APOD: Crescent Enceladus (2017 Feb 09)

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APOD: Crescent Enceladus (2017 Feb 09)

Post by APOD Robot » Thu Feb 09, 2017 5:12 am

Image Crescent Enceladus

Explanation: Peering from the shadows, the Saturn-facing hemisphere of tantalizing inner moon Enceladus poses in this Cassini spacecraft image. North is up in the dramatic scene captured last November as Cassini's camera was pointed in a nearly sunward direction about 130,000 kilometers from the moon's bright crescent. In fact, the distant world reflects over 90 percent of the sunlight it receives, giving its surface about the same reflectivity as fresh snow. A mere 500 kilometers in diameter, Enceladus is a surprisingly active moon. Data collected during Cassini's flybys and years of images have revealed the presence of remarkable south polar geysers and a possible global ocean of liquid water beneath an icy crust.

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Ann
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Re: APOD: Crescent Enceladus (2017 Feb 09)

Post by Ann » Thu Feb 09, 2017 6:40 am

Looks like the perfect place for skiing.

Though you might get out of breath pretty soon.

Ann
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Re: APOD: Crescent Enceladus (2017 Feb 09)

Post by heehaw » Thu Feb 09, 2017 1:27 pm

Good to know "North is up..." That lets me know ....

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Re: APOD: Crescent Enceladus (2017 Feb 09)

Post by neufer » Thu Feb 09, 2017 2:31 pm

Ann wrote:
Looks like the perfect place for skiing.
Though you might get out of breath pretty soon.
With gravity just one eighty ninth that of Earth they would probably all be bunny slopes.
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Ann
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Re: APOD: Crescent Enceladus (2017 Feb 09)

Post by Ann » Thu Feb 09, 2017 2:40 pm

neufer wrote:
Ann wrote:
Looks like the perfect place for skiing.
Though you might get out of breath pretty soon.
With gravity just one eighty ninth that of Earth they would probably all be bunny slopes.
Thanks for the info! :D

And thanks for teaching me a new word! :D

I just meant... Don't hold your breath while skiing on Enceladus, or rather, do hold your breath, because trying to breathe while skiing on Enceladus will be a breathless experience!

Ann
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Re: APOD: Crescent Enceladus (2017 Feb 09)

Post by Chris Peterson » Thu Feb 09, 2017 3:18 pm

Ann wrote:I just meant... Don't hold your breath while skiing on Enceladus, or rather, do hold your breath, because trying to breathe while skiing on Enceladus will be a breathless experience!
You had it right the first time. Don't hold your breath! The first rule for surviving a (brief) exposure to vacuum is to open your mouth and blow all the air out of your lungs as hard as you can. Otherwise, your lungs will rupture. Which is not the kind of breathless experience you want!
Chris

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Re: APOD: Crescent Enceladus (2017 Feb 09)

Post by neufer » Thu Feb 09, 2017 4:05 pm

Chris Peterson wrote:
Ann wrote:
I just meant... Don't hold your breath while skiing on Enceladus, or rather, do hold your breath, because trying to breathe while skiing on Enceladus will be a breathless experience!
You had it right the first time. Don't hold your breath! The first rule for surviving a (brief) exposure to vacuum is to open your mouth and blow all the air out of your lungs as hard as you can. Otherwise, your lungs will rupture. Which is not the kind of breathless experience you want!
http://www.uh.edu/engines/epi2691.htm wrote:
The University of Houston presents this series about the machines that
make our civilization run, and the people whose ingenuity created them.
Click to play embedded YouTube video.
No. 2691 : THE BODY AT VACUUM
by NASA astronaut Michael Barratt

<<In a famous scene in Stanley Kubrick’s “2001 A Space Odyssey”, astronaut Dave Bowman ejects himself through open space into an unpressurized airlock of the mother ship — this after the HAL 9000 computer refuses to let him in. After a couple of bounces, he manages to close the outer hatch and pressurize the airlock. A desperate survival move, but is it plausible? Hollywood and science fiction vary considerably in depicting the body in open space, from almost negligible effects to messy whole body explosions. Here’s some reality.

Because the lungs communicate freely with the outside, a sudden pressure drop causes the air in the lungs to rapidly expand, seeking an outlet. Keeping an open airway ensures that the air rushes outward, better than the alternative of trying to hold it in and causing a traumatic rupture of the lung. But of course respiration is no longer possible. With no circulation and no air to provide oxygen anyway, about 12 seconds of useful consciousness is available.

