[meteorite-list] Witnessed fall lunar's?

From: cdtucson at cox.net <*cdtucson_at_meteoritecentral.com*> Date: Thu, 9 Sep 2010 2:51:16 -0400 Message-ID: <20100909025116.83F4S.803885.imail_at_fed1rmwml46> · cdtucson_at_meteoritecentral.com

--
Carl or Debbie Esparza
Meteoritemax
---- "Sterling K. Webb" <sterling_k_webb at sbcglobal.net> wrote: 
> Hi, Lunar Gang, and List,
> 
> We have a situation here that needs straightening
> out.
> 
> Escaping from the Moon is one thing. Getting
> to the Earth is another. Here's how it starts.
> 
> An object is propelled off the lunar surface
> (doesn't matter how). As soon as it's no longer
> in contact with the force that impelled it, its
> speed can't increase.
> 
> It can decrease, though, and it does. Lunar
> gravity will pull down on it, reducing its speed
> at the same rate it would gain if it fell. It goes
> slower and slower. Eventually, its speed will fall
> to zero and it will reverse course and start to
> fall back.
> 
> UNLESS its starting velocity is above or at the
> Moon's escape velocity. It takes 2380 meters/sec
> to escape to the point 38,000 miles from the Moon's
> center to where the gravitation pull of the Earth
> and the Moon are equal. If the rock started with
> 2381 m/sec, it will get there moving at 1 m/sec,
> a crawl. After that, the important thing is: which
> way was it headed?
> 
> Surrounding the Moon is a distorted spherical
> (parabolic) envelope with its "pocket" pointing
> directly at Earth that outlines that balancing
> point between the Earth's and the Moon's "pull."
> It's called the Hill Sphere (for any body). The Hill
> Sphere, or equipotential point for the Moon, is
> at a radius of about 38,000 miles, still over 200,000
> miles from earth.
> 
> If a Lunar escapee has enough speed to reach the
> Moon's Hill Sphere and cross over, it will be under
> the control of the Earth's gravitational field. The
> Moon has only 1/81.3 of the mass of the Earth, so
> the balance point between them is much closer
> to the Moon than the Earth.
> 
> Oh, if it was going very fast, it could escape the
> Earth too, but the odds against that are great. No,
> that rock is dam lucky to have made it to the
> Translunar Gravitational Equipotential Point for
> its flight.
> 
> In general, since Lunar escape velocity is low
> compared to the Earth's, if a rock just barely escapes,
> by the time it crosses the Border, it would be moving
> very slowly, almost standing still. From the viewpoint
> of the Earth, it's like someone carried a rock 'way out
> there and while "standing still" far from Earth, dropped it.
> 
> Like so many borders, once you cross it, you're in
> another jurisdiction. The Moon no longer has any
> say in what happens to the rock that crosses the
> Hill Sphere Border.
> 
> Slowly at first, it begins to fall toward Earth, but it moves
> faster and faster, eventually acquiring (up to) 11,233
> meters/sec, plus any starting speed, blah, blah...
> Will it curve and swerve and head straight for the
> Earth's central spot?
> 
> No, not often. There are a variety of outcomes and
> few of them will get a rock to land on Earth. Many will
> end up co-orbiting the Sun along with the Earth and
> will eventually tangle with the Big Mother Planet again.
> 
> Some, that are headed more or less toward the Earth
> to begin with will scream past in an asymptotic pass,
> whipping around the Earth, changing direction and
> picking up speed, in a home grown version of the
> "gravity well" maneuver. They will tossed far and gone,
> in a gentler version of what Jupiter does to anything
> gets near it.
> 
> But only if they miss...
> 
> Some of those headed our way, a small percentage,
> will actually "strike" the Earth, or come in at a steep
> angle. They might survive to the ground... or they
> might not.
> 
> A few, we lucky few, will graze the top of the Earth's
> atmosphere tangentially, in a flat trajectory roughly
> parallel to the surface of the planet, at about zero
> degrees of altitude (relative to us). They will be moving
> between 11,186 meters/sec and 13,466 meters/sec
> and their chances of landing are As Good As It Gets.
> 
> That's the simple view from Physics 101. It turns out
> to be more complicated, however.
> 
> NOW, we have to turn the question around and look
> at it from the Moon's and the Rock's perspective. If you're
> a rock looking to get the Earth, what's the best way to
> leave home? That will determine what happens to you
> in the long run.
> 
> So, imagine you're an indecisive rock staring at the
> black Lunar sky... If you aim for where the Earth is
> NOW, it won't be there when you arrive. so which way
> do I go?! There are no signposts and no obvious solution...
> 
> Now, it's time to introduce you to Barbara E. Shute. Her
> work can be found at the NASA Technical Reports Server:
> http://ntrs.nasa.gov/search.jsp?No=10&Ne=35&N=4294963886&Ns=ArchiveName|0&as=false
> 
> I suggest "Dynamical behavior of ejecta from the moon.
> Part I - Initial conditions," a PDF of which can be found at:
> http://hdl.handle.net/2060/19660021054
> 
> It's just what that rock is looking for --- a road map to
> Earth! However, this is pretty heavy lifting if your orbital
> mechanics are rusty, like mine, although no doubt Rob
> Matson will eat it up and ask for please, another bowl, sir?
