We could do with as much information as possible about near-Earth asteroids. A manned mission is a natural step, both for investigating a class of object that could one day hit our planet, and also for continuing to develop technologies in directions that will be useful for our future infrastructure in space. You would think we would know much of what we needed from examining meteorites, which generally are chunks of asteroid material, but that assumption turns out to be erroneous.
A recent paper in Nature has the story. Richard Binzel (MIT) and colleagues have been considering the properties of asteroids for a long time, looking at the spectral signatures of near-Earth asteroids and comparing them to spectra obtained from meteorites. And it turns out that most of the meteorites that fall to Earth represent types of asteroid that are different from the great bulk of near-Earth asteroids. In fact, the varied types of meteorites we find here generally resemble the mix of asteroids found in the main belt.
How can this be? Binzel’s team posits our old friend the Yarkovsky effect, mentioned surprisingly often in our archives. Uneven heating of an asteroid surface and the subsequent radiation of that heat during rotation can create imbalances that accumulate over time, adjusting an object’s orbit. And if this study is to be believed, the Yarkovsky effect works more strongly on smaller objects and much more weakly on larger ones. So efficient is the effect at small scale that it can readily move boulder-sized objects out of the asteroid belt onto a path that leads to Earth, while larger asteroids are moved much more slightly.
The largest near-Earth asteroids come from the innermost edge of the main belt, according to this theory, possibly remnants of a larger asteroid that was shattered aeons ago by collisions. In the aggregate, two-thirds of all such asteroids correspond to the type of meteorites known as LL chondrites, which represent only about eight percent of meteorites. They’re rich in olivine and poor in iron. Knowing this means we can put most of our attention onto deflecting this type of object. “Odds are,” says Binzel, “an object we might have to deal with would be like an LL chondrite, and thanks to our samples in the laboratory, we can measure its properties in detail. It’s the first step toward ‘know thy enemy.'”
So most meteorites come not from the population of near-Earth asteroids but the main belt, on a track that is made possible by the Yarkovsky effect. How we defend against an incoming asteroid may well depend upon its type, so this is a result that should be weighed carefully. Let’s hope it can also be backed up by a mission to a near-Earth asteroid in the not too distant future. The paper is Vernazza, Binzel et al., “Compositional differences between meteorites and near-Earth asteroids,” Nature 454 (14 August 2008), pp. 858-860 (abstract)