Re: Stray asteroids from other systems or galaxies out there?

"Spokes" <spokesman123@xxxxxxxxxxx> wrote in message
news:1159005519.743820.250130@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I'm new to this NG. One thing that I've often wondered about is the
possibility of a stray asteroid entering our system or even hitting our
planet.
Do asteroids from other systems or galaxies come our way? Is is
possible or common? Any examples?


It would probably be difficult to tell the difference
between an object perturbed in from the Oort cloud and
one that has an extra-solarsystem origin. But there's
no reason why bits from other solar systems shouldn't
manage to scatter here and there.

Getting hit by something from another galaxy is certainly
possible if unlikely. The gravitational well that an
asteroid would have to climb out of to escape its parent
galaxy would be formidable. An estimate would go as follows.

An asteroid at the periphery of a galaxy of, say,
200 billion sun masses and 100,000ly radius would have
an escape velocity of approximately

Vesc = sqrt(2GM/r)

(I say approximately because the mass distribution of
the galaxy may not be spherical)

Putting some numbers to this,

G = 6.67 x 10^-11 m^3 kg^-1 sec^-2
M = 200x10^9 x 2x10^30 kg = 4x10^41 kg
r = 100000ly = 9.46x10^17 km

Vesc = ~ 8.6 x 10^5 km/hr

Not an impossible speed for a massive body to attain
via near-miss stellar interactions. Of course, having
managed to escape the parent galaxy the object would
then have to find ours. This may not be too great a
problem for galaxies within our local group, as they
are gravitationally bound anyways. There's a *lot*
of empty space out there between galaxies, though.


.

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