Bad astronomy | Why does the asteroid Bennu launch small rocks into space?
Why does Bennu spit pebbles into space?
It’s one of the weirdest things he does, and that’s saying something. Bennu is a small asteroid just over 500 meters across. It has the shape of two cones glued together base to base. It’s not a solid rock, but rather a pile of rubble, like a bag of rocks held together by gravity. It has a huge rock that protrudes 22 meters high called Benben Saxum.
And, also, well, it spits rocks into space.
Bennu is a near-Earth asteroid, moving around the Sun in a slightly elliptical orbit the same size as Earth’s orbit. It can approach up to half a million miles from us, so it’s classified as a potentially dangerous object, although at least for the next three centuries it won’t come close enough to us to pose a real threat. . Still, we want to know more about these rocks that could potentially hit us and ruin our day, so NASA sent the OSIRIS-REx mission there to map Bennu and possibly send samples back to Earth for scientists to study.
One of the many discoveries of this mission is that Bennu somehow pulls small rocks from its surface into space. It was one of the biggest surprises of the mission, and it’s unclear how it turns out. A likely cause is small micrometeorite impacts hitting the surface and detonating shrapnel. Another is heat stress: the day/night cycle when Bennu rotates causes the rocks on the surface to expand and contract as they move in and out of sunlight, eventually cracking them. It may throw small pieces.
The size of these spittoons seen ranged from very small to about 10 centimeters in diameter. Gravity on Bennu is little more than a whisper, just a few millionths of Earth’s, but if you can go back to the dark memories of high school science, you might remember van der force Waals, where some tightly packed molecules attract or repel each other. others due to their electron clouds. This force is weak, but in an asteroid whose surface is made up of jagged rock, it can help build a cohesive force that holds the asteroid together. That too has to be overcome, and throwing a boulder the size of your fist at speeds of a few centimeters to a few meters per second takes some effort, so it’s impressive that Bennu can muster that kind of punch.
But is this the only way to eject the stones? Just because we can think of two that cover most bases doesn’t mean they cover them all. A team of scientists has therefore looked into another possible launch mechanism: electrostatic charge. [link to paper].
The Sun’s ultraviolet light is very powerful, and when a UV photon hits Bennu’s surface, it can knock an electron away from an atom. As more hits and more electrons are lost, the rocks acquire a positive charge. If enough charge builds up, a rock can feel a force that pushes it away from its fellows, and that might be enough to overcome low gravity and launch it into space.
The Sun also blows a wind of subatomic charged particles, and this solar wind can also hit the surface and build up a static charge, and in the end can also launch pieces of rock into space. It’s the cosmic equivalent of rubbing a balloon over your hair and sticking it to a wall, although in this case you get an attractive force instead of a repulsive force, but it’s the same physics.
This buildup of electrostatic charge almost certainly occurs on Bennu, but what scientists have been investigating is the possibility that it is actually strong enough to spit rocks. What they discovered is that it is possible, but the force is quite weak, so it only works if the rock is small enough; assuming that no van der Waals cohesion can be projected up to about a centimeter. So that’s probably not why we’re seeing the bigger ones being ejected, but it might keep the smaller ones away.
Interestingly, some of the rocks seen ejected were on the night side of Bennu. Sunlight obviously can’t explain them at all, but solar wind still can. When the particles blow in front of Bennu, they block them on the day side, leaving a shadow behind them, a hole, as if you had to stand downwind of a building to protect yourself from the wind. It’s called a plasma wake, and the physics are pretty fierce, but the wind’s magnetic field can still connect to the surface on the downwind side of the asteroid, allowing particles to hit it. . However, by modeling this, the scientists discovered that the force was much weaker, too weak to explain rocks being thrown during the local night on Bennu.
So it seems that electrostatic charge is at best a minor force at work here, but it’s still valuable research. On the one hand, it can still work on the smallest bits and is probably very good at removing dust from the surface. Also, the electrostatic cohesion properties of rubble-pile asteroids are not well known, so a study like this is a good step in understanding that.
There will be a time in the future when a small bag of rocks like Bennu will be aimed at us, and we will have to do something to keep it from touching. In this case, the more we know about it, especially the forces that hold it together, the better. We probably wouldn’t want to blow it up, but rather set it aside, but even then we need to understand its structure to be able to do that.
It’s not every day that astronomy can literally save the world, but that day will come.