🚀 go-pugleaf

Frank Crary (fcrary@rintintin.Colorado.EDU) wrote:
: 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

--
****************begin r.s. response**************

	re.
		"... because of Jupiter ..."
		(posting cited above ...)

	how then,
	were the moons of jupiter formed?
	(and the moons of mars)
	(and the moons of saturn)
	?

	also ...
	why no asteroid belt past jupiter?

****************end r.s. response****************
Ralph Silverman
z007400b@bcfreenet.seflin.lib.fl.us