[Dayseed]

Constantinople Constantinople:

Long-winded as well.

Beats being simplistic.

[Constantinople]

Dayseed Dayseed:

Constantinople Constantinople:

Long-winded as well.

Beats being simplistic.

But not conciseness.

[Blue_Nose]

I'm glad you're here to educate me, Dayseed... fluid mechanics wasn't the easiest course I took, so I may have missed something.

I brought up friction between the jet and the atmosphere (or lack thereof, in the case of the moon) because it's the "salient" point in this instance.

Lateral diffusion as the hot gas exits the aperture certainly occurs. Of course, exposing these gases to the extreme cold outer space will very quickly reduce this pressure, and the diffusion will decrease.

The fact is, this lateral diffusion does nothing to affect the momentum of the jet stream.... that's where the atmosphere, and air resistance, would come into play. No air resistance means no external forces acting on the jet means no decrease in momentum.

The diffusion whch occurs causes the jet to increase in size as it exits the aperture, like a cone. I would suggest that due to this decrease in density (pressure), the jet will not have as much of an effect on an individual particle of moon dust/rock. That's because the cone is so large. While still providing the deceleration (same mass of gas is hitting the surface at a given rate), it doesn't have the energy required to uplift the rocks.

I know it's not your nature to concede a point, so I'm not offended by the Denny's remark.... I haven't eaten there in years; I'm a Smitty's man myself.

(edit: oops, that last comment doesn't make much sense, since Canadaka washed your mouth out with soap....)

[Dayseed]

Bluenose,

I don't need to concede a point; you just ended up agreeing with me. Here's what you originally wrote:

$1:

Otherwise, the gases and other particulate from the jets will obey the law of conservation of momentum and travel in a straight line "in a column downwards" (neglecting secondary flow) until hitting the surface of the moon.

Now you've written:

$1:

The diffusion whch occurs causes the jet to increase in size as it exits the aperture, like a cone. I would suggest that due to this decrease in density (pressure), the jet will not have as much of an effect on an individual particle of moon dust/rock. That's because the cone is so large.

Here's what I wrote originally:

$1:

On the moon, without atmosphere, the expulsion bleeds every which way and is rather innocuous to the moon.

Thanks! Unless I've completely misunderstood how you've gone from saying that in a vacuum, jet expulsion will travel in a column to being a diffused cone and instead you meant all of your talk about the cone to be supporting your erroneous assertion that it'll travel in a column.

If you're still on the skeptical side, just watch the flames coming out of the Space Shuttle on its way to orbit. The flame goes from columnar to conical as the density of the atmosphere decreases during the shuttle's ascent.

I've never been to a Smitty's. I'm actually on kind of a Cora's kick right now.

[Blue_Nose]

See the bit about secondary flow? That's we call fluid flows which occur parallel to the cross section of the flow, or laterally....

The point is, the flow isn't "bled everywhere"; it still travels directly towards the surface (assuming the jet is upright) at the same velocity.

If it's just a matter of sketchy terminology, then we're cool

[Jaime_Souviens]

Blue_Nose Blue_Nose:

...The fact is, this lateral diffusion does nothing to affect the momentum of the jet stream.... that's where the atmosphere, and air resistance, would come into play. No air resistance means no external forces acting on the jet means no decrease in momentum...

Your last sentence really gets at why there was no 'blast crater'. Given the weak gravity and non-existent atmosphere, all it took was a short blast to get the lander moving up to lunar orbit.

(And if you remember, only half the lander returned, the base of it remained on the moon. ---And didn't someone, NASA, the Eurpoeans, someone, recently were able to view the lander bases by telescope or somehow otherwise confirm that they're still where we left them?)

[Jaime_Souviens]

Jaime_Souviens Jaime_Souviens:

...(And if you remember, only half the lander returned, the base of it remained on the moon. ---And didn't someone, NASA, the Eurpoeans, someone, recently were able to view the lander bases by telescope or somehow otherwise confirm that they're still where we left them?)

Check that; I was wrong.

I remembered something about the topic, but I guess we're not there yet :

$1:

Better pictures are coming. In 2008 NASA's Lunar Reconnaissance Orbiter will carry a powerful modern camera into low orbit over the Moon's surface. Its primary mission is not to photograph old Apollo landing sites, but it will photograph them, many times, providing the first recognizable images of Apollo relics since 1972.

The spacecraft's high-resolution camera, called "LROC," short for Lunar Reconnaissance Orbiter Camera, has a resolution of about half a meter. That means that a half-meter square on the Moon's surface would fill a single pixel in its digital images.

Apollo moon buggies are about 2 meters wide and 3 meters long. So in the LROC images, those abandoned vehicles will fill about 4 by 6 pixels.

What does a half-meter resolution picture look like? This image of an airport on Earth has the same resolution as an LROC image. Moon buggy-sized objects (automobiles and luggage carts) are clear.

"I would say the rovers will look angular and distinct," says Mark Robinson, research associate professor at Northwestern University in Evanston, Illinois, and Principal Investigator for LROC. "We might see some shading differences on top from seats, depending on the sun angle. Even the rovers' tracks might be detectable in some instances."

Even more recognizable will be the discarded lander platforms. Their main bodies are 4 meters on a side, and so will fill an 8 by 8 pixel square in the LROC images. The four legs jutting out from the platforms' four corners span a diameter of 9 meters. So, from landing pad to landing pad, the landers will occupy about 18 pixels in LROC images, more than enough to trace their distinctive shapes.

Shadows help, too. Long black shadows cast across gray lunar terrain will reveal the shape of what cast them: the rovers and landers. "During the course of its year-long mission, LROC will image each landing site several times with the sunlight at different angles each time," says Robinson. Comparing the different shadows produced would allow for a more accurate analysis of the shape of the objects.

