It's even worse then that; the heavy elements accumulated in the lunar core,
but there weren't many heavy elements there in the first place. Apparently
the Moon originated when something hit the Earth and knocked a big chunk of
it into orbit. Since the Earth had already separated into core and mantle
at that time, the stuff that got knocked off was all mantle, and consisted
mostly of the boring elements O, Si, Mg and Al.
>We now know more than 2 k objects in orbits
>between Mars and Jupiter.
Lots more than that, actually. To be officially 'numbered', a minor planet
has to have an orbit that is known with sufficient accuracy that we can find
it again when we look any time in the next few decades. There were 6300
numbered asteroids two years ago; probably by now there are more than 7000
(I can't find recent data). There are also more than 112,000 observations
of minor planets with too little accuracy to be numbered; typically any
given minor planet accumulates several such preliminary observations before
its orbit is pinned down and it gets a number.
>More than 30 objects have orbits
>with the perihelion within Mars orbit. About 20 objects
>have their perihelions within the Earth orbit, this are
>the Apollo asteroids.
I just counted 27 inside the orbit of earth *now*; those with perihelia
inside earth orbit must be substantially more numerous. Look at the amazing
and wonderful charts
http://cfa-www.harvard.edu/cfa/ps/lists/InnerPlot.html
and
http://cfa-www.harvard.edu/cfa/ps/lists/OuterPlot.html
to see what's up in our neighborhood.
>Asteroids are commonly classified
>in Hirayama families by their orbit data; about 40 such
>families are known.
90% of asteroids do not belong to any known Hiriyama family. The families
originate when a body is smashed into smaller pieces by an impact. The
pieces have similar compositions and pursue similar orbits for a few million
years. Eventually the orbits drift apart from each other and the family
resemblance is lost.
>Additionally, there are the Trojans,
>which move in 60 deg distance onto Jupiter orbit, as seen
>from the Sun (as predicted by Lagrange).
Probably there are as many bodies in the Trojan clouds as in the main belt,
but since they are further away, less brightly lit, and covered with
(mysterious) coal-black stuff, not as many of them have been observed.
>These are but the largest primeval debris pieces, smaller ones
>being inaccessible to astronomic observations.
More precisely, nobody wants to pay for the astronomic observations. An
astronomer with a professional-grade telescope (cost ~$300,000) can find new
asteroids at a rate of one or two a night. Finding a new asteroid costs
about $1000, and there are thousands available at this cost. Once these are
found, there are hundreds of thousands available at $5000 each. Automation
could bring these prices down substantially.
I'm just making up these costs, but I think they're right to a factor of
two. Back when I was in grad school, I spent a lot of time studying
asteroids and hanging out with people who looked for them. It was cool
stuff, but it didn't pay the bills, so I went back to doing computer stuff.
--CarlF