Space rock on my mantlepiece

[M Holmes]

Arjan Hulsebos writes:

> M Holmes wrote:

> >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.

Since anything hit from the Earth/Moon orbit around the Sun into solar
orbit is imparted a one-off acceleration, it will take up an orbit that
intersects the Earth/Moon orbit twice a year, with appropriate
opportunities for being hit by either.

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

Only if it's slowed down from the Mars orbit so that the semiminor axis
of the new orbit is inside of Earth's orbit. remember that any debris
from Mars in solar orbit will cross the orbit of Mars twice per Martian year.

> 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).

Yup, I think those may be major factors in all this.

> </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.

The Earth was hit by a large planetoid early in its history. The Moon is
made out of stuff knocked from Earth into Earth orbit at that time. The
evidence is the similarity of Moon rock to what the Earth is composed
of.

Presumably this also means that there's a bunch of Earth meteorites on
other planets and moons.

FoFP