Space rock on my mantlepiece

[Arjan Hulsebos]

M Holmes wrote:

>Hawkfan writes:
>
>
>
>>Hmmm, according to
>>http://internt.nhm.ac.uk/cgi-bin/earth/metcat/metsPerGroup.dsml Lunar
>>meteorites are rarer than Martian meteorites, but not by that much ...
>>
>>
>
>I'm puzzled by that. Surely anything knocked off the Moon by meteoric
>impact would already be in an Earth-crossing orbit. Seems therefore much
>more likely that these would hit Earth than the same situation on Mars.
>
>
<action "Arjan dusting off his physics Ph.D">

Well, maybe not. Because the moon is so close to the earth, anything
that is chipped off the moon will have to either be close to the lunar
orbital around the earth, or to the earth's orbital around the sun in
order to stand a chance to hit the earth.

Anything that's chipped off Mars can fly off in just about any direction
and still have a chance of hitting the earth.

Then again, the moon is a whole lot smaller than Mars, so it'll get hit
less often, but due to the moon's gravity being about half of Mars',
it's easier to escape from the moon than it is to escape from Mars (not
even considering Mars' atmosphere).

</action>

If I remember correctly, the earth and the moon were formed roughly
around the same time. Furthermore, the moon's way too big to be a true
moon. I think the consensus nowadays is that the earth and the moon form
a bi-planery system. But I'm not an astronomer, so I may be wrong on this.

Gr,

Arjan H