# Thread: Takes more than gravity to keep rubble together

1. ## Takes more than gravity to keep rubble together

Laboratory Equipment magazine

Previous research has shown that asteroids are loose piles of rubble held together by gravity and friction. However, the UT team found that 1950 DA is spinning so quickly that it defies these forces. Ben Rozitis, a postdoctoral researcher, Eric MacLennan, a doctoral candidate and Joshua Emery, an assistant professor in the Department of Earth and Planetary Sciences, wanted to know what keeps the body from breaking apart.

Looking at thermal images and orbital drift to calculate thermal inertia and bulk density, the team detected the action of cohesive forces in an environment with little gravity.

“We found that 1950 DA is rotating faster than the breakup limit for its density,” said Rozitis. “So if just gravity were holding this rubble pile together, as is generally assumed, it would fly apart. Therefore, interparticle cohesive forces must be holding it together.”

2. How do we know it's a loose pile of rubble?
Can it be rubble that somehow the surface fused with other rubble (like a layer of ice or something)?

I'm imagining a piece of rubble crashing int the other piece just enough for a slight surface melting to fuse them, and so on.

Or something like that bag of peas in the freezer that become hard as a rock that I have to slam against the counter to break them up. (just to have the remainder fuse again)

3. I was told many years ago that the asteroid belt was created by the collision/destruction of an entire planet or moon. I suspect that is not the only source of asteroids, though. I look at a chunk of granite and clearly see it is not held together by gravity alone. So not all asteroids are alike? Why is that surprising? If a few grains of sand, rocks, and whatever clump together because their orbits are closely matched, and gravity is sufficient, then they do. Others don't match orbits. Some smash into one another and create more smaller rocks. Some just bounce away. Some rocks are spinning rapidly. Some not so rapid. Some are big. Some are not. I guess I am just confused why this is news.

4. Originally Posted by mkline55
I was told many years ago that the asteroid belt was created by the collision/destruction of an entire planet or moon.
That origin story has not been part of the mainstream astronomical view for at least four decades.

5. Originally Posted by StupendousMan
That origin story has not been part of the mainstream astronomical view for at least four decades.
Well I never claimed to be young.

6. I'm not sure what you are saying, nor do I know what was previously known.

But, the way I picture it is with this analogy:
A mop gets wet, you spin it to dry it out. There are some parts of that mop that stay together, while the water, dirt and maybe even some threads fly off.

I know a lot of that has to do with friction, surface tension, etc. But; I found an article that mentions this is about the Van Der Waal force. Since I am not too familiar with VDW, nor the math behind it, I really am not sure what's going on which is why I asked what I asked.

7. Originally Posted by NEOWatcher
I know a lot of that has to do with friction, surface tension, etc. But; I found an article that mentions this is about the Van Der Waal force. Since I am not too familiar with VDW, nor the math behind it, I really am not sure what's going on which is why I asked what I asked.
The wikipedia article about van der Waals forces is pretty good.

It is pretty important in molecular bonding and that is where I am personally most familiar with it. It is a group of weak intermolecular bonding mechanisms that arise from the electrostatic interactions between dipoles. It is much weaker than the attraction in ionic bonds (say between a positively charged sodium ion and a negatively charged chlorine ion). But when you make an ionic molecule like HCl, the hydrogen end will have a partial positive charge and the chlorine end will have a partial negative charge, forming a dipole. The positive end of one of these dipoles will have some attraction to the negative end of another molecule, and that is what gives you van der Waal attraction.

It can also take place in non-ionicly bonded systems, as long as there is any partial charge so as to form a dipole.

There are examples given in the article about where it becomes important in macroscopic systems. I didn't know it was being considered as a mechanism in these "rubble" asteroids, but thinking about it now, I'm not surprised.

As to the "how do they know these are piles of rubble", I should leave that answer to the astronomers here, but I thought one way was direct observation - that Japanese probe that visited an asteroid, for one.

8. Originally Posted by Swift
Yes; I tried reading that, and because I'm not all that versed on ionic, electrostatic, dipoles etc. I have a hard time with it.
I know about ionic bonds, but when it comes to other bonding, I start to get lost.
But; now that you gave me another description, it's a lot clearer. Thanks.

And; yes, given that, I also can't understand why this is new. Maybe it's the first example with the conditions that passed the threshold of gravity bonding where they were able to do the math.

Originally Posted by Swift
As to the "how do they know these are piles of rubble", I should leave that answer to the astronomers here, but I thought one way was direct observation - that Japanese probe that visited an asteroid, for one.
We have only visited very few close up. I don't know if that's enough or understand why it's enough. So; I await.

9. sounds like a job for Dark Matter

10. Originally Posted by transreality
sounds like a job for Dark Matter
We don't need dark matter to make the stuff of low-density asteroids sticky.

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Hm. Do you suppose that last year's comet came apart when the holding forces failled due to solar heating? Rotational forces would strew material widely and nothing would be detectable afterwards as observed.

12. What about "vacuum welding"? Or maybe there are frozen volatiles inside the object cementing particles together with ice?

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Here is a recent paper regarding the pebbles to planetismals problem:

Formation of pebble-pile planetesimals

http://arxiv.org/abs/1408.2535

To be considered is if the current too fast spinning pile may have been chipped off from part of a larger planetismal that offered stronger cohesive forces that formed the current fragment.

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This could help explain what appear to be dust plains in the apparently near vertical regions comet 67P/Churyumov–Gerasimenko. Something is keeping all the little stuff from pooling down near the neck.

15. The spinning does not tell us what binds 1950 DA together; it constrains the net force to be greater than gravity. Since Deep Impact had minimal effect upon surface features, this is also a minimal constraint. We don't know if these are 'rubble piles' as they appear to be, or a tightly welded nuclei with the appearance of a rubble pile on the surface.

Fun stuff.

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(29075) 1950 DA is a asteroid. Deep Impact impacted on a comet (Tempel 1). 67P/Churyumov–Gerasimenko is a comet.

ETA: As for why asteroids are likely to be rubble piles, it is because the observed density of asteroids is less than that of meteorites, some of which have origins in asteroids.
Last edited by Reality Check; Yesterday at 04:20 AM.

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