What went wrong with NASA's moon crash?

The idea was to crash a spacecraft into the Moon at a chosen spot where ice might be lurking. Hitting the Moon at twice the speed of a bullet would send a plume of debris miles high that could then be analysed spectroscopically for water.

But there appears to have been no plume - only a brief white flash. Why not?  Here's this blogger's explanation for what it's worth, published first on New Scientist and now the Times.

From the latter:

"Maybe a plume was formed but settled too quickly to be seen by the following probe, 4 minutes behind. Don't forget that although gravity is lower on the Moon, making things slower to fall, there's no air to slow the descent - even of dust particles or those hoped-for ice crystals.

There's a classic lab experiment in which a stout glass tube has all the air pumped out. A feather inside then falls as quickly as a ball-bearing.

Let's hope the NASA scientists did not forget their school physics."

Update: Sunday 18 October   Well, halleluja, a plume was captured on camera after all, although the photograph in New Scientist ("Elusive lunar plume caught on camera after all")  was somewhat disappointing. I mean to say - given it was said to be 6-8 kilometres wide, why show us a mere smidgeon of white on a so-called "zoomed image"? Small wonder the conspiracy theorists began falling out of the woodwork!

The fact that a plume was briefly formed lends support to the hypothesis I've ventured above. The plume didn't last long, because the particles fell back faster than many might suppose (given that the Moon's lower gravity seems to dominate most thinking, with the absence of an atmosphere frequently overlooked).

The absence of an atmosphere alters the dynamics of an impact and its aftermath considerably. Dust and other particles (ice?) may well be ejected much higher and further than on Earth, due to absence of a cushioning atmosphere - no air molecules to be pushed aside- but their return to "earth" would look entirely different to an observer or camera. Initially the descent would seem gentle - due to 1/6th Earth's gravity- but acceleration would be continuous, without anything comparable to "terminal velocity" (approx 120 mph on Earth in "old  money"). A quick back- of- envelope calculation using school physics says that while it would take some 30 seconds for dust or ice particles to reach 120mph (which is our earthly terminal velocity), they would go on accelerating indefinitely until they finally hit the surface. There's another factor to consider - which another scribe on NS comments has pointed out :  much of the ice -if present- would be vaporised by the kinetic energy (heat) of impact. The  molecules would be unlikely to re-coalesce in the Moon's near-vacuum, and simply disappear from view - except perhaps to a highly sensitive spectrometer that was set up specifically to look for them - and certainly fail to drop back. Molecules generally don't "drop back" in a vacuum, especially in a weak gravitational field. They would simply spread out, ie diffuse rapidly.

No genuinely free water on the Moon, it would seem...

The ambiguity of "free" in my title is deliberate. The traces of water detected by that inspired Indian lunar probe are not of course  free water, in a chemical sense, but chemically-bound, non-wettening "water".

 Is that a water-diviner in his hand? If so, then happy-hunting..

Even my non-scientific wife was quick to realize that, watching last night's news, even if the media reports elsewhere conjure up visions of future moon colonists tapping into an abundant supply.  Moonshine!

Being chemically-bound, whether weakly or strongly,  it's hardly free for the taking either. The expression "getting blood out of a stone" springs to mind.

Here's what I sent last night to the Times in response to that  trumpeting  headline re there being a litre of water per tonne of lunar soil. (One feels that "water" should have been enclosed in quotation marks in the Times's article).

To say there is "water" on the Moon when it's there as the hydroxyl radical/ion is a little misleading, as others have pointed out. It's like saying that farmer's lime, calcium hydroxide, is a potential source of water. True, in a manner of speaking, but you would have to get it red hot to drive off water as steam, as in lime-furnaces, and then have a means of cooling and condensing to get liquid water. It may sound easy in principle, but the practical aspects of attempting this on the Moon are formidable. One would need an abundant supply of energy - probably tens or hundreds of square metres of solar panels to generate megawatts of electricity.

Why not get the Sahara green first - to create biomass and reduce CO2? Huge problems to overcome, yes, but if we can't lick that problem, what hope is there of colonising the Moon or Mars on a realistic timescale?

September 24, 2009 11:10 PM BST on community.timesonline.co.uk

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"Hydroxyl", note, is not a molecule, as the report would have one believe. Being simply OH, with an unpaired electron, it would be a free radical, with no independent existence, and a lifetime of seconds at most if generated in the atmosphere by, say, cosmic ray bombardment. Most "OH" in rocks is, of course, present as negatively-charged hydroxyl ions, accompanying positively-charged metal ions, eg aluminium.

Aside: well done, btw,  Times for finally adding a time and date stamp to readers' comments,  and for a much-speedier moderation than before. The BBC-style facility for expressing approval is also cute.  It even survives cut-and-pasting here, apparently as a live-link - see the blue font above!  Shame though that one's comments are not accessible to search engines. The world wide web depends on linking, you know, if only for information-retrieval. Vanity has nowt to do with it, of course...

Update: Friday 15:15 pm.  Had some further  thoughts on the inconclusive nature of the chemistry after reading the New Scientist's feature:  Have just sent the following:


"Hmmm. So the signal could have come from water or from the "OH molecule". Leaving aside the faulty nomenclature - OH is not a molecule, but is either a free radical if electrically neutral, or a negatively- charged ion -  it seems a bit of a liberty to lump together two entirely different chemical species in this manner. The discovery of  H2O would indeed  be exciting, even if strongly adsorbed to minerals. But "hydroxyl", presumably as mineral hydroxides, would be an entirely different matter, requiring somewhat high temperatures in most cases if  desiring to drive off molecular water, which would then have to be cooled and condensed. Most of the hydroxides of predominant minerals in the Earth's crust - magnesium, calcium, aluminium etc-  hang onto their oxygen and hydrogen quite firmly, needing red heat or higher to dissociate into oxides and steam.

So it's somewhat premature surely to report that "water" has been discovered. On the basis of available evidence, none of which can be described as "hard",  what's been discovered are  oxygen atoms that are bonded to one or possibly two hydrogen atoms with a strong attachment to a mineral matrix given they are able to survive solar heating in a vacuum. Alternatively, and less usefully from the point of view of harvesting lunar water, the signal is picking up a temporary association, ie  the turnover model, which would explain why the discovery did not come earlier from study of Apollo-mission rocks"

Update  Sep 25 21:17 : See also "How could astronauts harvest water on the Moon" - latest article in New Scientist.