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Cost to build Gerard K. O'Neill's "Island Three"
On Oct 2, 10:27*pm, Carey wrote:
This seems like the way to go if deorbiting Phobos is in fact a desirable event. I don't like the notion of de-orbiting Phobos. Some Mars enthusiasts like to day dream of a martian beanstalk being our gateway to the resources in the main belt. They correctly point out that Mars has an angular velocity similar to earth (in other words an ~24 hour day) and less mass. Mars synchronous is about 17,000 kilometers above Mars surface which is less than half earth's ~36,000 kilometers geosynch altitude. They correctly point out that since stress is much less, less exotic materials can be used. No carbon nanotubes needed, T-1000 or even spun basalt does the job. While they demonstrate a Mars elevator is much less implausible than an Earth elevator, they still don't demonstrate a Mars elevator is doable in our world where financial realities impose limits. To hurl stuff to earth, Ceres or beyond, a Mars beanstalk would have to greatly exceed 17,000 kilometers in length. Building even a primitive industrial infrastructure on Mars is a stretch. Such a huge mega project as a beanstalk on Mars I don't see for centuries if not millenia. Now take another look at this Phobos tether: http://clowder.net/hop/TMI/PhobosTether.jpg About 14,000 km in length, less than the distance to Mars synchronous orbit. It offers trans earth insertion as well as trans Ceres insertion. It offers most the powers a martian beanstalk confers with a small fraction of the length and material. The chief difference is that from Mars' surface a small suborbital hop is needed to dock with the tether foot. A 300 km, .6 km/sec hop. This Phobos stalk is shorter and suffers less stress than a Martian beanstalk. So spun basalt is also a possible material and an even smaller taper does the job. Phobos could not only be our beach head to Mars -- It could be the beach head to the solar system. But even a Phobos tether is a stretch. Sometimes humanity moving beyond LEO seems implausible. But it's fun to daydream. |
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Cost to build Gerard K. O'Neill's "Island Three"
On 10/3/2010 12:20 PM, Hop wrote:
On Oct 2, 10:27 pm, wrote: This seems like the way to go if deorbiting Phobos is in fact a desirable event. I don't like the notion of de-orbiting Phobos. Some Mars enthusiasts like to day dream of a martian beanstalk being our gateway to the resources in the main belt. They correctly point out that Mars has an angular velocity similar to earth (in other words an ~24 hour day) and less mass. Mars synchronous is about 17,000 kilometers above Mars surface which is less than half earth's ~36,000 kilometers geosynch altitude. They correctly point out that since stress is much less, less exotic materials can be used. No carbon nanotubes needed, T-1000 or even spun basalt does the job. But we don't need all of Phobos there to make that idea work; a chunk of it a few thousand feet in diameter would be plenty big to do that concept with. For that matter, there's always Deimos to do it with also. Being in a higher orbit, Deimos is even more favorably situated to toss things around the solar system with due to having less gravity well to climb out of. Pat |
#13
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Cost to build Gerard K. O'Neill's "Island Three"
On Oct 3, 6:00*pm, Pat Flannery wrote:
For that matter, there's always Deimos to do it with also. Being in a higher orbit, Deimos is even more favorably situated to toss things around the solar system with due to having less gravity well to climb out of. Pat To fling stuff from Mars, you need so called centrifugal force to greatly exceed Mars gravity. Centrifugal force is w^2 r, where w is angular velocity in radians and r is distance from Mars' center. Phobos makes a rotation every 7 hours 40 minutes, Deimos 30 hours 20 minutes. Phobos' angular velocity is about 4 times that of Deimos, therefore w^2 is almost 16 fold difference. Gravity gradient is steeper deep in Mars' gravity well. Gravity is falling off much more gradually by the time you reach Deimos' 20,000 km altitude. For trans earth insertion, the Oberth effect is more helpful deeper in Mars gravity well. A tether extending ~28,000 km from Deimos could hurl stuff to earth. In contrast it takes only 6,155 km of tether from Phobos for trans Earth insertion. |
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Cost to build Gerard K. O'Neill's "Island Three"
On Oct 4, 12:14*am, Pat Flannery wrote:
