Hi,
Whoops! The ultimate in late-night dopiness, replying
to my own post.
Some amplifications and clarifications occur to me right away.
First, the object that crosses the zero potential sheet with very little
residual velocity heads straight into the Earth and burns up in a
near vertical descent: no meteorites will be left from that encounter.
Simulations show that only a small fraction of one percent of lunar
debris make it to the Earth, about half the number of Martian
meteorites. Since the number of Lunaites is greater than half the
number of Martians, I think that shows we have underestimated
the number of impacts that occur in near Earth space. For all
practical purposes, the Earth and the Moon are co-targets,
proportional to their gravitational and geometric "target size,"
so the Earth is being impacted more than we think also...
Which in turn brings me back to my "greater fall rate"
argument...
The lunar object that has entered an Earth co-orbit has orbital
velocities very similar to the Earth's, the differences being mostly
in the geometry of the orbit. When there is a close encounter
between the Earth and the lunar object, the small rock is merely
deflected into an orbit that grazes the upper atmosphere enough to
re-enter it.
An example would be a co-orbit with a period of 362 days instead
of the Earth's 365.25 days. The Earth and that object would pass by
each other every 104.35 years and maybe, just maybe, after 100
passes (10,000 years) of assorted closeness, they would get tangled
up and we'd get a new Lunaite. The object's velocity, relative to the
Earth's velocity, would be very small, a little faster or a little slower,
so it wouldn't be "falling from a great distance" as in the example of
direct transfer from the Moon to the Earth, but merely having its orbital
vectors adjusted into an atmospheric encounter... From there on in, it
takes the same chances as any meteoroid.
In such encounters, the object wouldn't be close to the Earth for more
than a few hours on each pass-by, so you'd be radar searching for
104 years in the hope of spotting something during a 100 hour passage,
not really practical, if you could do it at all, that is. Of course, the
Earth
is probably being passed by junk of all kinds all the time, slowly, in
co-orbits, without optical detection either, since the only way co-orbiters
could be detected would be viewing close to the horizon at dawn
and dusk -- the most impossible viewing angle imaginable from
the Earth's surface and the lousiest viewing conditions. And, when
co-orbiters are far enough away to be in a dark place in the sky,
they're too faint to be seen. Perfect.
The Earth has a blind spot...
Sterling K. Webb
---------------------------------------------------------------------
----- Original Message -----
From: "Sterling K. Webb" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>; "Meteorite List"
<meteorite-list@meteoritecentral.com>
Sent: Sunday, February 12, 2006 3:00 AM
Subject: Re: [meteorite-list] Orbital debris watching radar
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" <[EMAIL PROTECTED]>
To: "Meteorite List" <meteorite-list@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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