Diamonds in the rubble

Abstract

Two ancient asteroids with distinctive diamond-like shapes are revealing clues not only about their own formation, but that of the solar system as well. For the past three years, two spacecraft have been studying a pair of the most primitive solar system objects ever explored. The asteroids, Ryugu and Bennu, born of violent collisions in the asteroid belt, are some of the smallest objects ever visited by spacecraft. Ryugu is just over a kilometer across, and Bennu is about half as wide. The asteroid Bennu ejects particles from its surface in January 2019—a phenomenon captured in an image composed from multiple photos. Image credit: NASA/Goddard/University of Arizona/Lockheed Martin. But these small bodies are promising a huge bounty of data to astronomers over the next few years. Although the findings could revolutionize our understanding of the history, formation, and composition of asteroids, they may also tell us a little about the solar system during its infancy. That’s because these carbon-rich (1) asteroids, with their coal-dark surfaces, are also among the darkest bodies known in the solar system. Although that can make it difficult to observe them from Earth, studying them could tell us about the building blocks of life that such asteroids may have delivered to Earth early in the history of the solar system. Before researchers could hope to study these asteroids, they first had to reach them. This, by itself, was an enormous challenge. Only a handful of spacecraft have visited asteroids, and fewer still have managed to orbit one of these tiny, irregularly shaped objects. The Hayabusa spacecraft, launched by the Japanese Aerospace Exploration Agency (JAXA) in 2003, became the first to collect a sample of an asteroid. But the mission was beset by engine problems. Hayabusa’s prospects for returning with a sample remained uncertain for much of …