Larger craters require larger impactors, and those become more rare as time goes on. But it’s not an exact science, because there are plenty of other things that can determine how big a crater is. For example, if an impactor hits a soft surface, the crater will be larger. But if it hits a porous surface, the crater will be smaller. We’re still figuring out some details about Bennu’s texture, so like with almost everything else, we will have to wait for more data to come back to say anything for sure. So, these papers have talked about Bennu’s chemistry, its size, and its age. But they’ve also talked about its shape and spin, and the results there aren’t any more normal. Bennu is shaped like a top, and it spins like one, too. Before we launched OSIRIS-REx, we had evidence that the rate at which it spins has also been increasing, but new results from the mission have been able to confirm that. The researchers also concluded that this increase is likely caused by something called the YORP effect. This is where the heat an asteroid absorbs from the Sun gets re-radiated at angles that generate torque.
This is a small effect, but it adds up over astronomical time, so now, Bennu is rotating once every 4.3 Earth hours. And that isn’t just a cool stat. That spinning creates a centripetal force, which combines with Bennu’s gravity to affect all of the particles it’s made of. For one, it means that the overall force pulling particles toward Bennu’s center of mass is roughly three times greater at the poles than it is at the equator. In other words, you would weigh three times more at Bennu’s north or south pole than you would if you were on its equator! That is very weird! This spin also means that a pebble rolling down from the north pole to the equator would build up enough velocity to literally escape Bennu forever. That seems like it should be a mechanic for a Super Mario Galaxy game, but it’s a real thing. Gravity is nowhere near as strong on Bennu as it is on Earth, so seemingly weird stuff can happen. Also, this phenomenon can most likely explain the asteroid’s weird shape.
Rocks that roll down Bennu starting at lower latitudes could still fly off the asteroid, but instead of disappearing forever, they’d likely stick around in orbit for a while and then fall back down. That would help redistribute Bennu’s matter over time, contributing to its shape. So far, OSIRIS-REx has proven that there’s nothing boring about Bennu, and that even small objects in the solar system have a bizarre and fascinating history. And if we’re learning all of this from only the preliminary results, we’re gonna have a lot to talk about once the spacecraft brings us a sample home.