🚀 go-pugleaf

In article <57cncl$2re@nntp.seflin.lib.fl.us>,
Ralph Silverman <z007400b@bcfreenet.seflin.lib.fl.us> wrote:
>: >All indications
>: >> : are that they formed in weightless environments (except for martian and
>: >> : lunar meteorites of cources). 

>: >How do small rocks form without gravity?  

>: Easily. They hit each other and stick. There is no direct evidence of
>: how asteroids formed, but several moons of the outer planets and
>: Shoemaker/Levy-9 indicate that some moons and comets are loosely
>: bound collections of material. So hitting and sticking is quite
>: reasonable. Gravity only becomes relevant when an object has
>: grown to a size of ~100 km.

>	re..
>		"...Easily..."
>	(posting cited above) ...
>	how long would that take?

The theories say this would take something like 100,000 years
before the bodies got big enough for their gravity to really
make a difference. At that point, the biggest would be about the
size of asteroids. After that, they would continue to grow
at a slower rate (as mass collects in sizable objects, the
time between collisions decreases) until the biggest ones
reached the size of large asteroids or possible lunar size.
Then the growth rates slow even more dramatically (this is
the point were the objects have accumulated everything close
to their orbits, and can only grow by having their gravity
perturb more distant objects into crossing orbits.)

>	if the earth formed
> 	that way ...
>	why are these not planets too?

You mean a planet in the asteroid belt? Basically because of Jupiter.
If Jupiter weren't there, they would have. But Jupiter got big
(started acquiring its gas envelope and becoming really massive)
around the time the asteroids were, well, about the size of
asteroids. The influence of Jupiter was enough to perturb their
orbits so that they collided at much higher velocities than they
would have without Jupiter. The higher impact velocities meant
that the odds of hitting and staying together were lower, and
the odds of hitting and braking up were greater. That prevented
the asteroids from growing to larger sizes. The terrestrial
planets (and the material which formed them) were far enough from
Jupiter that its gravity didn't prevent further growth.

                                                     Frank Crary
                                                     CU Boulder