# How is asteroid 101955 Bennu possible?

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• alva3
In summary, the conversation discusses five questions about the structure of asteroid 101955 Bennu. The questions revolve around the possibility of small boulders and gravel being held together by gravity, the potential of a satellite landing on the surface, the impact of a tether on the micro-gravity, the role of van der Waals forces, and the agglomeration of debris as a violation of the second law of thermodynamics. The expert summarizer explains the concept of rotational velocity and potential energy in relation to the asteroid's structure and answers each question with scientific explanations. The conversation ends with one participant expressing difficulty in imagining the concept despite understanding the math behind it.
alva3
TL;DR Summary
Five questions about the structure of asteroids; specifically 101955 Bennu.
Summary: Five questions about the structure of asteroids; specifically 101955 Bennu.

Please forgive my ignorance, but I need help reconciling a few things...
I'm curious about the images I've seen of asteroid Bennu:

1. If gravity is proportional to combined masses divided by their distance, how is it even possible that small boulders and gravel are being held together by each other on a rotating body?

2. If it is possible, once the asteroid's speed and rotation were matched, couldn't a satellite land on its surface simply by getting closer (without intentionally attempting to land) i.e. "be pulled"?

3. Wouldn't the force of a tether overcome the micro-gravity in a huge way and send a large portion of the gravel flying off into space? https://www.sciencedirect.com/science/article/abs/pii/S009457651731682X

4. How do van der Waals forces account for what friction and gravity cannot by themselves? https://www.sciencedaily.com/releases/2014/08/140813132037.htm

5. Lastly, how is the agglomeration of a bunch of debris like this not a violation of the second law of thermodynamics?

Sincere thanks for replies.

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alva3 said:
1. If gravity is proportional to combined masses divided by their distance, how is it even possible that small boulders and gravel are being held together by each other on a rotating body?

you seem not to realize that it is a solid mass with a small bit of loose material on its surface
so the main mass IS NOT being held together by gravity

alva3 said:
2. If it is possible, once the asteroid's speed and rotation were matched, couldn't a satellite land on its surface simply by getting closer (without intentionally attempting to land) i.e. "be pulled"?

Someone else maybe able to answer that

alva3 said:
3. Wouldn't the force of a tether overcome the micro-gravity in a huge way and send a large portion of the gravel flying off into space? https://www.sciencedirect.com/science/article/abs/pii/S009457651731682X

alva3 said:
Summary: Five questions about the structure of asteroids; specifically 101955 Bennu.

4. How do van der Waals forces account for what friction and gravity cannot by themselves? https://www.sciencedaily.com/releases/2014/08/140813132037.htm

alva3 said:
5. Lastly, how is the agglomeration of a bunch of debris like this not a violation of the second law of thermodynamics?
Again, for all these Q's, refer to my answer to your Q #1

as you have a basic misunderstanding of the makeup of that asteroidDave

alva3 said:
If gravity is proportional to combined masses divided by their distance
The force of gravity on a particular chunk of gravel on the [hypothetically spherical] surface would be proportional to the combined mass of all the other chunks divided by the radius squared. It would also be proportional to the mass of the chunk in question.

The gravitational potential deficit of a particular chunk of gravel on the [hypothetically sperical] surface would be proportional to the combined mass of all the other chunks divided by the radius [not squared this time]. It would also be proportional to the mass of the chunk in question.

If the rotational velocity is low enough so that the kinetic energy of a chunk of gravel on the surface is less than half of the potential energy deficit then that chunk can rest on the surface. If more than half and less then all then the chunk will go into orbit instead. If the kinetic energy exceeds the potential energy deficit then the chunk will escape.

alva3
It doesn't rotate that fast. You don't need a large force to keep things on the surface.
The critical rotation rate depends on the density only if we approximate the asteroid as sphere: ##\frac{GMm}{r^2} = m \omega^2 r## where G is the gravitational constant, M is the mass of the asteroid minus a small mass m (e.g. a rock on the surface), r is the radius of the asteroid, ##\omega## is the angular velocity. For a sphere the mass is ##M=\frac 4 3 pi r^3 \rho## with the density ##\rho##, so plug that in: ##\frac{4 G \pi r^3 \rho m}{3r^2} = m \omega^2 r##. Simplify: ##\frac{4}{3} G \pi \rho = \omega^2##. For Bennu, ##\rho=1190 kg/m^{3}##, leading to a maximal angular velocity of 0.000313 per second, or a rotation period of 5.4 hours. It rotates slightly faster, 4.3 hours, but it is not a perfect sphere, and some of it is held together simply by being a solid object.
alva3 said:
2. If it is possible, once the asteroid's speed and rotation were matched, couldn't a satellite land on its surface simply by getting closer (without intentionally attempting to land) i.e. "be pulled"?
The position of spacecraft is controlled closely. You don't land unintentionally, unless something breaks in the spacecraft .
The asteroid is so small that the gravitational attraction and the escape velocity are tiny. To land you need to approach it very, very slowly, otherwise you might bounce off.
alva3 said:
5. Lastly, how is the agglomeration of a bunch of debris like this not a violation of the second law of thermodynamics?
Why would it? Things collide, sometimes they stick together and release the impact energy as heat.