Okay, I understand the concept now, but why do you need all of Phobos there to do this? Even 1/10,000 of its total mass would be more than enough to give you a very massive and stable anchor to attach your tether to. Pat That would be 1/10,000 of the tether's momentum bank. If it enjoys a lot of use, that may not be enough. That's also all but 1/10,000 of the resources sent to the bottom of a deep gravity well. Why are you so fired up to send Phobos down? Phobos' low density indicates its a rubble pile with lots of voids and/ or volatile ices. Well before it reached Mars, it'd exceed its Roche limit and smear into a string of pearls like Shoemaker Levy-9. You can't use pusher plates to send down an elongated dust cloud. Nor can you drag down a dust cloud with a tether. This would be an orbital debris nightmare that would pretty much make Mars off limits. If you were able to send down a fraction of this dust cloud, you wouldn't have a cannonball plunging into the north pole, but a dust storm burning up in the upper atmosphere. Even if you could send down a cannonball I haven't seen any convincing models that this would add substantially to Mars atmosphere. |
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Cost to build Gerard K. O'Neill's "Island Three"
On 10/3/2010 6:43 PM, Hop wrote:
To fling stuff from Mars, you need so called centrifugal force to greatly exceed Mars gravity. Centrifugal force is w^2 r, where w is angular velocity in radians and r is distance from Mars' center. Phobos makes a rotation every 7 hours 40 minutes, Deimos 30 hours 20 minutes. Phobos' angular velocity is about 4 times that of Deimos, therefore w^2 is almost 16 fold difference. Gravity gradient is steeper deep in Mars' gravity well. Gravity is falling off much more gradually by the time you reach Deimos' 20,000 km altitude. For trans earth insertion, the Oberth effect is more helpful deeper in Mars gravity well. A tether extending ~28,000 km from Deimos could hurl stuff to earth. In contrast it takes only 6,155 km of tether from Phobos for trans Earth insertion. Okay, I understand the concept now, but why do you need all of Phobos there to do this? Even 1/10,000 of its total mass would be more than enough to give you a very massive and stable anchor to attach your tether to. Pat |
#16
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Cost to build Gerard K. O'Neill's "Island Three"
On 10/3/2010 9:01 PM, Hop wrote:
On Oct 4, 12:14 am, Pat wrote: Okay, I understand the concept now, but why do you need all of Phobos there to do this? Even 1/10,000 of its total mass would be more than enough to give you a very massive and stable anchor to attach your tether to. Pat That would be 1/10,000 of the tether's momentum bank. If it enjoys a lot of use, that may not be enough. That's also all but 1/10,000 of the resources sent to the bottom of a deep gravity well. Why are you so fired up to send Phobos down? Because I can melt a whole pile of subsurface ice on the surface, create water clouds, and increase the surface atmospheric pressure - making it a lot easier to colonize the place. Considering the havoc a three-mile wide asteroid wreaked on Earth when it hit Yucatan at the end of the Cretaceous age (like creating tidal waves in Hudson's Bay and Hawaii and blowing most of the water out of the Gulf of Mexico) dropping something far larger on a far smaller planet should really create some impressive effects in regards to changing its climate. Phobos' low density indicates its a rubble pile with lots of voids and/ or volatile ices. Well before it reached Mars, it'd exceed its Roche limit and smear into a string of pearls like Shoemaker Levy-9. You can't use pusher plates to send down an elongated dust cloud. Nor can you drag down a dust cloud with a tether. It's probably indeed a rubble pile, but not a dustball. For starters, if it was all dust or gravel, even its low gravity would have formed it into a sphere, not the oblong shape it is. So there are some honking big chunks of rock in it, and those will do nicely as they descend under it's orbital path as it breaks up under tidal stresses at low orbital altitude. In fact, having its mass hit all over the place in a ring around the planet is better than having it hit all in one place as far as total atmospheric heating goes. This would be an orbital debris nightmare that would pretty much make Mars off limits. Only at that altitude; they now think Phobos may be a result of a major asteroid impact on Mars throwing debris into orbit, like a small scale version of what created Earth's moon: http://www.msnbc.msn.com/id/39399235...science-space/ Even if it did turn into a dust ring of some sort, any spacecraft encountering it in Mars orbit would be moving at the same relative speed it would be, and given its low orbital altitude, could easily avoid it by orbiting at a higher altitude and different orbital inclination. If you were able to send down a fraction of this dust cloud, you wouldn't have a cannonball plunging into the north pole, but a dust storm burning up in the upper atmosphere. And you start figuring out how much atmospheric heating that amount of dust hitting the atmosphere and burning up would generate, because it's going to get mighty toasty down there if it comes in in a fairly short period of time. Even if you could send down a cannonball I haven't seen any convincing models that this would add substantially to Mars atmosphere. Well then, you may think there's lots of water ice on the Moon, but none on Mars, although NASA is very keen on pointing out that there are vast frozen water deposits lurking just under the Martian surface. ;-) You melt a lot that into water vapor and yes, the atmospheric pressure rises, just like fog is more dense than air over deserts. Pat |
