Starship Asterisk*

by **Cousin Ricky** » Thu Dec 25, 2014 1:41 pm

Ron-Astro Pharmacist wrote:That brings up a very good point. If a collision occurred something had to hit it. If a split occurred it could be the striking object (asteroid or comet) reconnecting to a part of the original comet, the two pieces reconnecting or some other three-way scenario. Either way – what's left is an odd duck.

IMG_0345.jpg

Odd duck.

by **rstevenson** » Thu Dec 25, 2014 12:43 pm

I'm not sure we know enough about the history of an object like this to conclude that, Bruce. It's tempting to think of the boulders as tumbling slowly down a slope and coming to rest "down" there. But perhaps they were always roughly where they are now, and have just been exposed by erosion caused by outgassing. The boulders may have moved very little after the presumed impact that brought the large chunks together. It's intriguing to imagine how a comet like this came to its current condition and shape, but I don't think our Earth-honed senses and expectations can help us much, based only on pictures. I think we monkeys will have to be there, doing monkey things like dropping and throwing rocks and seeing where they go, to really get a feel for what happens. Or maybe we'll just get really good at building more monkey-like robots -- but what fun would that be.

Rob

by **BDanielMayfield** » Thu Dec 25, 2014 3:54 am

But we do have an indication of where the center of mass is Rob; the field of boulders that have come to rest at the bottom of what looks like a kind of powdery talus slope at the base of the cliff face. Where this smaller material has settled is telling us where down is in this image.

Bruce

by **rstevenson** » Thu Dec 25, 2014 1:28 am

I was kind of thinking the same thing. After all, that rock face only looks like a cliff if you view it as shown in the APOD. But as you orbit the comet (in your imagination, alas) the rock face will at times look more like a rough plane with an odd sand dune off to the side...

Without an indication of where the main center of mass is, there's no reason to think of it as a cliff at all. It would only be a cliff, in the usual sense of the word, if the center of mass was directly below ("below" as viewed in the APOD image) this rock face, which it probably isn't.

Rob

by **BMAONE23** » Wed Dec 24, 2014 11:34 pm

Reading through these posts and looking at the cometary body's physical shape, I see 2 separate globes that are conjoined by a land bridge. I see both globes having separate but distinct centers of mass with the bridge coalesced around a line between them. In this case I shouldn't imagine that "cliff diving" from the edge of a globe to the bridge would yield the desired effect, no more than it would be possible to cliff dive down the hall of your house. What I see happening is likely the ability to step off the top surface and around to the wall of what might feel like a 45° slope that gradually lessens as you again approach a surface point closer to the center of mass.

by **Chris Peterson** » Wed Dec 24, 2014 10:17 pm

ta152h0 wrote:why the big thing about cliff on a comet ? Could just be desert tipped up 90 degrees and the sand goes to the bottom. By the way, has anyone dug down far enough in, lets say, the Sahara desert and reached bottom ?

The Sahara isn't very sandy. Most of it is just bare rock. Where there's sand, its average depth is probably around 150 m. Obviously, where there are dunes it may be locally thicker.

by **ta152h0** » Wed Dec 24, 2014 8:52 pm

why the big thing about cliff on a comet ? Could just be desert tipped up 90 degrees and the sand goes to the bottom. By the way, has anyone dug down far enough in, lets say, the Sahara desert and reached bottom ?

by **Ron-Astro Pharmacist** » Wed Dec 24, 2014 6:11 pm

In case anyone is curious. I just asked one of our patients who happens to be a wildlife biologist. He said that mallards, too, often intermingle with other than their own type resulting in a composite object begging attention from an "Astro-Golfer". Happy Holidays!!

by **Ron-Astro Pharmacist** » Wed Dec 24, 2014 4:45 pm

Chris Peterson wrote:
Ron-Astro Pharmacist wrote:That is an interesting theory that the shape is from a collision and now it's the two halves reconnected. I'm sure the comet has been digitally mapped its surface and ESA should be able to try to match the halves should any common surfaces remain.
It need not be two halves of the same. It could also be two separate bodies now connected. We know from meteoritic evidence that some small bodies in the Solar System are formed from a mixture of materials from very different origins.

That brings up a very good point. If a collision occurred something had to hit it. If a split occurred it could be the striking object (asteroid or comet) reconnecting to a part of the original comet, the two pieces reconnecting or some other three-way scenario. Either way – what's left is an odd duck.

by **Chris Peterson** » Wed Dec 24, 2014 4:01 pm

Ron-Astro Pharmacist wrote:That is an interesting theory that the shape is from a collision and now it's the two halves reconnected. I'm sure the comet has been digitally mapped its surface and ESA should be able to try to match the halves should any common surfaces remain.

