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

[Ron-Astro Pharmacist]

It seems like there will also be interest to look deep into a comets interior to see how it formed over time. I would think it may have been considered to stab it like a dart then withdraw out a sample like they do with ocean sediment and exam the core. I was kind of wondering why they didn't do that on 67P? Philae with a tube through its center, it might even be an effective landing shock absorber or anchoring device? Then, to get at the goodies inside, use a retractable inner straw. It might even be able to move up and down on the outer pole to view from different heights. I'd be curious to know what types of designs were considered for Philae.

[Chris Peterson]

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.

I was using a similar animation the other day to try and explain to my wife why it is that near the solstices (the extremes of daylight length) the actual change from day to day is very small, and near the equinoxes you can notice the change in daylight over just a few days.

[Steve Dutch]

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 ! )

Where have we seen them? Not on the Moon, not on Mars, or Titan, or Vesta. Miranda has a pretty impressive cliff but not jagged and spiky. The reasoning behind jagged space art was that young mountains on Earth are jagged. True, but they're jagged because of glaciation and water erosion. Nobody really asked what features would look like on a world that never, ever had erosion. And long before we had landers, astronomers could calculate the real slopes of lunar mountains from their shadows, and see lunar mountains along the limb, and showed that they had gentle slopes. And they were completely ignored, just like the ones who said there were no canals on Mars and that Venus was too hot for life. Just not as interesting as the fantasy.

[Chris Peterson]

It seems like there will also be interest to look deep into a comets interior to see how it formed over time. I would think it may have been considered to stab it like a dart then withdraw out a sample like they do with ocean sediment and exam the core. I was kind of wondering why they didn't do that on 67P? Philae with a tube through its center, it might even be an effective landing shock absorber or anchoring device? Then, to get at the goodies inside, use a retractable inner straw. It might even be able to move up and down on the outer pole to view from different heights. I'd be curious to know what types of designs were considered for Philae.

The surface was so hard that the drill on Philae couldn't operate. So no straws, I think.

The most practical way of learning about the interior is with a kinetic impactor, like used with the Deep Impact mission, or like Hayabusa 2 will use on an asteroid.

[JohnD]

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?

If you go to the thread about "Wanderers" that I linked to above, you will find the previous discussion on cliff-jumping. It includes all you need to calculate how fast you would be going at the bottom of the cliff, and how long it would take you to get there. It's just arithmetic!
The point is that this comet has an almost micro-gravity environment. G is very low, so that you would accelerate very slowly, even in free-fall and vacuum, and arrive at the bottom going quite slowly and safely. Orientating yourself to land safely, presumably on your feet would be quite a feat [sic!].

But, Tzabeau, going through the CoG? I doubt very much if the CoG is in free space. It will be inside the body of the comet. Also, Rosetta found that surface gravity on the comet varied from place to place in direction as well as quantity, indicating that there are 'mass-cons' in the comet that will impose a 'local' CoG in such a weak gravity field.
In theory, suppose that you could fall into the centre of a much larger body, such as the Earth, that has a much greater and more homogeneous gravity field. G would fall as you approached the centre, where it would be zero. By that time you would be falling at a significant velocity (!) so would pass right through the centre, but as G would then, for you, be negative, you velocity would begin to fall. The result would be a prolonged to and fro oscillation across the centre, until you came to rest there in a zero gravity field. If that ere possible in the comet, I can see you oscillating between the mass-cons, like one of those magnetic toys that seem never to come to rest.

JOhn

Ooops! ToSeeked by Chris!

[Ron-Astro Pharmacist]

It seems like there will also be interest to look deep into a comets interior to see how it formed over time. I would think it may have been considered to stab it like a dart then withdraw out a sample like they do with ocean sediment and exam the core. I was kind of wondering why they didn't do that on 67P? Philae with a tube through its center, it might even be an effective landing shock absorber or anchoring device? Then, to get at the goodies inside, use a retractable inner straw. It might even be able to move up and down on the outer pole to view from different heights. I'd be curious to know what types of designs were considered for Philae.

