Been watching "For All Mankind" footage of the Apollo Mission, and have one question

seasnake

I understand that the moon has no atmosphere and so the flag that was planted on the moon stays stretched out and streight, but this point I am rather confused by the footage of the documentary as when it was first planted the bottom edge of the flag was quivering back and forth while the upper most portion was still like it had some sort of bracing holding it that way. I assume that perhaps the quivering bottom portion of the flag is due to momentum from being planted, but then later in the documentary the flag is shown again, all stretched out and not moving. Again no problem here, except even later in the documentary the perfectely stretched out flag isn't perfectly stretched out at all but rather a bit more bunched together.

My question then becomes this, given such footage as put out by NASA, does the behavior of the flag in the documentary correspond to how a flag should act in such an environment? Again, I don't understand the quivering at the bottom of the flag when it was first shown, nor the different positions as to how the flag is spread out at different times of the documentary.

Pengwuino

You're correct that the moon has no atmosphere but the implications are incorrect. A flag placed on the moon will fall (the fabric part that is) because gravity is still there from the moon pulling it down. NASA knew this and to be more picturesque, a bar was actually installed on the flag along the upper edge from what I remember hearing. This is why it acted in the way it did. Fluttering occurs when the poll is not only planted, but twisted, for obvious reasons.

DaveC426913

I just watched the Mythbuster ep where they duplicated the flag itself and the vacuum, (alas, they couldn't dupe the low gravity).

The flag actually waved significantly more and significantly longer in the vacuum than it did in their control test with air.

seasnake

I accept your bar premise, and your implications statement as I imply nothing, but am still a bit troubled by the footage that I watched. For the bottom of the flag to flutter as it did at first, and then not to would imply that the flag poll was first twisted as you put it, and then became untwisted later as the flag became stiff, still, perfectly spread out and non-moving, while later on the poll tilted, twisted, or whatever in such a way that the perfectly spread out flag became instead a bit bunched. Without an atmosphere, and with a relatively constant gravital pull and the short amount of time the Apollo was on the moon's surface when the flag was in the footage, I'm not exactly sure that the actions of the flag was that as what physics would otherwise predict, and so am puzzled.

"I just watched the Mythbuster ep where they duplicated the flag itself and the vacuum, (alas, they couldn't dupe the low gravity).
The flag actually waved significantly more and significantly longer in the vacuum than it did in their control test with air."

If this were so, then the still shots of the flag would have made no sense. I'm confused by what I saw in the documentary.

cephron

I saw the mythbuster episode, as well! It was a blast. I thought they were a little obvious in their over-excitement at the vacuum footprint, though; it wasn't particularly good :p

I have a guess about why it may not have worked as nicely, aside from not having real moon dust ("regolith", I think they called it) - and aside from perhaps not having 60+ pounds of weight applied to it: vacuum cementing?
It's a wild guess at best, because all I know about vacuum cementing is gleaned from Larry Niven books and a wikipedia stub (http://en.wikipedia.org/wiki/Vacuum_cementing).
But, from what I understand, vacuum cementing is prevented by gas molecules being adsorbed onto the surface. And, clearly, the dust they were using was exposed to air before it was placed in the vacuum chamber and had the air pumped out. I don't know how long they left it before they tried the footprint in the vacuum, but perhaps it wasn't long enough for the gas molecules to fully disassociate from the dust.

Edit: hey, it just occurred to me - perhaps that could also be a reason for the flag to scrunch in on itself as time passes? Not actual cementing, but simply bits of organic matter sticking to other bits of organic matter in the absence of the lubricating presence of air?

Creator

Aren't you guys aware that there is a wire mesh sewn into the fabric of the flag to stiffen it so it can stay extended.??

http://astroprofspage.com/archives/162

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ralilu

why arent any stars visible from the moon?

Pengwuino

Why aren't any stars visible from the Earth? Think about it... the moon landing didn't happen at "night".

DaveC426913

This is a matter of exposure. When we look up at the night sky, our eyes are night-adjusted. On the Moon, the cameras were set for the sunlit ground and the astronauts. The stars were far too dim to be seen.

Take a flash picture of some people in your dining room near a window at night. Would you expect to see stars through the window?

ralilu

so the camera flash cancelled out the light from the stars. i always thought the stars would appear bright on the moon, i guess i was wrong.

Pengwuino

There is no flash, it's daylight photography. My astronomy professor back in the day always emphasized it when you're dealing with the moon. Whenever you're looking at the moon or thinking about what happens on the moon, remember the sun is shining bright in the sky on the moon just like it does during the day on Earth.

ralilu

i see

DaveC426913

Yes, I didn't mean to suggest there was a flash, just setting up similar circumstances of exposure but without complicating it by the presence of sky light in daylight.

russ_watters

Note that it isn't possible to demonstrate the stars-in-the-lunar-sky problem on earth, so Dave's example of a camera flash at night (no need for the window) is a good way to simulate the problem.

At the same time, you can see stars during the day on the moon if you point your camera or eyes away from the surface or brightly lit objects and shield them from extraneous light. That allows a longer exposure/larger pupil dilation to let more light in.

Also: Earth's atmosphere, when it is clear out, absorbs very little incident light for objects high in elevation. In other words, a star viewed from earth on a nice clear night is more than 90% as bright as it would appear from space. What makes stars hard to see isn't how bright the stars are, it is the fact that everything else you see is much brighter, except at night.

ralilu

so stars when viewed from space are less bright than the stars when viewed from earth on a clear night ,because in space there are more bright objects?

Pengwuino

No, he means the brightness will be 90%+ of the brightness the star would have when viewed from space. If a star has the brightness on some arbitrary scale of say, 10, then on Earth on a clear night at high elevations it would have a brightness of 9 to 10.

ralilu

ohhh ok, coz the atmosphere absorbs very little incident light...silly me