In a recent study by researchers at MIT, one of the biggest questions in planetary science may have been answered. For years we’ve known that the composition of a majority of meteorites didn’t match the composition of most of the near-Earth asteroids (NEA) we’d seen. This means that even though we worry about having an asteroid slam into us out of the blue, those that cross our paths haven’t really hit us that often in the past. So then where do they come from?
As reported in the August 14 issue of Nature, the smaller rocks that most often fall to Earth come straight in from the main asteroid belt out between Mars and Jupiter, rather than from the NEA population. How do we know? Studying the spectral analysis of an object can tell you it’s chemical makeup. Each chemical gives off a ‘spectral signature’ in the light it emits. Turns out that about two-thirds of the NEAs fall under a category of asteroid known as LL chondrites. However, only eight percent of the meteorites found on Earth are of this type. The rest resemble the mix of asteroids that are sparsely scattered in the area between Mars and Jupiter, commonly called the asteroid belt.
“Why do we see a difference between the objects hitting the ground and the big objects whizzing by?” asks MIT professor Richard Binzel. “It’s been a headscratcher.” As the effect became gradually more and more noticeable as more asteroids were analyzed, “we finally had a big enough data set that the statistics demanded an answer. It could no longer be just a coincidence.”
So why would we see more objects hit us from a distant starting point than from the stuff right in our neighborhood? There’s a phenomenon discovered long ago but only recently recognized as significant, the Yarkovsky effect. That’s when solar heating causes perturbations in a rotating body’s path. The side facing the sun absorbs radiation. As it rotates, it radiates the energy in different directions, and over time this causes the orbit to change. All objects orbiting the sun experience this, but what’s really important is that it affects smaller objects more than larger ones. So a small, distant asteroid has a greater chance of getting shoved into a collision course with Earth than the large ones near us. Add to that the gravitational nudges caused by Jupiter and the Earth itself, and the odds increase greatly.
Now comes the task of assessing the risk of collision based upon the properties of the asteroids. If eight percent are of a certain type, then maybe we should only devote eight percent of the strategic planning resources into preparing for that likelihood. “Odds are, 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,” he says. “It’s the first step toward ‘know thy enemy’.”