The surface was so hard that the drill on Philae couldn't operate. So no straws, I think.

The most practical way of learning about the interior is with a kinetic impactor, like used with the Deep Impact mission, or like Hayabusa 2 will use on an asteroid.

It must be quite the process planning space missions. I did a quick search and found this link that was from a while back.

http://www.open.ac.uk/personalpages/a.j ... apcom.html

I suspect there are many scenarios that involve probability statistics. I'd feel pretty stupid if the pole bounced off the comet like a dart hitting the wire framework of the board.

[Guest]

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?

I am curious too about the terminal velocity, and weight, but I am too lazy to search the formulas and compute. Anyway, I assume that not only the velocity matters, but weight too, because if your weight is there few grams, sudden stop will be less strong than falling here from a chair.

Empirically, from what I remember, moon is one quarter diameter of earth, and weight is one sixth, this comet is 5 k km so is 2k smaller, so maybe weight 3k smaller. I am 100 kg here, and a mere 33 grams there, less than a beach ball

[Tszabeau]

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).

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.

Assuming that would that also hold true on a mass with such a relatively low gravity-well as a comet... the long sought perpetual-motion machine could be realized as a frictionless-generator or a whackamole starship launching platform.

[Chris Peterson]

Assuming that would that also hold true on a mass with such a relatively low gravity-well as a comet...

It would... although the situation is complicated by reality, which never offers the perfect world of abstract physics problems. The comet isn't spherical, it doesn't have a uniform density, it is rotating, the environment you're falling in isn't a perfect vacuum.

the long sought perpetual-motion machine could be realized as a frictionless-generator or a whackamole starship launching platform. ;)

Perpetual motion isn't a problem (outside those practical realities mentioned above). The Earth is in nearly perpetual motion around the Sun. Satellites in high orbits are in nearly perpetual motion around the Earth. The problem is if you want to do something useful with your motion- to extract energy that can be used to do work. That's were the system falls apart.

[Boomer12k]

You know.....first thought about this pic.....remember those old Sci-Fi movies...where they go to some asteroid, or something...there is one where they shrink in size....well...this pic tells me....They got it RIGHT....barren and craggy....that looks like some of the mat-job sets from some of the old movies...amazing!

Awesome pic, from a Comet!!!

:---[===] *

[Chris Peterson]

You know.....first thought about this pic.....remember those old Sci-Fi movies...where they go to some asteroid, or something...there is one where they shrink in size....well...this pic tells me....They got it RIGHT....barren and craggy....that looks like some of the mat-job sets from some of the old movies...amazing!

Yes, it looks fake. There are too many little disconnects from a natural Earth scene- the sharpness, the lack of scale. Our brains don't quite know how to deal with this.

[Jim Armstrong]

My first thought, too, was whether there is an "up and down" on such a small body. Perhaps the illusion is just perspective.
Then I thought the spot looked very much like a beach on California's north coast.
Then I gave myself a surprise by clicking by mistake on another page I had open, expecting to see again the stark cliffs of Comet CG.

http://epod.usra.edu/blog/2014/12/eyjaf ... eland.html

Ah, our wonderful Earth!

[Ron-Astro Pharmacist]

Assuming that would that also hold true on a mass with such a relatively low gravity-well as a comet...

It would... although the situation is complicated by reality, which never offers the perfect world of abstract physics problems. The comet isn't spherical, it doesn't have a uniform density, it is rotating, the environment you're falling in isn't a perfect vacuum.

the long sought perpetual-motion machine could be realized as a frictionless-generator or a whackamole starship launching platform.

Perpetual motion isn't a problem (outside those practical realities mentioned above). The Earth is in nearly perpetual motion around the Sun. Satellites in high orbits are in nearly perpetual motion around the Earth. The problem is if you want to do something useful with your motion- to extract energy that can be used to do work. That's were the system falls apart.