> 
> First, the Moon, OUR Moon, is odd. It's a long way from
> the Earth and its orbital velocity (1022 m/sec) is much
> slower than its escape velocity (2380 m/sec), so when
> a rock does escape the Moon's gravity, it's in for a wild
> ride, as it's often going too fast or too slow for where it is.
> 
> First, to actually escape the Moon, the rock's speed has to
> be greater than mere escape velocity. Escape velocity will
> only get you to the Hill Sphere Border. It seems that velocities
> of 2600 to 2700 meters/sec are needed to actually escape the
> gravitational environment beyond the Moon's Hill Sphere..
> 
> Second, given that you're going fast enough, the one
> critical factor is the angle at which you leave the Moon's
> surface. There is one critical angle for each spot on the
> Moon's surface that guarantees you'll get to Earth if
> your speed is right. That ideal angle for the minimum
> possible velocity varies depending on where on the
> Moon you are, but other angles will do the job if you
> are going faster.
> 
> An intriguing conclusion that it is just as easy to get
> to the Earth from the "back" side as it is from the "front"
> (or facing) side. That means that all our breathless
> speculation about whether a Lunar meteorite COULD
> have come from the Backside is wasted. It makes
> NO DIFFERENCE. Each side is an equally likely
> source.
> 
> However, the Eastern Hemisphere is heavily favored, and
> it seems likely that everything that makes it to the Earth
> came from the Moon's "East Coast." When the rock leaves
> the East Hemisphere, its velocity is added to the Moon's
> orbital velocity. If it's pointed right, it's on a "fast return
> trajectory" toward the Earth.
> 
> But if it pops out of the Moon's gravitational control from
> the West Hemisphere, it's suddenly running too slowly
> in a retrograde orbit that can't be sustained. It makes a
> sharp right turn and crashes back into the Moon's surface
> and makes a new (smaller) crater!
> 
> If Shute's math is too much for you (show of hands?), skip
> to the charts and diagrams at the end. They make things
> much clearer. Shute did numerical integrations to sample
> impacts, ejecta-producing events, and concludes that as
> much as 3.3% of the ejecta could get to Earth.
> 
> Surviving the landing is another matter. (Isn't it always?)
> After reading this, it's my impression that the Moon likely
> produces much more material that arrives at the Earth
> than we usually think it does, and that the short supply
> of Lunaites is a "collection selection" effect, as has been
> suggested.
> 
> Another impression is that it may only be the more
> powerful impacts that produce Lunaites. In that case,
> deliveries to the Earth may only occur at intervals and
> there may be a multitude of Lunaites delivered from
> each impact (although they may be scattered), in contrast
> to the steady rain of meteoroids from far beyond the Earth.
> 
> I'm too Googled out to check, but is there "clustering"
> of the terrestrial ages of Lunaites at irregular intervals?
> 
> 
> Sterling K. Webb
> ---------------------------------------------------------------------------------
> ----- Original Message ----- 
> From: "Randy Korotev" <korotev at wustl.edu>
> To: <meteorite-list at meteoritecentral.com>
> Sent: Tuesday, September 07, 2010 4:06 PM
> Subject: Re: [meteorite-list] Witnessed fall lunars?
> 
> 
> >
> >>MikeG asks:
> >
> >>"Is there a theory for why there have been no witnessed falls of lunar
> >>meteorites?  It seems odd to me that we have 4 Martian witnessed falls
> >>(Shergotty, Chassigny, Zagami, Nakhla, and almost Lafayette) and no
> >>lunars."
> >
> > One issue is that these 5 meteorites are 5 kg, 4 kg, 18 kg, 10 kg, and 
> > 0.8 kg in mass.  Only 3 lunars are >4 kg in mass.
> >
> > Another issue (probably more important) is that lunar escape velocity 
> > is only 2.4 km/s and very little material ejected from the Moon is 
> > going much faster than that.  This velocity compares with 20-40 km/s 
> > for asteroidal meteorites.  Is a rock entering the atmosphere at 2.4 
> > km/s going to noticeably incandesce?  I don't know.  I believe that 
> > the space shuttle hits the atmosphere at ~7.7 km/s.
> >
> > Melanie asks:
> >
> > "I asked this a while ago on Greg Catterton's forum, and I was told 
> > that rocks
> > from the moon aren't as solid (tough) as Mars rocks, and therefore are 
> > less
> > likely to survive entry... yet what about all these Howardites?"
> >
> > Although breccias, most of the lunar meteorites are very tough rocks. 
> > Any rock that survives being blasted off the Moon isn't going to 
> > disintegrate in Earth's atmosphere any more than an asteroidal or 
> > martian meteorite.
> >
> > Steve says:
> > "The moon is close to the earth and material knocked off the moon has 
> > a relatively short time to reach the earth."
> >
> > Compared to what?  Some lunar meteorites took a million years or more 
> > to reach Earth.
> >
> > "Mars is farther away and not protected by a companion and its closer 
> > to the asteroid belt so it receives many more impacts than the moon."
> >
> > Not "many more."  Only a factor of two greater for Mars, but the 
> > average velocity of the impactors is only 60% as great.
> >
> >
> >
> > Randy Korotev
> > Washington University in St. Louis
> >
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