Second author

Understanding the nature and origin of asteroids that pass by — and occasionally hit — Earth may help us to identify future impact threats. Richard Binzel, a planetary scientist at Massachusetts Institute of Technology (MIT) in Cambridge, and his colleagues match meteorites (objects that fall to Earth from space) to their asteroid origin on the basis of their mineral compositions. Surprisingly, the authors find that near-Earth asteroids are most similar to a rare class of meteorites (see page 858). Binzel tells Nature more.

How did your approach evolve?

Three years ago, my co-authors and I started a joint programme between MIT and the University of Hawaii using NASA's InfraRed Telescope Facility to observe near-Earth asteroids one or two nights per month. Measurement of each object's near-infrared spectra gave clues to its mineralogy, but we needed a way to quantify the underlying mineral chemistry. Pierre Vernazza developed a model to determine the percentages of common minerals, such as olivine and pyroxene, in meteorites for his PhD dissertation. We challenged his model to decode the chemistry of known meteorites. It did so well that we adopted it for decoding asteroid fingerprints.

How did you explain your results?

Using this telescope, we can check the composition of bigger asteroids — those about 1 kilometre in size. Meteorite samples are typically only about 1 metre in size. Despite this difference, we expected the mineral composition of near-Earth asteroids to match the bulk of meteorites. But we found they best matched a class that represents only 8% of meteorites, which probably originate from the inner edge of the asteroid belt. So how do small meteorites from elsewhere in the asteroid belt reach Earth? The Yarkovsky effect — a gentle force that results when an asteroid radiates the Sun's heat — can alter a small object's orbit over time, so could slowly nudge smaller asteroids from throughout the belt towards Earth. The effect is minor for large asteroids, so only those at the belt's inner edge have a good chance of reaching Earth.

Could this help us respond to potential problem asteroids?

We analysed the entire asteroid population, and also assessed potentially threatening objects, such as Apophis — a 300-metre-diameter asteroid that will first come close to Earth on 13 April 2029. By determining its meteorite class, we were able to establish its size, likely mass and energy in just a couple of days. These parameters help us estimate whether an object will reach Earth — the first step for any possible course of action.