It's not the same but it would increase efficiency to be able to capture the energy of an atmosphere entry in some way to use it for lift-off on the way home or to slow re-entry. Say to catalyze an endothermic reaction to create a useful fuel from a waste product.

http://antoine.frostburg.edu/chem/senes ... fuel.shtml

I think I'd better let an actual chemist to figure this out for future heat shields. Of course it also could be a way to get rid of undesirable people here on Earth. Hey – I'm not going to test out my own ideas. That would be crazy.

[Cousin Ricky]

I am curious too about the terminal velocity, and weight, but I am too lazy to search the formulas and compute. Anyway, I assume that not only the velocity matters, but weight too, because if your weight is there few grams, sudden stop will be less strong than falling here from a chair.

Empirically, from what I remember, moon is one quarter diameter of earth, and weight is one sixth, this comet is 5 k km so is 2k smaller, so maybe weight 3k smaller. I am 100 kg here, and a mere 33 grams there, less than a beach ball

In this context, it's not a good idea to confound mass and weight.

[Chris Peterson]

I am curious too about the terminal velocity, and weight, but I am too lazy to search the formulas and compute.

It would take about an hour and a half to fall to the bottom, and you'd hit at less than 0.5 m/s. That's about as fast as you hit when you jump off a deck of playing cards on the Earth.

EDIT: oops, I was using 0.0001 m/s² for the acceleration of gravity on this comet. It's actually about ten times that. So it would take a little over a half hour to fall, and you'd hit at 1.4 m/s, or the same speed you land when you jump off a brick.

Anyway, I assume that not only the velocity matters, but weight too, because if your weight is there few grams, sudden stop will be less strong than falling here from a chair.

Best to think in terms of mass, not weight.

If a low mass person and a high mass person jump off the cliff together, they will land at exactly the same time, going exactly the same speed. The energy of impact is proportional to their mass times their velocity squared. So yeah, the more massive guy is going to have a more energetic landing. But in this case, not very energetic for any plausible human mass.

[davepotter]

If you SLOWLY stepped off the 300 meter-high cliff, you would land at about 0.2m/s (certainly not fatal!) The fall would take about 20 minutes. You would have to be very careful about walking off the cliff - a horizontal speed of only 0.6m/s would put you in orbit around the comet, and 0.8m/s would give you escape velocity (you would never return!) Be careful up there!

[BMAONE23]

But the More Mass Guy will rebound higher after hitting the surface due to that "More Energy" which will impact on him or have a higher likelihood of bouncing on impact

[Chris Peterson]

But the More Mass Guy will rebound higher after hitting the surface due to that "More Energy" which will impact on him or have a higher likelihood of bouncing on impact

That depends on the nature of his body and what kind of suit he's wearing. The collision could be highly elastic (like billiard balls), or highly inelastic (like a bug hitting a windshield).

[davepotter]

oops - just saw that the cliffs are about 1000m high - so you would land at about 0.5[m/s] (someone else said that earlier) and it would take about 40 minutes. Still not fatal.

[Prospero]

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 ! )

Where have we seen them? Not on the Moon, not on Mars, or Titan, or Vesta. Miranda has

I was attempting humour by pretending to mis-construe the ambiguous structure of your statement ("seen one", - which one ?) following on from Ann's in post number oh er ? well about 6 in from the top in which she alludes to the Moon conspiracy.

I wish I had'na bothered th'noo ! !

[Prospero]

and it would take about 40 minutes.

but how far would 67P have moved during those 40 min ?
Doesn't it all depend upon in which direction you jump and how much effort you put in to it ?

Could you (a human) ( in the average space suit ) jump "upwards" with enough vigour to reach escape velocity, or run sideways fast enough to gain orbit ?

Edit oh, just seen davepotter "slowly step off " post, !!
what tangled web we do weave !

[davepotter]

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

[Jimmie]

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...

[davepotter]

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!

[SouthEastAsia]

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.