Asteroid Bennu A New Surprises In September 2016, NASA’s OSIRIS-REx spacecraft began its two-year journey to the asteroid Bennu. And this past December, it finally entered Bennu’s orbit. Because scientists are pretty smart, though, they started collecting data on Bennu long before OSIRIS-REx officially got there, so now, they are ready to start sharing their preliminary results. This week, researchers published seven papers about Bennu across four journals: Nature, Nature Astronomy, Nature Geoscience, and Nature Communications. There’s a lot to talk about there, from updates on the asteroid’s density to its chemical composition. And while we don’t have time to unpack everything, we will give you some highlights. Because it turns out that Bennu is not entirely what we expected. Bennu is kind of special, as far as asteroids go. We knew before we got there that its chemical composition is similar to that of meteorites that formed at the beginning of the solar system, but Bennu is a lot more pure. Those meteorites changed as they got knocked off whatever parent body they came from, and they became further contaminated by interacting with Earth’s atmosphere and the ground. But that hasn’t happened to Bennu. So when OSIRIS-REx returns a sample of its surface in 2023, we will be able to say more confidently what the chemistry of the early solar system was like. Except, this mission has also revealed that taking the sample will be harder than we thought. As part of their research, scientists refined their estimates about Bennu’s mass, volume, and density.
Bennu isn’t very dense at all, which supports the idea that it’s just a pile of rubble instead of a hunk of rock. That part wasn’t surprising, because other near-Earth asteroids around Bennu’s size are like this, too. During their lifetimes, these objects were likely obliterated through one or many collisions. Then, over time, those rocks came together until they were one asteroid, tenuously tied together by gravity. What was more interesting about Bennu is that not all of its rocks seem to have come from the same place. OSIRIS-REx found that they come in a variety of colors and shapes and reflectivities, which suggests that they came from different places and underwent different evolutionary processes. Some of them likely came from whatever parent body Bennu originally belonged to, while others could have come from rocks that collided with the asteroid over the years.
At first, that sounds awesome for sample collection, since it means that we could study multiple rock types with one mission. The problem is, some of the rocks OSIRIS-REx detected are more than 20 meters tall, and there are a lot of them. That means that there are very few safe regions large enough to bring the spacecraft down to the surface to take a sample. Realistically, though, this doesn’t mean it’s all over. Japan’s Hayabusa2 mission is currently studying an asteroid with boulders like this, and it managed to find a safe place to take a sample. So OSIRIS-REx will likely be fine. Sample collection is just gonna take some time. Still, if and when we figure it out, one thing is for sure: It’s gonna be worth it. Because among many other things, our closer view of Bennu has also allowed scientists to figure out the asteroid’s age. And this thing is old. After analyzing a set of what are probably large impact craters, researchers estimate that parts of Bennu’s surface are anywhere from 100 million to 1 billion years old. That’s around a fifth as old as the solar system itself. One of the ways they figured this out was by counting the number of craters of different sizes.