[Blue_Nose]

Jaime_Souviens Jaime_Souviens:

Your last sentence really gets at why there was no 'blast crater'. Given the weak gravity and non-existent atmosphere, all it took was a short blast to get the lander moving up to lunar orbit.

Also, though I'm not certain of the design of the landing base, it would provide a sort of launchpad from which the pod took off (I hadn't considered the departure in the previous posts), thereby reducing the effect on the surface itself.

Another point: even if there was a significant amount of material disturbed by the jets, it wouldn't stay airborn (a bad term, considering the lack of air) because there is nothing to keep it there. Dust stays in the air because of the viscous forces exerted by air. In a vacuum, the particles would drop like stones.

[Jaime_Souviens]

Blue_Nose Blue_Nose:

Another point: even if there was a significant amount of material disturbed by the jets, it wouldn't stay airborn (a bad term, considering the lack of air) because there is nothing to keep it there. Dust stays in the air because of the viscous forces exerted by air. In a vacuum, the particles would drop like stones.

See for yourself :

[video width=200 height=150]http://nssdc.gsfc.nasa.gov/movie/as17_liftoff.mov[/video]

[RUEZ]

Jaime_Souviens Jaime_Souviens:

Blue_Nose Blue_Nose:

Another point: even if there was a significant amount of material disturbed by the jets, it wouldn't stay airborn (a bad term, considering the lack of air) because there is nothing to keep it there. Dust stays in the air because of the viscous forces exerted by air. In a vacuum, the particles would drop like stones.

See for yourself :

[video width=200 height=150]http://nssdc.gsfc.nasa.gov/movie/as17_liftoff.mov[/video]

Who filmed that when it was taking off?

[Jaime_Souviens]

RUEZ RUEZ:

Jaime_Souviens Jaime_Souviens:

Blue_Nose Blue_Nose:

Another point: even if there was a significant amount of material disturbed by the jets, it wouldn't stay airborn (a bad term, considering the lack of air) because there is nothing to keep it there. Dust stays in the air because of the viscous forces exerted by air. In a vacuum, the particles would drop like stones.

See for yourself :

Who filmed that when it was taking off?

They left a camera behind, pre-set to pan up, (actually, the pre-set seems to have been a little slow, because the lander creeps off the top of the screen).

Not hard to do.

And also worthwhile, just so physicists could argue about the effects of explosions on the lunar surface.

[ShepherdsDog]

$1:

Who filmed that when it was taking off?

Some Nigerian astronaut. Like the guy stuck on the secret Russian Space Station, they didn't have enough room to take him back with them (those damn samples from the moon were so bulky). Only thing is, unlike his compatriot who has been stuck on the station for 14 years, Yankin Yerchayn prefers living on the moon to living in Nigeria.

[Dayseed]

Blue_Nose Blue_Nose:

See the bit about secondary flow? That's we call fluid flows which occur parallel to the cross section of the flow, or laterally....

The point is, the flow isn't "bled everywhere"; it still travels directly towards the surface (assuming the jet is upright) at the same velocity.

If it's just a matter of sketchy terminology, then we're cool

Bluenose,

I don't know what you've been talking about. You clearly said it would come out in a column because of the law of conservation of momentum (which, by the way, is the wrong law Clarence Darrow, you meant Newton's first law of motion ~ the law of inertia) and lack of atmospheric friction.

It's not a matter of sketchy terminology, it's you being wrong, correcting yourself to my initial supposition and then blaming it on your lexicon.

However, if you believe that rocket expulsion in a vacuum will become a cone instead of a column because of diffusion, we're cool.

[Blue_Nose]

Dayseed Dayseed:

I don't know what you've been talking about. You clearly said it would come out in a column...

I did say that it was a column, neglecting secondary flows, because I didn't bother to consider its shape. There's a multitude of vortices, etc, which will determine the shape, and it wasn't the crux of my point (the 'downward' part, as opposed to 'every which way' was the one with which I was most concerned), so I didn't consider it. You're very correct in stating that diffusion of pressure will create a conic shape, I don't deny it, and I even supported it when you brought it up. It was obviously a oversimplification on my part, but it was one that I didn't feel took away from the intended message.

Dayseed Dayseed:

...because of the law of conservation of momentum (which, by the way, is the wrong law Clarence Darrow, you meant Newton's first law of motion ~ the law of inertia) and lack of atmospheric friction.

Momentum is conserved, energy is conserved, and the law of inertia is obeyed. In this case, they all indicate that the gas doesn't decelerate. There's more than one way to skin a cat. Chacun a son gout.

Dayseed Dayseed:

It's not a matter of sketchy terminology, it's you being wrong, correcting yourself to my initial supposition and then blaming it on your lexicon.

I'm blaming it on your lexicon. Really, since you're so knowledgeable in this area, why couldn't you come up with a more accurate description than "billow outwards" and "bleeds in every which way"? That is what I intended to call you on, because "every which way" isn't what you ment. Also, look up secondary flow if you think I made it up...

Dayseed Dayseed:

However, if you believe that rocket expulsion in a vacuum will become a cone instead of a column because of diffusion, we're cool.

We are cool then. I'm not disagreeing with what you're saying, just how you said it initially, and how I interpreted it.

[Dayseed]

Bluenose,

Your intention was that since there isn't an atmosphere, the expulsion will travel in a column downwards, in a straight line, due to Newton's first law of motion. Again, I don't see how your new explanation that you didn't consider its shape explains you describing it as a straight line column, which seems to consider the shape.

However, there isn't much sense travelling 'round and 'round in circles.

My choice of words were due to not every one possessing the same scientific acumen. Diffusion doesn't bring to mind the same image as "gases billowing outwards".

The bottom line to all this is: Man was on the Jesus Monkey-Balls Moon.