Plus or Minus

[Bert]

I have a question to throw out into the waters and I imagine that the
question itself may give the audience a sense of my ignorance of
science in general as well as the matters specific to the question.
I have read, in several places, that no one has come up with very
specific details of the events to unfold and I am at a loss as to
why.

The known’s of the impact are the mass and velocity of the missile
and, what I assume (this could be my mistake), we know about the
moon’s surface. With this information should we not be able to
calculate to any greater degree of certainty the magnitude of
brightness and the altitude above the mean surface the plume reaches?
I have seen between 1 and 26 miles in altitude and various, less
certain, ideas about magnitude of brightness.

I was lurking and collecting personal information when I decided that
my ignorance was in greater need than was my curiosity.

Thank You in advance,

Robert Sobotor

[Jim Mosher]

Bert,

The mass and velocity of the missile tell you with great certainty how
much energy will be dissipated in the impact. Likewise, the amount of
sunlight that would be reflected off a cloud containing a known number
of particles at each height is pretty well known, at least in
principle.

As I understand it, the big uncertainty is *how* the known amount of
energy will be dissipated, and hence what kind of cloud it will
produce. One can imagine anything from the missile bouncing off a
rubber-like surface as a single crumpled wreck that would be even
harder to see than the original missile -- to a vast shower of
extremely tiny particles, which would be readily visible. The height
the particles reach will be determined by the velocity with which they
are ejected from the surface, but (if I understand correctly) that,
too, could be high or low depending on how much total mass the impact
energy is transferred to.

Hope that helps.

-- Jim

[jim phillips]

No help at all if you ask me. So, you are saying we have no idea and the question is, as I read it, "Shouldn't we really have a good idea of what is going to happen since we have been to the lunar surface with manned and unmanned vehicles Multiple times and therefore know what the surface is like and we know what the mass and velocity of the space vehicle is?"

Jim Phillips

> Date: Thu, 8 Oct 2009 14:39:39 -0700
> Subject: [LCROSS_OBS: 1183] Re: Plus or Minus
> From: jimm...@gmail.com
> To: lcross_ob...@googlegroups.com

[Jim Mosher]

Hi Jim,

Thank you for your candid criticism of my answer.

I continue to think that uncertainties in how the lunar surface would
respond to the known energy of the impact were the weakest link in
making predictions of size and brightness of the plume. I agree that
manned and robotic visits to selected sites near the Moon's equator,
and experience with previous impacts, should have provided some
insight; but I am no expert on how these things work. I'm glad no one
asked me to estimate how much dust, and of what size, would be kicked
up by the impact of the same projectile at a random place on Earth.

Now that the experiment is concluded, the apparently low observed
reflectance of the plume will presumably allow scientists to place
some upper limit on the column density of particles that was ejected
in this particular case -- although a quantitative answer would
undoubtedly require information about the size of the particles, and I
don't know if there is enough data to determine that with any
certainty.

-- Jim M.

On Oct 8, 2:50 pm, jim phillips <thefamil...@hotmail.com> wrote:
> No help at all if you ask me. So, you are saying we have no idea and the question is, as I read it, "Shouldn't we really have a good idea of what is going to happen since we have been to the lunar surface with manned and unmanned vehicles Multiple times and therefore know what the surface is like and we know what the mass and velocity of the space vehicle is?"
>
> Jim Phillips
>
>
>
> > Date: Thu, 8 Oct 2009 14:39:39 -0700
> > Subject: [LCROSS_OBS: 1183] Re: Plus or Minus

> > From: jimmos...@gmail.com

> > > Robert Sobotor- Hide quoted text -
>
> - Show quoted text -

[Michelle Nichols]

Just to put some perspective on this, from my opinion -

The fact that this mission occurred at all tells us that we don't know enough about the Moon's surface and that missions like LCROSS and LRO help us to figure out quantifiable numbers.  Exploring a few sites on the lunar surface with manned and unmanned missions does not make anyone, even NASA, an expert on the entire lunar surface.  My favorite analogy is to think about trying to quantify the entire Earth's surface by landing 10 spacecraft near the equator and having them travel less than a mile (or not moving at all).  Or thinking about the wide variety of Martian surfaces that we have seen thanks to the rovers, landers, & orbiters. Another example is the MESSENGER images of Mercury - we really don't know everything about Mercury, and there are surprises even in the most recent flyby images.  Mercury is not the Moon, and we have to start thinking about it that way.  I think the LCROSS mission's optimistic ideas of the plume & its visibility relied on the fact that we *thought* we knew about the Moon's surface, and it turns out we might not have such a good idea about the conditions at the poles, after all.  That's interesting, in and of itself - at least to me.  

So, we didn't see it with our smaller telescopes.  That is important scientific information.  LRO did see the plume, and Keck & Hubble haven't released their information, yet, so that might turn out to be intriguing, as well.  If they saw it and all of the rest of the smaller telescopes on the Moon-facing side of the planet didn't see it (even scopes that we would call massive), that can help the scientists put quantifiable numbers on what the type of material was that LCROSS hit, along with how much of it went into space, at what angle, etc.

