of the lunar surface is covered by soils - the result of bombardment by tiny meteors and solar wind particles over millions of years. On a microscopic scale, the pummelling has mangled minerals, turned crystals to glass and produced chemical and physical changes that influence the way the surface scatters and reflects light. The result is that Moon rock looks redder and the absorption bands of its silicate minerals are weakened or erased altogether.
But many people doubted whether space weathering could significantly alter the appearance of asteroid surfaces. After all, tiny meteors strike at lower speeds in the asteroid belt because orbital speeds are slower farther from the Sun, and the solar wind is less powerful. Also, asteroid "soils" are too short-lived to become highly weathered: asteroids would not have enough gravity to hold onto surface material knocked about by impact.
But results that have been coming in over the past year or two suggest that space weathering might be the answer after all. Lyuba Moroz and her colleagues at the Vernadsky Institute in Moscow crushed an ordinary chondritic meteorite to pieces in the lab. Then they zapped the powder repeatedly with a high-power laser to simulate bombardment by tiny meteors. Sure enough, the reflection spectrum of the powdered meteorite gradually changed to look like the spectrum of an S-type asteroid.
More evidence that space weathering occurs on asteroids came from the Galileo spacecraft, now orbiting Jupiter, which caught a fleeting glimpse of the S-type asteroid Ida in August 1993 on its way to the giant planet. On Ida, the freshest, youngest craters - perhaps less than 100 million years old -have spectra that are less red and have deeper absorption bands than the rest of Ida's surface. Perhaps meteor impacts throw up pristine asteroidal bedrock and there has not yet been enough time for space weathering to redden their spectra and greatly weaken their absorption bands.
Support for this hypothesis came last year from a computer model, developed by Paul Geissler of the University of Arizona, which simulated what happened when pulverised bedrock from the youngest of the larger craters on Ida, Azzurra, was forced to the surface. His simulations showed that the material thrown up should form an irregular pattern, that matches almost exactly the terrain that shows deeper absorption bands and less red spectra than most of Ida. These regions have spectra part-way between ordinary chondritic material and the older, redder terrain that gives Ida its typically S-type spectrum.
Galileo also provided another clue that Ida might be an ordinary chondrite in disguise. It found that Ida has a little moonlet, now named Dactyl, just over half a kilometre in diameter. By watching Dactyl move through part of an orbit around Ida, Michael Belton of the National Optical Astronomical Observatories in Tucson, Arizona, was able to work out a rough estimate of Ida's mass.
The pictures gave a precise measure of the volume of Ida's misshapen figure. Together, the mass and volume give Ida a density of roughly 2500 kilograms per cubic metre. If Ida were the stripped metallic core of a melted body, its density should be two or three times that. And even if collisions had broken Ida apart and it had reassembled as a pile of rubble with internal voids, its density - though obviously lower - would still not be as low as that measured.
My suggestion is that because ordinary chondrites have densities of around 3500 kilograms per cubic metre, Ida is in fact chondritic with a rubble pile internal structure that would lower its bulk density to roughly the value measured.
While the Galileo evidence is a help, it does not prove that S-type asteroids are responsible for the ordinary chondrites. During its hurried flybys, Galileo simply did not pick up enough information to solve the conundrum once and for all. It zoomed past Ida at many kilometres per second, affording only a few minutes of good observing time altogether. Even the best Galileo pictures showed surface features no smaller than a very large building.
In contrast, the NEAR spacecraft will hover close to its target, Eros, and will slowly close in on it for a whole year. Eros is an S-type asteroid, whose spectrum from Earth looks similar to that of Ida. So Eros could well be an ordinary chondrite. But its spectrum seems to vary, at least slightly, from one side to the other. It is possible that the variations on Eros indicate some ancient differentiation, the beginnings of what happens when a body starts to melt and alter its original mineralogy.
NEAR has instruments that will repeat Galileo's asteroid observations, but with much more detail. Its cameras may distinguish features less than a few metres apart through several spectral filters designed to explore the mineral composition of Eros. And the Near Infrared Spectrograph (NIS) should make it possible to be even more specific about the minerals present by making regional maps at 64 separate wavelengths which extend much further than the camera's filters into the infrared.
Unlike Galileo, NEAR also can study the detailed chemistry of the surface. It has a spectrometer that detects gamma rays emitted from the decay of radioactive elements such as potassium, uranium and thorium. NEAR also has an X-ray fluorescence spectrometer that will pick up X-rays from the major mineral-forming elements like magnesium, silicon, calcium and iron, which all emit X-rays in response to solar radiation.
In all, NEAR will provide an unprecedented attack on the S-type conundrum, and should provide a clear answer to the question of whether Eros, at least, is an ordinary chondrite. And combining the data from Eros, Ida and the ongoing studies of asteroids could well solve the question of the S-types generally. NEAR should also reveal whether Eros has moons, yield the first very precise measurement of an asteroid's density, and provide extraordinary detail about the geology of Eros.
Accumulating such detailed knowledge about Eros may even have an ironic payoff for our descendants. Last year, Patrick Michel and Christiane Froeschlé of the Nice Observatory and Paolo Farinella at the University of Pisa ran computer simulations that suggest that Eros has about a fifty-fifty chance of ending its existence by crashing into Earth many millions of years from now. Such an impact would dwarf even the asteroid or comet impact 65 million years ago that ended the reign of the dinosaurs. If in the future humans do face this assault, perhaps some will think back to the beginning of the third millennium, when a small NASA spacecraft named NEAR told us everything we need to know about this approaching world.
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18:53 05 September 2008
