Asteroids

[kilzok]

Hello everyone, this is my first post. Please forgive me if this question is too simple. I have often wondered why the asteroids dont clump together thru the force of gravity. Have any asteroids ever shown evidence of clumping?

[antoniseb]

If they were all on some gooey surface that damped their movement they probably would do some clumping, but when they move toward each other because of gravity, they pick up speed sufficient to later also move away.

They were probably created in a disk, which have have had the effect of damping their accelerations toward each other.

[Ilya]

It's not as simple as that. Solar system started out as a huge disk of debris, and it certainly did clump -- into what we now know as planets. Two rocks will stick together if they collide at very low relative velocity -- IOW, if they were on very similar orbits. The reason some debris never accreted is that at some point the planets (primarily Jupiter) became big enough to perturb the orbits and thus make all collisions too energetic to stick.

I know this is oversimplification, but close enough.

[SAMU]

That the asteroids have too much relative velocity to stick together is an old and has always been an inadequate idea. It was postulated before we had an idea that collisions occur as often as we now know they do. It had been pointed out even before that that even small collisions convert relative kinetic energy to heat which escapes from the system slowing the relative velocities and allowing them to stick together. That's the acretion theory in a nutshell. The most accepted idea these days is that a large body was struck hard, pulverized and the asteroids are the pieces. A similar idea is used to explain the rings of Saturn and Jupiter with the rings of saturn being from a more recent collision.

[01101001]

The most accepted idea these days is that a large body was struck hard, pulverized and the asteroids are the pieces.

Most accepted by whom?

I'd say the mainstream theory is offered by Wikipedia: Asteroid belt:

A common hypothesis agreed upon by most astronomers, called the nebular hypothesis, is that during the first few million years of the solar system's history, planets formed by accretion of planetesimals. Repeated collisions led to the familiar rocky planets and to the gas giants. However, if the average velocity of the collisions is too high, the shattering of planetesimals dominates over accretion, and planet-sized bodies cannot form. The region lying between the orbits of Mars and Jupiter contains many strong orbital resonances with Jupiter, and planetesimals in this region were (and continue to be) kicked around too strongly to form a planet. The planetesimals instead continue to orbit the Sun as before.

And,

An old hypothesis, much less favored nowadays, was that the asteroids in the asteroid belt are the remnants of a destroyed planet called Phaeton. The key problems with this hypothesis are the staggering amount of energy required to achieve this kind of effect, and the low combined mass of the asteroid belt (less than that of Earth's moon).

[Kaptain K]

Many asteroids show evidence that they are "clumps" (orbiting rock piles), rather than being solid objects.

[SAMU]

Quote:
Originally Posted by SAMU
The most accepted idea these days is that a large body was struck hard, pulverized and the asteroids are the pieces.

Most accepted by whom?

By those of us who understand solar system dynamics in terms of gravity, velocity, impact and clumping. The article you cite from wikipedia contains the disfavored idea that the asteroid belt has been maintained by "orbital resonances" with Jupiter and Mars. This is an old idea and is disfavored because it demands an enormous and complex degree of chance, that no one has been able to describe, to maintain the asteroid belt since the time of original solar system formation.

Not that it could not have happened by that process but, since collision is the simpler explanation, the application of Occam's Razor says that it is probably the right one.

[01101001]

The article you cite from wikipedia [Wikipedia: Asteroid Belt] contains the disfavored idea that the asteroid belt has been maintained by "orbital resonances" with Jupiter and Mars. This is an old idea and is disfavored because it demands an enormous and complex degree of chance, that no one has been able to describe, to maintain the asteroid belt since the time of original solar system formation.

Let's ask an astrophysicist!

NASA: Ask an Astrophysicist: Origin of the Asteroid Belt

[Q] Has it ever been considered that the asteroid belt was maybe a destroyed planet?

[A] The fact that the asteroid belt has such a well-defined, high concentration of asteroids suggests two things. One, that they are fragments of a planet that broke-up long ago, or two, that they are rocks that never managed to accumulate into a genuine planet. Currently, scientists tend to favor the latter explanation.

Do you have some cite that doesn't shatter your fractured planet contention?

[SAMU]

This site

http://neo.jpl.nasa.gov/neo.html which describes
"Near-Earth Objects (NEOs) are comets and asteroids that have been nudged by the gravitational attraction of nearby planets into orbits that allow them to enter the Earth's neighborhood.

