[meteorite-list] Lunar/Martian controversy

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 10:18:02 2004
Message-ID: <3FE4E79D.4062E6F5_at_bhil.com>

Hi,

    Just as it's hard to get to the Moon (from Earth, that is, I haven't made it
yet), it's even harder to get TO the Earth FROM the Moon.
    Yeah, I know, it flies in the face of the obvious (Earth LOOKS flat)! There's the
Moon just hanging around up there in the heavenly neighborhood, always right next
door. Surely if you dropped a rock off of it, the rock would go plonk! straight down
to my backyard, crushing the BBQ pit or splashing into the swimming pool! Wouldn't
it? Did you hear a noise? Go look...
    OK, you've thrown The Rock. You had to throw it as fast or faster than 2340 m/sec
or it would never escape the Moon. (Quite an arm you got there, kid!)
    That means The Rock will reach the "neutral point" where the pull of the Moon and
the tug of the Earth equal each other and cancel out. If you had really great control
of that arm and managed to toss The Rock at exactly the right speed, it would reach
the neutral point with ZERO velocity and it would STOP!
    Yes, it would just hang out there, going nowhere and enjoying a fine view of both
planetary bodies for the rest of Eternity (that's where I want to build my retirement
home) or until the first little perturbation came along and then it would then fall,
either back towards the Moon or on towards Earth.
    Since the "neutral point" is very close to the Moon, the fall towards the Earth
will convert all The Rock's potential energy of position into good old kinetic
energy. The Rock will achieve the full escape velocity of Earth in falling that it
would take to get the heck off the planet if you were going the other way: 11,200
m/sec.
    Since in "real life" The Rock will probably have cruised past the "neutral point"
with some residual velocity all its own, its terminal velocity will always BE GREATER
THAN the Earth's escape velocity!
    Hey wait! Slow down! You missed the turn! You drove right past it! It's back
there... That's life on the gravity freeway. So, unless The Rock is hideously
unlucky, it's going somewhere but that somewhere ain't the Earth!
    Not only that, The Rock will have some vector given to it by the circumstances of
its ejection and it WON'T be heading toward the one tiny spot of space where the
teensy-tiny Earth is going to be five days, two hours, 11 minutes and 43 seconds
later. (Kid, do you throw a curve ball?)
    Since the big fat Earth only takes up 1/32,400 of the Moon's sky, the odds of
hitting the Home Planet are small (I won't say "astronomically" small, I won't, I
won't...)
    And the "lucky" Rock, the one that heads straight and unerringly towards the
Earth dead center on target, right for your backyard? Well, since its entry angle is
90 degrees, straight screeeeeming vertically down through the atmosphere, it's going
to burn up a long, long time before it reaches your BBQ pit or your swimming pool.
    So, almost all the Rocks thrown from the Moon will go their own way in the solar
system, distaining a visit to the Big Brother planet, so close and yet so far away.
Maybe, in 100,000 years, we and our planet might acidentally bump into some Lunar
Escapees that have been wandering around in generally Earth-like solar orbits and we
can get re-acquainted, but the odds are against it.
    Bottom Line? The answer to "How do we get from the Moon to the Earth?" is same as
the answer in the very old and not very funny joke about asking directions:
    "You cain't get there from here."


Sterling K. Wdebb
---------------------------------------------------------------

j.divelbiss_at_att.net wrote:

> Q: Do we think that it is possible for the moon's impact ejecta into space to
> escape Earth's gravity? I would think not, or very-very little.
>
> Cautious this time,
>
> John
>
Received on Sat 20 Dec 2003 07:21:51 PM PST