The asteroid Bennu is covered in Styrofoam-like rubble.
The conclusion of a team of scientists looking at Bennu is that it is a 500-meter wide asteroid. Bennu is a collection of millions of small rocks that are held together by their own gravity.
It is thought that rubble pile asteroids may have once been more solid, but when an asteroid chunk gets hit by another asteroid it can break into many pieces. The gravity of a small asteroid like Bennu isn't very strong, but it's enough to hold it together
The images taken by OSIRIS-REx were looked at by the astronomer. There are a lot of impact craters. There are many small ones and few big ones. It is true for planets and moons.
There are 1,560 craters on Bennu, and they found a very interesting thing. There are a few big craters. There are a lot of small ones. The number of craters increases as they get smaller until they become less in number.
What's the reason?
There are rocks.
There is rubble covering the surface of Bennu. The rocks can be very small and large. The rocks on Bennu are different from the ones on Earth. They are very porous and easy to break. The big boulders on the surface of the asteroid could fall under their own weight here on Earth.
OSIRIS-Rex collected samples on the surface of Bennu. The rock was held together by a whisper despite moving at a leisurely 10 centimeters per second.
You would think that a kitten sitting on something would make it worse, but it is not. Small rocks moving through space at high speed make craters when they hit a solid surface as the energy of motion is converted into mechanical energy, which causes the surface material to be ejected and dug out. If the surface is made of rocks, a lot of the impactors energy goes into crushing those rocks rather than making a crater.
It has major implications for the science of asteroids and the task of moving one out of the way should it be headed for earth. One idea is to hit the asteroid hard with a massive space probe, so that the probe will push the asteroid onto a different path. The small moon Dimorphos of Didymos will be impacted by the DART mission in October of 2022.
If the asteroid is a rubble pile, a lot of the impact energy will go into crushing and rearranging the surface material. Understanding how they behave under impact could be the difference between life and death.
Science is also cool. You can estimate the age of the surface by looking at the distribution of craters on an asteroid and knowing how much junk is put there in space. Small craters can be erased by smaller impacts, while large craters can last a long time. For Bennu, the scientists estimate craters bigger than 100 meters across can survive for 10 to 65 million years before being eroded away. The numbers were thought to be 15 times higher, but Bennu's natural armor means the erosion is much more rapid.
The surface of Bennu is very young compared to the asteroid itself. Understanding how asteroids change over time is a step in the right direction. On asteroids, the skin on your body can make you look older than you actually are.
Practical benefits may be added to this knowledge. A friable layer of material under the ship's skin would protect it from smaller micrometeorites. It has been used in spacesuits for a long time. Future engineers might want to look at it in action.
If we spot a rubble pile on its way to Earth, there are other options besides using a nuke, though you may be dismayed that using a nuke is a good option. The more we study these asteroids, the more likely we are to find a way to stop them from ruining our day.
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