The vapor pressure of water at body temperature is about one 16th of atmospheric pressure. Below this pressure, which you find above an altitude of about 63,000 ft, body fluids begin to boil away. Moist surfaces experience this immediately, such as the eye, mouth and throat, and airways. Deeper inside, body water in low pressure areas also turns to its gas phase, water vapor. This occurs rapidly in the lung and beneath the skin. Bubbles of water vapor also form in venous blood; these essentially vapor lock the circulation. We call the syndrome associated with the formation of water vapor bubbles in the body at vacuum ebullism. An interesting occurrence of partial ebullism occurred during a high altitude balloon flight, in which Joseph Kittinger experienced a glove pressurization failure while ascending well above the 63,000 feet where ebullism occurs. His hand was described as swelling to about twice normal size and was quite painful. He jumped from a height of over 102,000 feet, free falling for over four minutes and eventually opening a parachute at 18,000 feet. While falling back into the atmosphere, his hand repressurized and returned to normal size and full function.

There are at least a couple of human exposures to whole body vacuum that ended happily. In 1966, a technician testing a space suit in a vacuum chamber experienced a rapid loss of suit pressure due to equipment failure. He recalled the sensation of saliva boiling off his tongue before losing consciousness. The chamber was rapidly repressurized, he regained consciousness quickly, and went home for lunch. Another man was accidentally exposed to vacuum in an industrial chamber; it was at least three minutes before he was repressurized. He required intensive medical care, but eventually regained full function. These instances show that ebullism is not inevitably fatal — and the body holds together just fine.

In the case of explosive decompression, there are of course other hazards and events. Whatever water vapor is in the air instantly condenses due to the rapid temperature drop. Some of us experience this during chamber training, where we undergo a rapid decompression from simulated altitudes of say 8000 feet to over 20,000; ... it definitely gets your attention when some of the air rushes out of your lungs and fog instantly appears. This was also shown in the 2001 scene. The fog rapidly dissipates as the water turns back into vapor in the surrounding vacuum. For the case of decompressing explosively to complete vacuum, it is possible that even keeping an open airway (mouth open, glottis open) may not present a big enough pipe to allow the air to escape the lungs with an injury causing pressure buildup. In addition, whatever gas may be present in the stomach will instantly expand, and may induce vomiting by forcing stomach contents back up the esophagus as a path of escape. Overall, such events are good to avoid.

And so we return to Dave Bowman’s predicament. From my rough timing of the movie’s events, it was about 8 sec between explosive decompression and activating the handle to repress the airlock, then an additional 4 – 5 seconds to close the door and begin repressurizing. Could he have managed the feat? The answer is probably. With the presence of mind to exhale and keep an open airway, he may have performed this necessary task without injury. But his was a unique scenario of planned sudden decompression with a pressurized endpoint.

So if you should ever face decompression — well, don’t hold your breath.>>
Art Neuendorffer

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Re: APOD: Crescent Enceladus (2017 Feb 09)

Post by pferkul » Thu Feb 09, 2017 5:33 pm

Here is a version with the shadows enhanced. Presumably Saturn is illuminating the dark side.
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Re: APOD: Crescent Enceladus (2017 Feb 09)

Post by MarkBour » Thu Feb 09, 2017 11:56 pm

Ann wrote: Looks like the perfect place for skiing.
Ann, this is how I've always pictured you.

So with such low gravity, it would take a very long drop to get any speed, but then, if you could get some air ... I wonder what level of McTwist Shaun White and friends could achieve. McTwist x 20? https://www.youtube.com/watch?v=oF-jmHTYLBQ
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Re: APOD: Crescent Enceladus (2017 Feb 09)

Post by Ann » Fri Feb 10, 2017 1:23 am

MarkBour wrote:
Ann wrote: Looks like the perfect place for skiing.
Ann, this is how I've always pictured you.

So with such low gravity, it would take a very long drop to get any speed, but then, if you could get some air ... I wonder what level of McTwist Shaun White and friends could achieve. McTwist x 20? https://www.youtube.com/watch?v=oF-jmHTYLBQ
I wish - I'm a desk junkie! I posted that picture just because the APOD caption on February 8 said that tomorrow's picture would be "Ski Enceladus"...

But your avatar suggests that you are no desk junkie, as does the fantastic link you provided! Image

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Re: APOD: Crescent Enceladus (2017 Feb 09)

Post by MarkBour » Thu Feb 16, 2017 11:43 pm

Ann wrote:But your avatar suggests that you are no desk junkie, as does the fantastic link you provided! Image
Ann
Ha! Well, I'll keep picturing you that way, just the same. Yes, your post fit "Ski Enceladus" very well.
I have to admit to being a mostly-desk-junkie. I ski, but not all that well, never competitively.
As for relating to Shaun White, I tried snowboarding, spent a couple of days falling and falling, and then went back to skiing.
I'm sure I'll never do a McTwist on either skis or a snowboard. :-)
Mark Goldfain