Bennu spits rocks into space.
This is one of the strangest things it does. Bennu is a small asteroid. It is shaped like two cones stuck together. A rubble pile is a bag of rocks held together by gravity. It has a boulder named Benben Saxum that is 22 meters high.
It spits rocks out into space.
Bennu is a small asteroid that is moving around the Sun in the same way as Earth. It can get as close as half a million kilometers to us, so it is classified as a Potentially Hazardous object, though at least for the next three centuries it won't be a real threat. NASA sent the OSIRIS-REx mission there to map Bennu and return samples to Earth to study the rocks that could potentially ruin our day.
Bennu is shooting small rocks from its surface into space, one of the many discoveries from that mission. That was one of the biggest surprises from the mission, but it is not clear how this is happening. Small micrometeorite impacts hitting the surface are likely to be the cause. The day/night cycle as Bennu spins makes the rocks on the surface expand and contract as they enter and leave sunlight, which eventually cracks them. Small bits can be thrown away by this.
The spitballs were seen as small as 10 centimeters across. The gravity on Bennu is barely more than a whisper, just a few millionths of Earth's, but if you recall the van der Waals force of high school science, you might recall it. The force is weak but in an asteroid with jagged rocks it can help hold the asteroid together. Bennu can muster that kind of oomph if it can be overcome and launched at speeds of a few centimeters to a few meters per second.
Is that the only way rocks are ejected? We can think of two that cover most of the bases, but that doesn't mean they cover all of them. A team of scientists looked into the possibility of an electric charge.
When a UV photon hits the surface of Bennu, it can flick away an electron from an atom. Rocks gain a positive charge when more hit and more electrons are lost. If enough charge builds up, a rock can feel a repelling force from its fellow rocks, and that could be enough to overcome the meager gravity and launch it into space.
The solar wind can hit the surface and build up a static charge, and in the end can also launch bits of rock into space. It's the same as rubbing a balloon on your hair and sticking it to a wall, but with an attractive force instead of repelling it.
The scientists looked at the possibility that the charge could be strong enough to spit rocks away. The force is small so it only works if the rock is small enough. This is not the reason we see the bigger ones ejected, but it could ping away smaller ones.
The night side of Bennu was where some of the rocks were ejected. The sun can't explain them all, but the solar wind can. As the particles blow past Bennu it blocks them on the day side, leaving a shadow behind it, like if you were to stand in the middle of a building to escape the wind. Particles can hit the lee side of the asteroid because the magnetic field of the wind can still connect to the surface. The scientists found that the force is too weak to explain the rocks launched away during local night on Bennu.
This is still a valuable research and it looks like the force of electrostatic charging is a small one. It can work on the smallest bits and is very good at lofting dust off the surface. A study like this is a good step in figuring out the properties of rubble pile asteroids.
When a small bag of rocks like Bennu is aimed at us, we have to do something to stop it. The more we know about it, the better. We probably wouldn't want to blow it up, but we need to understand its structure to be able to do that.
It isn't every day that astronomy can save the world.
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