It need not be two halves of the same. It could also be two separate bodies now connected. We know from meteoritic evidence that some small bodies in the Solar System are formed from a mixture of materials from very different origins.

by **Ron-Astro Pharmacist** » Wed Dec 24, 2014 3:28 pm

Wayne Jepson wrote:
Ron-Astro Pharmacist wrote:Why cliffs on a comet? ...

...Anyway, if I may engage in wild speculation, comet CG appears to be the remnant of a much larger icy body shattered by a collision with a similar object, and two large fragments became gravitationally bound together along with pulverized material from the collision. Exposure to the sun has probably preferentially evaporated away the pulverized rubble connecting the two large fragments, which is consistent with the jets currently emanating only from the comet's waist and not the two lobes.

That is an interesting theory that the shape is from a collision and now it's the two halves reconnected. I'm sure the comet has been digitally mapped its surface and ESA should be able to try to match the halves should any common surfaces remain.

by **MarkBour** » Wed Dec 24, 2014 6:26 am

There are places somewhat like this on Earth and the Moon, and Mars. From what I'm reading here, we are looking at a valley that is quite comparable in size to Yosemite Valley. Of course climbers would love this place! They could scale this cliff one-handed. In fact, if the calculations listed earlier are correct (that from the top of the cliff you'd only land as hard as falling from a brick), then a corollary is that you could jump from the bottom (like Keanu Reeves), to the top of the cliff. And indeed, I think your greatest concern with jumping would be achieving escape velocity.

by **geckzilla** » Wed Dec 24, 2014 6:04 am

SouthEastAsia wrote:I think you're sounding a bit jealous not being part of the design team, if you ask me. No? Why such the fuss? This is pretty incredible and game-changing science truly... and this is all you can come up with? May you be more inspired in 2015's science breakthroughs across the Universe

Not at all. Chances are if I have something to criticize then it's just something that I've taken some time to look at and care about. I can simultaneously appreciate something and yet not be gushing over with praise about it.

by **Wayne Jepson** » Wed Dec 24, 2014 5:50 am

Ron-Astro Pharmacist wrote: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.

They are impressive cliffs. What strikes me most about the image are the obvious laminations in the cliff face, rising "vertically" from the sandy waist of the comet right to the "summit". They look like they could be sedimentary layers exposed in some cliff face on a similar-sized mountain on earth. I'm not saying the layering in the comet had to have formed via sedimentation or accretion (any other ideas would be welcome), but if they did, then the parent body would have to have been significantly larger than the comet now is... perhaps a Kuiper Belt object shattered by a collision eons ago, after which some large fragments and rubble later agglomerated together into what we now know as comet CG? The comet certainly has the appearance of two boulders held together by an icy bridge... not unlike comet 103P/Hartley, although that one appeared more like two dirty snowballs connected by an ice bridge.

Anyway, if I may engage in wild speculation, comet CG appears to be the remnant of a much larger icy body shattered by a collision with a similar object, and two large fragments became gravitationally bound together along with pulverized material from the collision. Exposure to the sun has probably preferentially evaporated away the pulverized rubble connecting the two large fragments, which is consistent with the jets currently emanating only from the comet's waist and not the two lobes.

by **SouthEastAsia** » Wed Dec 24, 2014 2:09 am

geckzilla wrote:
SouthEastAsia wrote:And to Geck... you got me, things of this magnitude just sound better if given some personality! But allow me to recant: Hopefully He/she (Philae) will shine again, another day in 2015 if 'the' panels do in fact recharge. God speed!
It's just an it. To be honest, the way they have these cartoons anthropomorphizing it into a timid little baby who boldly jumps onto a comet and is now peacefully sleeping is bizarre to me. But it also seems to be having a positive effect on their PR so I guess I can't complain. People are very attached to cute, widdle Philae as a result.

I think you're sounding a bit jealous not being part of the design team, if you ask me. No? Why such the fuss? This is pretty incredible and game-changing science truly... and this is all you can come up with? May you be more inspired in 2015's science breakthroughs across the Universe

by **geckzilla** » Wed Dec 24, 2014 1:45 am

SouthEastAsia wrote:And to Geck... you got me, things of this magnitude just sound better if given some personality! But allow me to recant: Hopefully He/she (Philae) will shine again, another day in 2015 if 'the' panels do in fact recharge. God speed!

