Friday, November 6, 2009

Cold and Dry

Moon Above Desert
This seems to be the current forecast for the Moon's polar craters, as it presumably has been for the last few billion years, and will continue to be for the next few billion.

Not much (only one heavily processed image) has come out officially from the L-CROSS team since their mission scored a lunar bull's eye, minus the photogenic plume that was supposed to be the media highlight of the show. But in the grand old aerospace industry tradition of using Aviation Leak to get the story out, the L-CROSS team dropped some hints in the online Sky & Telescope about what is going on behind the scenes.

The impact did produce a plume, but it was about 10 times less massive than expected. Why it was so sparse is not yet known and may never be fully known, but new hypervelocity impact modeling suggests that debris may have gone more 'out' than 'up.'

There's also mention of the Centaur booster stage 'collapsed into itself when it hit.' I am not quite sure what to make of that last part. You'd expect any tank structure to 'collapse into itself' when it slams into the Moon at 1.5 km/s. But fans of kinetic weapons, including me, take note: There is a lot that we do not know about uber-fast impacts.

As for what was in the faint plume, Sky & Telescope gives contradictory hints, noting that the IR signature of water vapor is conspicuous (and implicitly absent), but also broadly hinting that when an L-CROSS public announcement comes, in a couple of weeks, the team may reveal that it did detect water. But not, I suspect, very much of it. What they did detect, rather oddly, is mercury.

The article also has one other interesting tidbit, though not from L-CROSS. Apparently an instrument aboard the Lunar Reconnaissance Orbiter determined that the surface temperature on the floor of those permanently shadowed are only about 35 K - much lower than anticipated, and making those spots the coldest known place in the entire Solar System.

And right in our local 'hood. How cool is that?

11 comments:

Anonymous said...

Well, maybe the polar craters will one day be home to mineing colonies, but just not mineing for ice...

Ferrell

Citizen Joe said...

As the tank was collapsing, if there was any gas left inside, it may have super heated from the compression and then blown out sideways, while the tail end of the tank kept the plume from going straight up.

Of course, with mercury there, you can always measure the temperature.

Jim Baerg said...

"Of course, with mercury there, you can always measure the temperature."

Only if the temperature is above about -40 °C. Below that the mercury freezes.

Anonymous said...

Jim, in Saskatchewan we called that a normal winter's day.

Cold, dry, and lousy scenery. More and more the Moon resembles my childhood.

Ian_M

Jean Remy said...

"Of course, with mercury there, you can always measure the temperature."

Only if the temperature is above about -40 °C. Below that the mercury freezes.

I was going to say the same. Rather amusingly, I first learned of that little tidbit reading Jules Verne ( along long long time ago). The guy really knew his stuff, and he was writing Rocketpunk before rockets! Pretty accurate too. Well aside from the fact that his water buffer was too small. Von Braun calculated it should've been a couple of kilometers thick to dampen the brutal acceleration of Columbiad.

Tangent aside, on the collapsing tank:

Maybe they were expecting it to flower outward rather than implode? Those tanks are generally pretty flimsy and only keep their shape for the same reason a roman arch does (very loosely) meaning that as long as they are structurally intact pressures are spread and equalized along the surfaces, and the tank keeps its shape. As soon as that integrity is breached, it will fail in the direction of the highest pressure (e.g: high pressure inside will make it expand outward.) Does that mean then the pressure outside the tank when it first started to fail was higher than expected?

Carla said...

There were a few hints on the BBC's astronomy programme last week that the L-CROSS team would have something interesting to announce quite soon.

The very low temperature is interesting, if only because I had a vague idea that things like the amount of solar energy would be fairly well known and well modelled. Presumably not!

Rick said...

Maybe Canada has a big future on the Moon!

Regarding the tank impact, at 1.5 km/s, I would expect structural cohesion to be nearly insignificant, the tank effectively being 'pieces of sheet metal in formation.'

But perhaps not. The speed of sound in metal, IIRC, is on the same order, so the rear portion might have been experiencing loads from the impact before it hit.

The L-CROSS team is obviously dropping hints! But I was also surprised by the ultra cold temperatures in the shadowed regions. I thought they could predict that sort of thing pretty reliably. Evidently it is trickier than that.

Anonymous said...

As far as I understand it, based on my extensive studies of EAD metadata systems, heat dispersal in an airless environment would depend on energy propogating through surrounding materials. Areas in perpetual shadow would only receive whatever heat 'leaks' to them through the surrounding dust and rock, or is radiated away by those materials. Obviously we don't know as much about Lunar dust as we thought we did, or we wouldn't have crashed the probe in the first place. But the cold spots could also have been sheilded from heat propogation by some sort of rocky outcrop.

The hints out of the L-CROSS group are frustrating. I want my internet instant infotainment now!

Ian_M

Rick said...

Presumably heat leaks into these crater floors from the lunar interior at a really slow rate, for the surface temperature to fall to 20 K.

The hints out of the L-CROSS group are frustrating. I want my internet instant infotainment now!

LOL.

But perhaps spacefaring will teach our culture some patience. Much as I am an advocate of fast orbits for human travel, fast is relative, and short of magitech, space travel involves longer travel times than we've seen since the age of sail.

In fact, our current robotic deep space missions have astonishing durations, and while the 'crews' all remain on Earth, they can end up pretty much spending their careers on a mission.

Cassini was launched in 1997, but it started taking definite form about 1994, so there must be people who have been part of it for 15 plus years.

Anonymous said...

Rick-"In fact, our current robotic deep space missions have astonishing durations, and while the 'crews' all remain on Earth, they can end up pretty much spending their careers on a mission."

There were members of the Pioneer ground control team that started as graduate student interns at the start of the program and have recently retired.

I think that the possibility of future astronauts only making one or two interplanetary trips during a career (or even one-way, staying as permanent settlers), sounds more and more likely.

If the tank struck even slightly off center, then the mechanical forces could have caused it to 'explode' asymectrically, instead of evenly, and spreading the energy over a wide area instead of focusing it on the impact site.

Ferrell

Anonymous said...

Not sure if everyone got the news or not, but here we go:

http://www.nasa.gov/home/hqnews/2009/nov/HQ_09-265_LCROSS_Confirms_Water.html

- Sabersonic
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