#17
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Cost to build Gerard K. O'Neill's "Island Three"
Pat Flannery wrote:
Because I can melt a whole pile of subsurface ice on the surface, create water clouds, and increase the surface atmospheric pressure - making it a lot easier to colonize the place. Assuming the subsurface ice even exists... And even if it does, given the lack of thermal input to keep it liquid, it'll start freezing out soon enough. Not that your "whole pile" amounts to that much water on a planetary scale in the first place. Considering the havoc a three-mile wide asteroid wreaked on Earth when it hit Yucatan at the end of the Cretaceous age (like creating tidal waves in Hudson's Bay and Hawaii and blowing most of the water out of the Gulf of Mexico) dropping something far larger on a far smaller planet should really create some impressive effects in regards to changing its climate. Yeah, what that impactor did was *cool* the Earth - exactly the opposite of what you'd want at Mars. And you start figuring out how much atmospheric heating that amount of dust hitting the atmosphere and burning up would generate, because it's going to get mighty toasty down there if it comes in in a fairly short period of time. Um, no. There is neither enough mass nor orbital energy in Phobos to accomplish much in that department. Well then, you may think there's lots of water ice on the Moon, but none on Mars, although NASA is very keen on pointing out that there are vast frozen water deposits lurking just under the Martian surface. ;-) You melt a lot that into water vapor and yes, the atmospheric pressure rises, just like fog is more dense than air over deserts. But what doesn't rise is the partial pressure of 02. Nor does the minor (and temporary) rise in pressure give any especial advantage, in fact it makes things like suit design even harder. D. -- Touch-twice life. Eat. Drink. Laugh. http://derekl1963.livejournal.com/ -Resolved: To be more temperate in my postings. Oct 5th, 2004 JDL |
#18
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Cost to build Gerard K. O'Neill's "Island Three"
On Oct 4, 3:49*am, Pat Flannery wrote:
Because I can melt a whole pile of subsurface ice on the surface, create water clouds, and increase the surface atmospheric pressure - making it a lot easier to colonize the place. IF you vaporize enough dry ice and water ice, you might trigger a runaway green house effect. Is there enough Martian CO2 and water ice to do this? Not demonstrated. IF you were successful in triggering a runaway green house effect, would it result in an atmosphere thick enough you wouldn't have to wear pressure suits? Highly doubtful. For starters, if it was all dust or gravel, even its low gravity would have formed it into a sphere, not the oblong shape it is. Ummmm.... No. Try Googling "tidal elongation". This would be an orbital debris nightmare that would pretty much make Mars off limits. Only at that altitude; Pulling Phobos down to Mars would scatter debris throughout Phobos' orbital plane from ~6000 km up (Phobos' present altitude) to Mars upper atmosphere. Even if it did turn into a dust ring of some sort, any spacecraft encountering it in Mars orbit would be moving at the same relative speed Something incoming from earth would be following a hyperbola with regard to Mars. It would NOT be moving at the same relative speed as the debris it encounters. it would be, and given its low orbital altitude, could easily avoid it by orbiting at a higher altitude and different orbital inclination. Mars' equatorial plane would be filled with debris from roughly 6000 km altitude to the top of Mars' atmosphere. An inclined orbit would pass through the debris plane at its descending or ascending node. And you start figuring out how much atmospheric heating that amount of dust hitting the atmosphere and burning up would generate, Current guesstimates place Phobos' Roche limit at 5000 km altitude. This is when tidal forces would start breaking it apart. Nukes or trying to drag it down would probably start breaking it apart earlier. Most of it would remain in orbit about Mars as a plane of orbital debris. Even if you could send down a cannonball I haven't seen any convincing models that this would add substantially to Mars atmosphere. Well then, you may think there's lots of water ice on the Moon, but none on Mars, Um... No. That's not what I may think. There is good evidence of Martian water. Is there enough to trigger runaway greenhouse? Is there enough to substantially raise Mars' atmospheric pressure? Could de orbiting Phobos vaporize enough ice to do this? This you still haven't demonstrated. When I ask for a convincing model, I'm hoping for some math and physics. |
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