Clearly, the influence of nearby planets is far more likely to pull asteroids from the asteroid belt that to maintain them in it.

The "gravitational resonance" theory demands that as an asteroid circles the sun and is perturbed from its orbit by a planet it circles around and again passes the planet, or another planet, and is perturbed back into its original orbit. This is only stable in the long term if there are few bodies acting on the asteroid. This is not the case. There are many planets, moons and the other asteroids themselves acting on the asteroids which should, in a term much shorter than the formation of the solar system, clump them into a planet. In fact the time spans discussed are described like this. If the time since the original acretion disk formed to now is called 24 hours then most of the planets were formed (clumped together) in the first 8 minutes. That illustrates how strong the (clumping together) forces are.

[01101001]

This site
http://neo.jpl.nasa.gov/neo.html which describes
"Near-Earth Objects (NEOs) are comets and asteroids that have been nudged by the gravitational attraction of nearby planets into orbits that allow them to enter the Earth's neighborhood.

No. I meant cite some source that actually backs you up. Do you have a source that says current mainstream thought is that the asteroids were once part of a planet-sized body that was pulverized?

While you're struggling with that, here's some more that disagree with you:

StarrySkies.com: Origin of the Asteroids

It was once believed that the asteroids were the remains of a planet that was destroyed by a collision or some other tragedy. Astronomers now believe that these are the remnants of a planet that never was.

Answers.com: Asteroids

Current cosmogonical models for the origin of planets involve accretion from myriads of asteroidlike planetesimals. It is likely that asteroids are a remnant of the planetesimals that failed to accrete into a planet between Mars and Jupiter. Perhaps bombardment of the asteroid zone by large planetesimals scattered from massive, nearby Jupiter increased the relative velocities of asteroids to the present value of 3 mi/s (5 km/s) so that asteroids fragment rather than accrete when they meet each other. Instead of forming a planet, the asteroids have been smashing each other to bits.

Space.com: Asteroids Data Sheet

There are two hypotheses about how most of the asteroids formed. One says they broke off of a mother planet that existed between Mars and Jupiter. More likely, however, they represent what space was like before the planets formed, and they are the remnants of that process -- bits and pieces that never quite joined together.

Solstation.com: Main Asteroid Belt

Asteroids are primoridal objects left over from the formation of the Solar System. While some have suggested that they are the remains of a protoplanet that was destroyed in a massive collision long ago, the prevailing view is that asteroids are leftover rocky matter that never successfully coalesced into a planet.

AerospaceGuide.com: Asteroid Belt

Scientists believe the asteroids are the pieces of a planet that never formed. One possible theory is the ongoing gravitational tug-of-war between Jupiter and Mars has prevented the pieces from bonding together, hence, this planet was never created.

[Van Rijn]

Quote:
Originally Posted by SAMU
The most accepted idea these days is that a large body was struck hard, pulverized and the asteroids are the pieces.

By large body, do you mean a planet, or are you just saying that some of the asteroids are the result of collisions of various larger asteroids?

I remember that the destroyed planet idea was popular in many older science fiction stories from around mid twentieth century, but I don't know when it would have been generally accepted in the field. So, I looked it up in a couple of older introductory astronomy texts I happen to have. From "Pictorial Astronomy" printed in 1956 there is a short comment on page 139. It mentions two ideas, that they "were formed by cooling and condensation from incandescent matter ejected from the sun" (essentially an early form of the accretion model). But it also mentions "The suggestion has been made that the asteroids are fragments of a planet. It is not known how the planet was broken up, but it could have resulted from a close approach to a large planet like Jupiter."

So in 1956 it could go either way. What about later? From "Realm of the Universe," printed in 1980, on page 192, they say:

Most authorities regard it as probable that the minor planets were formed from the same material that formed the principal planets, and at about the same time. Most likely they formed from material that could not form a major planet, perhaps because too little material was there to begin with, or perhaps also because the disruptive tidal effects of Jupiter prevented such an object from forming.

They do suggest that some asteroids are the result of collisions between larger asteroids but there was no mention of the "destroyed planet" idea. I'm pretty sure that idea had faded out years before that, though.

[jlhredshift]

here

Obviously some low speed impacts and direct evidence of mass accumulation.

[SAMU]

"your fractured planet contention"
"you're struggling"

If you're going to continue in that malcious, nasty vien I'm dumping this discussion.

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