APOD: The Cliffs of Comet CG (2014 Dec 23)

Comments and questions about the APOD on the main view screen.
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APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by APOD Robot » Tue Dec 23, 2014 5:08 am

Image The Cliffs of Comet Churyumov–Gerasimenko

Explanation: These high cliffs occur on the surface of a comet. They were discovered to be part of the dark nucleus of Comet Churyumov–Gerasimenko (CG) by Rosetta, a robotic spacecraft launched by ESA which began orbiting the comet in early August. The ragged cliffs, as featured here, were imaged by Rosetta about two weeks ago. Although towering about one kilometer high, the low surface gravity of Comet CG would likely make a jump from the cliffs, by a human, survivable. At the foot of the cliffs is relatively smooth terrain dotted with boulders as large as 20 meters across. Data from Rosetta indicates that the ice in Comet CG has a significantly different deuterium fraction -- and hence likely a different origin -- than the water in Earth's oceans. The Rosetta spacecraft is scheduled to continue to accompany the comet as it makes its closest approach to the Sun in 2015 August.

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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by ta152h0 » Tue Dec 23, 2014 5:30 am

Can the Big Bang be considered a gamma ray event ?
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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by Ann » Tue Dec 23, 2014 5:33 am

This picture sure looks amazing.

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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by Nitpicker » Tue Dec 23, 2014 5:48 am

One half expects to spot the cast of Lord of the Rings tramping through the scene.

Does anyone know if Rosetta is still looking for Philae? Could Rosetta even get close enough to resolve an image of Philae?

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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by geckzilla » Tue Dec 23, 2014 7:30 am

Nitpicker wrote:Does anyone know if Rosetta is still looking for Philae? Could Rosetta even get close enough to resolve an image of Philae?
No, I don't think there is anything sure about where it's at. Someone thought they found a series of pictures of it glinting in the sun but I'm dubious. http://spaceexp.tumblr.com/post/1058751 ... lae-by-its
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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by Locutus » Tue Dec 23, 2014 9:42 am

Does anyone know why there are no stars visible in the background? Can't be light pollution from some bright city :D

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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by Ann » Tue Dec 23, 2014 9:53 am

Locutus wrote:Does anyone know why there are no stars visible in the background? Can't be light pollution from some bright city :D
Perhaps it is a photoshopped image produced by Hollywood! :wink:
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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by hoohaw » Tue Dec 23, 2014 10:12 am

Ann wrote:
Locutus wrote:Does anyone know why there are no stars visible in the background? Can't be light pollution from some bright city :D
Perhaps it is a photoshopped image produced by Hollywood! :wink:
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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by hoohaw » Tue Dec 23, 2014 10:13 am

ta152h0 wrote:Can the Big Bang be considered a gamma ray event ?
If you insist, yes, for a tiny, tiny fraction of a second near the beginning. Don't know why you would want to, though.

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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by RedFishBlueFish » Tue Dec 23, 2014 11:04 am

Considering that for so many millennia comets were thought to be harbingers sent from god(s) to foretell the fate of nations, it is difficult to imagine a scene showing more complete indifference to humans and to their concerns than this image of dark cliffs rising from the surface of this small world hurtling blindly through the bleak grandeur of space.

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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by RedFishBlueFish » Tue Dec 23, 2014 11:06 am

Locutus wrote:Does anyone know why there are no stars visible in the background? Can't be light pollution from some bright city :D
For the same reason that most of the moon walking images show no stars - the exposure latitude of the recording medium.

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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by JohnD » Tue Dec 23, 2014 11:49 am

We had a debate in a different thread on how survivable would be jumping down cliff faces in reduced gravity.
http://asterisk.apod.com/viewtopic.php?f=9&t=34201

I'm happy to concede that on this comet it would be survivable. Trouble would be that gravity might not, in fact would not, be parallel to that enormous cliff! Or even that it would change direction across its length. One kilometer is a significant part of the whole comet's dimensions, so you would be passing by the CoG as you travelled along (can't say "down"!) the cliff face.

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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by geckzilla » Tue Dec 23, 2014 12:23 pm

JohnD wrote:I'm happy to concede that on this comet it would be survivable. Trouble would be that gravity might not, in fact would not, be parallel to that enormous cliff! Or even that it would change direction across its length. One kilometer is a significant part of the whole comet's dimensions, so you would be passing by the CoG as you travelled along (can't say "down"!) the cliff face.
Any kind of ambulation on this comet I imagine would be quite annoying. The merest movements would result in an awkward tumbling and flying about and any misjudgement could cause you to leave the comet entirely. I think your legs would be pretty much useless. Maybe if you anchor a some segments of chain or rope all around the thing you could climb with your arms to wherever you'd like to go.
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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by Steve Dutch » Tue Dec 23, 2014 2:04 pm

Every Hollywood effects artist likes to depict celestial bodies with jagged, sharp, spiky landscapes. This is the first time we've actually seen one.

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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by Tszabeau » Tue Dec 23, 2014 2:19 pm

JohnD wrote:We had a debate in a different thread on how survivable would be jumping down cliff faces in reduced gravity.
http://asterisk.apod.com/viewtopic.php?f=9&t=34201

I'm happy to concede that on this comet it would be survivable. Trouble would be that gravity might not, in fact would not, be parallel to that enormous cliff! Or even that it would change direction across its length. One kilometer is a significant part of the whole comet's dimensions, so you would be passing by the CoG as you travelled along (can't say "down"!) the cliff face.