alva3
jbriggs444 said:
If the rotational velocity is low enough so that the kinetic energy of a chunk of gravel on the surface is less than half of the potential energy deficit then that chunk can rest on the surface. If more than half and less then all then the chunk will go into orbit instead. If the kinetic energy exceeds the potential energy deficit then the chunk will escape.

This I get. Apparently the rotation rate is speeding up for no apparent reason, and stuff is flying off:

rotation: https://news.agu.org/press-release/...-return-mission-is-rotating-faster-over-time/
particle plumes: https://Earth'sky.org/space/challenges-osiris-rex-asteroid-bennu

Despite the science, I still have to admit it seems almost fictional to imagine Stone Mountain in GA., hurling through space, with gravel attached to the surface. The math computes, the mental image does not.

mfb said:
The critical rotation rate depends on the density only if we approximate the asteroid as sphere...
Thank you for the reply. Like I said, mathematically I get it. Conceptually (as a physical reality) I just do not. But hey, I've never been in space before.

This article does say however, that it's basically a "pile of rubble... with a bulk density much smaller than would be expected for a solid object." Admittedly, that is difficult to imagine. And, being such a low density pile of rocks, wouldn't landing on it seriously compromise not only the rotation, but also the orbit?

alva3 said:
This article does say however, that it's basically a "pile of rubble... with a bulk density much smaller than would be expected for a solid object." Admittedly, that is difficult to imagine. And, being such a low density pile of rocks, wouldn't landing on it seriously compromise not only the rotation, but also the orbit?
This pile of rubble has a mass of ~70 million tonnes. That's enough inertia to remain completely unfazed by what mass-wise is comparable to a medium-sized car, even if the probe crash-landed onto the asteroid. At best, it would dislodge some surface material from the main body.
Picture 10 million elephants standing on very slippery ice, if you can. Whatever change to the motion of the entire group you'd want to make, be it push them or make them rotate, you'd need to overcome the inertia of each elephant to make the desired difference. It's a lot of work.

But the mission to Bennu doesn't even intend to land as such, just hover above and blow some nitrogen onto the surface with a robotic arm.

alva3 and anorlunda
Bandersnatch said:
But the mission to Bennu doesn't even intend to land as such, just hover above and blow some nitrogen onto the surface with a robotic arm.
That's nearly the same as landing for at an object with such a low mass. You need one minute to fall down 10 cm near its surface.

alva3 and sophiecentaur

## 1. How was asteroid 101955 Bennu formed?

Asteroid 101955 Bennu is believed to have formed from the remnants of a larger asteroid that was destroyed in a collision with another object. The debris from this collision eventually came together to form Bennu.

## 2. How big is asteroid 101955 Bennu?

Asteroid 101955 Bennu has a diameter of approximately 500 meters, making it one of the larger known asteroids in our solar system.

## 3. How is asteroid 101955 Bennu able to maintain its orbit?

Asteroid 101955 Bennu is able to maintain its orbit due to the gravitational pull of the Sun and other large bodies in our solar system. Its orbit is also influenced by the Yarkovsky effect, which is caused by the asteroid's rotation and the uneven heating of its surface.

## 4. Is asteroid 101955 Bennu a threat to Earth?

While asteroid 101955 Bennu does have a close approach to Earth every 6 years, its orbit is well-studied and it is not currently considered a threat to our planet. However, scientists continue to monitor its orbit and any potential changes that could pose a risk in the future.

## 5. What have we learned from studying asteroid 101955 Bennu?

Studying asteroid 101955 Bennu has provided valuable insights into the formation and evolution of our solar system. It has also given us a better understanding of the potential impact hazards that exist in our cosmic neighborhood and the importance of continued asteroid research and monitoring.