Solar System Historians
About 4.5 billion years ago, a giant disk of rocks, gas, and dust coalesced to form Earth and other planets. Asteroids are some of the leftover pieces from this process and can provide insights into how our solar system came to be. Most asteroids are found in the asteroid belt between Mars and Jupiter, a vast field that looks nothing like Hollywood’s portrayal of it. The asteroids are usually widely separated, and collisions are few and far between. Asteroids have changed far less than most other solar system bodies like Earth and the Moon, making these small rocky bodies a valuable window into the early solar system.
Despite this potential window into the past, researchers know little about asteroids. The first one wasn’t discovered until 1801. It wasn’t until 1959 that researchers realized that many meteorites came from asteroids. More than 22,000 meteorites have been identified on Earth, including some originating from Mars and the Moon. But researchers only began classifying asteroids by type in the 1970s and 80s, and definitively identifying a specific parent body has been a challenge. Then, in 1991, a spacecraft made the first flyby of an asteroid; since then, missions have buzzed more than a dozen of them. And so far, only the first Hayabusa mission has ever grabbed a piece of an asteroid (25143 Itokawa in 2005) and brought it back to Earth.
Like Ryugu and Bennu, Itokawa is a so-called rubble pile asteroid, the remains of a larger object that shattered in a collision. But whereas Ryugu and Bennu are rich in carbon, Itokawa is a stony asteroid, the source of the most common types of meteorites. The potato-shaped Itokawa has few craters or boulders on its surface. This finding led many to suspect that Hayabusa2 and OSIRIS-REx would encounter similar surfaces and have little trouble selecting samples from their targets. That turned out not to be the case. Both Ryugu and Bennu were covered with boulders that make collecting samples a challenge. And their boulder-strewn surfaces weren’t the only surprises.