John
I wonder if, as you passed through the CoG... would you just stop in mid-air, so-to-speak? What an awesome PoV you'd have.

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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by Prospero » Tue Dec 23, 2014 2:25 pm

Steve Dutch wrote:Every Hollywood effects artist likes to depict celestial bodies with jagged, sharp, spiky landscapes. This is the first time we've actually seen one.
No surely not the first time, we have seen plenty, this is classic Bonestell.

continuing Ann's theme :)
(nice one Ann ! )

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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by ghaegele2@gmail.com » Tue Dec 23, 2014 2:44 pm

Considering a distance of more than a kilometer, wouldn't a cliff jumper continue to accelerate until reaching terminal velocity which, in such thin "air," would be a very high rate of speed resulting in death on impact?

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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by Joules » Tue Dec 23, 2014 2:54 pm

Despite a density of 0.4 g/cc, less than half that of water, (http://sci.esa.int/rosetta/14615-comet-67p/), I'm still not seeing a whole lot of porosity in the structure.
Ice foam cliffs, low density clathrates?

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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by geckzilla » Tue Dec 23, 2014 3:21 pm

Steve Dutch wrote:Every Hollywood effects artist likes to depict celestial bodies with jagged, sharp, spiky landscapes. This is the first time we've actually seen one.
To be fair, we don't see very many landscapes up close. There are most definitely some jagged landscapes out there. But, yes, fiction does tend to focus on the more exotic possibilities, does it not? That's often why it's interesting.
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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by Ron-Astro Pharmacist » Tue Dec 23, 2014 3:30 pm

Why cliffs on a comet? I looked through the links (today) but did not see mention of why this comet has such an unusual shape? I believe, from before, there was discussion that it may be the joining of two comets but thoughts on comet-shaping would be welcome.
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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by Chris Peterson » Tue Dec 23, 2014 3:36 pm

Joules wrote:Despite a density of 0.4 g/cc, less than half that of water, (http://sci.esa.int/rosetta/14615-comet-67p/), I'm still not seeing a whole lot of porosity in the structure.
Ice foam cliffs, low density clathrates?
Most likely, a combination of high porosity and low density constituents. Assuming a rubble-pile structure, the porosity would be in the interior, so we wouldn't really see it. The outer surface has been modified by centuries of outgassing and interaction with the space environment.
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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by Chris Peterson » Tue Dec 23, 2014 3:41 pm

ghaegele2@gmail.com wrote:Considering a distance of more than a kilometer, wouldn't a cliff jumper continue to accelerate until reaching terminal velocity which, in such thin "air," would be a very high rate of speed resulting in death on impact?
In this near vacuum, terminal velocity would be absurdly high, maybe even relativistic. Given the very low surface gravity, on the order of 0.0001 G, you will reach the bottom of the cliff and be stopped by the surface long before you build up much speed.
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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by Chris Peterson » Tue Dec 23, 2014 3:49 pm

Tszabeau wrote:I wonder if, as you passed through the CoG... would you just stop in mid-air, so-to-speak?
The center of mass of this comet is in the interior, so you couldn't pass through it. As you fell, your position with respect to it would be changing, so you would follow a curved path (like an orbit) rather than a straight one (like a fall).

Actually, a fall and an orbit are very similar. Relevant to your question, consider a planet with a hole bored straight through it. What would happen if you jumped in? Ignoring secondary issues like air friction and the Coriolis effect, you'd accelerate until you reached the center, and then decelerate towards the opposite side, coming to rest at the other end of the hole. You'd then fall back, bobbing forever from end to end. How long would that round trip take? Exactly the time required for one orbit right at the surface. On Earth, a low orbit takes about 90 minutes, and that's how long it would take you to fall through the planet and back.
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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by geckzilla » Tue Dec 23, 2014 4:18 pm

Chris Peterson wrote:Actually, a fall and an orbit are very similar. Relevant to your question, consider a planet with a hole bored straight through it. What would happen if you jumped in? Ignoring secondary issues like air friction and the Coriolis effect, you'd accelerate until you reached the center, and then decelerate towards the opposite side, coming to rest at the other end of the hole. You'd then fall back, bobbing forever from end to end. How long would that round trip take? Exactly the time required for one orbit right at the surface. On Earth, a low orbit takes about 90 minutes, and that's how long it would take you to fall through the planet and back.
Interesting. Never thought about it like that but the relationship between a circle and sine is a simple beauty that is strangely easy to miss... or it was for me, anyway. I think I made it all the way through school without realizing it until I started tinkering with Flash and was applying trigonometry to animating some graphics. Quite unexpectedly gratifying, that. Of course, if my textbook could animate like this then I think I would have gotten it back then.
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Re: APOD: The Cliffs of Comet CG (2014 Dec 23)

Post by ghaegele2@gmail.com » Tue Dec 23, 2014 4:20 pm

Chris Peterson wrote:
ghaegele2@gmail.com wrote:Considering a distance of more than a kilometer, wouldn't a cliff jumper continue to accelerate until reaching terminal velocity which, in such thin "air," would be a very high rate of speed resulting in death on impact?
In this near vacuum, terminal velocity would be absurdly high, maybe even relativistic. Given the very low surface gravity, on the order of 0.0001 G, you will reach the bottom of the cliff and be stopped by the surface long before you build up much speed.
In that case seems like there's a greater chance of starving to death before reaching the ground. Better pack a few sandwiches.