It's just an it. To be honest, the way they have these cartoons anthropomorphizing it into a timid little baby who boldly jumps onto a comet and is now peacefully sleeping is bizarre to me. But it also seems to be having a positive effect on their PR so I guess I can't complain. People are very attached to cute, widdle Philae as a result.

by **Guest** » Wed Dec 24, 2014 1:06 am

Ok... let batman do his winged dive to the base of the cliff... But that is that at the bottom of the cliff. The stuff that appears smooth in the photo, allowing for the granularity thereof. Frozen liquids? (doesn't look like it) Or a 'rain' of ice crystals that have fallen to the surface? If ice crystals, why collect in the low areas if there is no 'weather'. Why not an even distribution across the whole surface? Unless there is a retrograde angle to the surface of the cliff being shown.

by **SouthEastAsia** » Wed Dec 24, 2014 1:01 am

rstevenson wrote:
SouthEastAsia wrote:Now, with respect to 'jumping off that 1km' cliff... how far could that human jumper travel if wearing a wing-suit??? That could be a cool experiment.
Er, um, wing suits work in an atmosphere, and a fairly thick atmosphere at that. There ain't much gas there to support a wing suit!

Rob

So... maybe that future wing-suit-jumper could circumnavigate?? Can't wait for the go-pro on that!

And to Geck... you got me, things of this magnitude just sound better if given some personality! But allow me to recant: Hopefully He/she (Philae) will shine again, another day in 2015 if 'the' panels do in fact recharge. God speed!

by **ta152h0** » Wed Dec 24, 2014 12:58 am

remember the Apollo moonlanding when a hammer and a feather was dropped, landing at the same instant ?

by **rstevenson** » Wed Dec 24, 2014 12:48 am

SouthEastAsia wrote:Now, with respect to 'jumping off that 1km' cliff... how far could that human jumper travel if wearing a wing-suit??? That could be a cool experiment.

Er, um, wing suits work in an atmosphere, and a fairly thick atmosphere at that. There ain't much gas there to support a wing suit!

Rob

by **geckzilla** » Wed Dec 24, 2014 12:45 am

SouthEastAsia wrote:No disrespect to Philae. Hopefully she will shine again, another day in 2015 if her panels can be recharged!

Philae is gender neutral.

by **SouthEastAsia** » Wed Dec 24, 2014 12:38 am

Truly increbible image taken by Rosetta here. I have to agree with Ann on that. The irony perhaps is that the orbiting Rosetta could actually end up being the more mind-blowing and science-gathering robot than the Philae lander itself? No disrespect to Philae. Hopefully she will shine again, another day in 2015 if her panels can be recharged! Salute to ESA on this one.

Now, with respect to 'jumping off that 1km' cliff... how far could that human jumper travel if wearing a wing-suit??? That could be a cool experiment.

by **davepotter** » Tue Dec 23, 2014 11:58 pm

From an earlier post: "Doesn't it all depend upon in which direction you jump and how much effort you put in to it ?"

You are correct.

First, even a very small horizontal speed at the top of the cliff will result in a landing point far away from the cliff face (because you would keep moving horizontally at that speed for the 40 minutes it takes to drop to the surface)

Second, Coriolis effect would play a significant role. If you hung from a rope and dropped "vertically", your trajectory would be curved, because the bottom of the cliff is not moving laterally (due to the comet's rotation) as fast as the top of the cliff.

Given the 12.4 hour rotational period, the comet's mass (1e13[kg], and the height of this cliff (easily 3000 meters from the comet's center of mass), you might be approaching orbital speed just standing on top!
(vsurface=2*(pi)*r/12.4/3600) vorbit=sqrt(G*1e13/r). Set vsurface=vorbit, and solve for r. Result: if the top of the cliff is 3229 meters from the comet's center of mass, the surface is moving at orbital speed, meaning if you are there and drop a ball it would not fall! And if you threw the ball horizontally at even a centimeter per second in the same direction as the comet's rotation, it would rise into its own orbit. And if you were standing on a scale, it would read zero.
Strange!

by **Jimmie** » Tue Dec 23, 2014 11:41 pm

I would think that terminal velocity would be less of an issue, than being impacted on an approaching and opposing cliff face. Regardless of the cliff divers terminal velocity, the rotational velocity would have a greater impact on the falling object. Depending on the direction the jumper chose. In any case, if he survives, he will still be in big trouble. I examined the photo and there is no Star Bucks in sight, and they are everywhere...

by **davepotter** » Tue Dec 23, 2014 10:59 pm

Doesn't it all depend upon in which direction you jump and how much effort you put in to it ?

Starship Asterisk*

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

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