[meteorite-list] Orbital debris watching radar

[Sterling K. Webb]

Hi, Darren,

    I gather from the phrase about having their orbits decay,
that by "Earth orbit," you mean "in orbit about the Earth."
Orbits around the Earth only "decay" because the orbit
touches the uppermost atmosphere enough to cause drag
which, however minute, reduces orbital velocity. It may seem
logical that materials kicked off the Moon would easily and
immediately end up in an orbit around the Earth, or at least
some of them would.

    But the truth is that it is nearly impossible to get from the
Moon to the Earth, and that lunar meteorites almost certainly
do not arrive at the Earth that way, however illogical that sounds.

    The many simulations of transfer of materials around the
solar system show the same result: impact materials from the
Moon mostly go into eccentric solar orbits. a small percentage
go into "co-orbits," that is, they enter solar orbits very similar
to the Earth's orbit, sort of wandering along with us, and it is
from that population that some get tangled up with the Earth's
gravity and get pulled in. "Short" transit times are 10,000 years.
When a lunar shows no cosmic ray exposure, that only means
that it was less than 25,000 years.

    The reason why it's so hard to get from the Moon to the Earth
is this:  any object that falls to the Earth from a "great distance"
achieves escape velocity by the time it gets very near to the
Earth.  And escape velocity is just that: you escape. No orbiting
for you...

   There is a point, between the Earth and the Moon where the
gravitational pull of the Earth and the Moon balance each other.
Since the Earth is heavier than the Moon that point is closer
to the Moon than the Earth. The point that lies in a straight line
between the Moon and the Earth is the first LaGrange Point,
BTW.

   But there are a multitude of points in every direction where
equal force vectors from the Moon and the Earth meet: a sheet
of zero gravitational potential.

    If an object is ejected from the Moon's surface toward the
Earth without enough velocity to reach the zero sheet, it falls
back toward the Moon.

    If it arrives at the zero sheet with just a smidge of velocity
more than zero, it will fall toward the Earth, ramping up to
escape velocity or near escape velocity at its closest approach
then roar on out into the solar system.

    If it arrives at the zero sheet with a good deal of velocity
more, it will fall on an Earth-influenced path and probably
ramp up to a lot more than escape velocity...

   So, you see, stranger, thar ain't no way to get thar from here...


Sterling K. Webb
----------------------------------------------------------------------
----- Original Message ----- 
From: "Darren Garrison" <cynapse at charter.net>
To: "Meteorite List" <meteorite-list at meteoritecentral.com>
Sent: Sunday, February 12, 2006 1:27 AM
Subject: [meteorite-list] Orbital debris watching radar


On a less argumentative subject, there is an idea I've been wondering about 
for
a while.  Thinking back to my wondering about what lunar meteorites do 
between
leaving the surface of the moon and reaching the surface of the Earth, there 
is
the idea that some of them enter Earth orbit and then have their orbits 
decay
until they fall.  Given the really fresh lunars found lately, that would 
seem to
imply that there could be more of them in orbit now.

So, not really a coherent question but more of a musing-- just how small an
object at what distance can the radars that constantly track orbital space
program junk around the Earth reliably track?  And would there be any way to
determine if a piece of orbiting debris was junk or an incoming lunar?
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