Space exploration is difficult, and we don't always get the numbers we think we will get.  Not seeing a plume does not make this mission a failure, and we should actually revel in the fact that LCROSS hit the object in the sky that is easiest for everyone to see - the Moon.  It is something we all can spot in the sky with no trouble, and getting all of us jazzed to do a little citizen science was actually quite cool.

Hope that helps assuage the initial disappointment....now the REAL fun begins - the scientists are poring over the data.  After a few years of planning, they finally have real numbers to play with.  I can't wait to see what they come up with.

Just my thoughts,

Michelle

Visit the Adler Planetarium Website

http://www.adlerplanetarium.org

[canopus56]

Ditto to Michelle's comments on the mystery of the plumeless crater

Over the next month, the LCROSS team will reconstruct the event in
more detail. The plume failure will turn out to be more of a p.r.
problem than a loss of science information problem. (The p.r. problem
is that the general public was expecting a plume and they are
stakeholders as the financiers of this and future NASA missions.)
Part of the LCROSS team event science reconstruction probably will
address the lack of observable plume.

So, how do you dig a 1 meter deep by 20 meter diameter crater (about
the size of a house foundation), excavate between 200 and 350 metric
tons of dirt and not have a plume? What we do know is that the impact
crater appears to be right-predicted size but no plume was observed. A
top 10 list of possible causes for a "good" crater but no plume are:

1) The plume was there at expected density and was just to faint,
because:

a) The sunlight-topography model was wrong and the curtain never
reached the sunlight.
b) The impacter hit a slope and the ejecta angle sprayed most of the
mass on the back of M1.
c) The impacter hit a boulder in a boulder field and broke up just
before hitting the surface.
d) The topography model was wrong and the plume was blocked from
Earth view.
e) The unknown site specific surface material was compressed rock,
not regolith (the rock ledge theory).
g) (My speculative contribution) They hit a thinly buried ice shelf
and most of the ejecta was in the form of gas (the ice ledge theory).

2) The plume was not there or was only a faint, less-dense-than
predicted curtain, because:

a) The plume model was wrong - the plume was much less dense than
modeled. Basic physics principles for optics controlled the plume's
visiblity. If it was less bright than the "black" shadow region, even
Palomar's 200 inch cannot take an image of the plume that is fainter
than the shadowed portion of the crater against which was the
background.
b) The plume model was right but the surface materials were not
what was anticipated. Same as 1(b)(c) and (d).

The science review with the additional LRO imagery probably will
address and answer these questions.

Clear skies - Kurt

On Oct 10, 7:23 am, "Michelle Nichols" <mnich...@adlerplanetarium.org>
wrote:

[Jim Mosher]

Kurt -

Regarding Item 2a in your list, simply being faint should not have
made the plume undetectable.

The only way I can imagine the plume being invisible would be if the
amount of light added by reflections off the newly added particles was
precisely balanced by the amount of pre-impact lunar background they
blocked. That could conceivably happen against a bright lunar
background -- where, like a dust-devil, the plume could appear either
bright or dark -- but in the shadowed areas of the Moon the pre-impact
level is most likely set by scatter in the instrument and in the
Earth's atmosphere. There is essentially no lunar signal to block,
and since the pre-impact sources of scattered light will continue to
be present, any reflection of sunlight off newly added particles will
add a lunar signal to this. Detection is "simply" a matter of the
exposure being long enough to record this slightly changed level with
adequate fidelity for successful subtraction of the pre-impact level.

I would assume there are addition photos taken by the Palomar
telescope, but I am concerned that the ones posted on the internet:

http://www.astro.caltech.edu/palomar/lcross.html

were taken so soon after impact (10 and 15 seconds) that the maximum
number of ejecta particles may not have had time to reach sunlight.
Subtracting the 10 second photo from the 15 second photo and enhancing
the difference brings up no obvious plume-related signal, but I'm not
convinced they are deeply enough exposed to record the very faint
signal that might be present at 2.1 microns. Especially in the 8-bit
(256-level) versions available on the internet, a plume signal that
adds a signal equivalent to, say, one-half percent of the sunlit lunar
surface brightness, or less, would likely go undetected. But lack of
detection doesn't mean a signal wasn't present or that it couldn't
have been detected in a longer exposure, or by an instrument optimized
for low light-level photography.

-- Jim

> > Visit the Adler Planetarium Websitehttp://www.adlerplanetarium.org- Hide quoted text -

[canopus56]

Jim, Good points all. There is a Palomar mpg compressed movie
covering the time of impact. http://www.astro.caltech.edu/palomar/animations/lcross_palomar.mpg
A. Bouchez, who took the Palomar images can be found at
http://www.astro.caltech.edu/~abouchez/ . - Kurt

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>
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