Living on Mars?

[Lvl 2 Expert]

I only know one thing: when that time comes there will be plenty of people living in the Gobi desert.

If we want to make Mars habitable we might at least try settling the Sahara desert first.

Team desert! (Or ocean surface, or sea floor, or...)

[Lord Joeltion]

I don't disagree with any of this, except in a general "we really should stop taking over ecosystems and turning them into our private farms"... but I make an explicit exception to the Sahara, because It'd be nice to turn its expansion around.

Just want to point out that the Sahara probably has a role on other ecosystems of the planet. I'd rather not mess around with the single biosphere equilibrium we have ATM, and suggest to perfect crop growth and try to make synthetic proteins economically viable. Until we can successfully terraform a single massive body, at least.

Now, I am sympathetic to the reality that civilization-killing asteroids and other single-planet dangers do exist, but if one of those happens, and the Mars colony depends on Earth for survival, we haven't actually put our eggs in more than one basket. And the bottom line right now is that Mars does not sound independently viable.

Most planetary level cosmic threats are probably a lot easier to solve than the economical sink-hole colonizing a deserted planet would ever be. And the death of the sun is so far from us as species that we may as well be concerned about the heat-death of the universe itself

[hamishspence]

... they don't? The tether is absurdly light. That's the point, and the problem of why we can't build it. It's like being hit with a feather in freefall... except it is much, much, much lighter than a feather.

Ben Bova's Solar System series (Mercury novel) depicted "falling space elevator" as extremely devastating - because, while it's low density, it's huge. The term "skytower" is used for it.

Some elevators might burn up:

https://en.wikipedia.org/wiki/Space_elevator_safety

but this one was on the order of 100 or more metres in diameter. And that might have been at its thinnest point.

The end coming down from nearly 25000 km up, would also have more time to accelerate to dangerous speeds, with air resistance not being able to mitigate those speeds enough. (The end above the geostationary platform, when the whole thing is severed, would go spinning off into space.)

Relevant bits from the novel:

Although buckyball fibers are lighter in weight than any material that is even half their tensile strength, a structure of more than thirty-five thousand kilometers' length weighs millions of metric tons.
The skytower wavered as it tore loose from the geostationary platform, disconnected from the centrifugal force that had pulled it taut. One end suddenly free of its mooring, its other end still tethered to the ground, the lower half of the tower staggered like a prizefighter suddenly struck by a knockout blow, then began its long, slow-motion catastrophic collapse.
...
The lower half of the skytower slowly, slowly tumbled like a majestic tree suddenly turned to putty. Its base, attached to the rotating Earth, was moving more than a thousand kilometers per hour from west to east. Its enormous length, unsupported now, collapsed westward in a long, long, long plunge to Earth.
...
Slowly at first, but then with ever-increasing speed, the skytower's lower half collapsed to the Earth. Its immense bulk smashed into Ciudad de Cielo, the tethers at its base snapping like strings, the shock wave from its impact blowing down those buildings it did not hit directly. The thunder of its fall shattered the air like the blast of every volcano on Earth exploding at once. Seconds later the falling tower smashed down on the northern suburbs of Quito like a gigantic tree crushing an ant hill. The city's modern high-rise glass and steel towers, built to withstand earthquakes, wavered and shuddered. Their safety-glass facades blew out in showers of pellets. Ordinary windows shattered into razor-sharp shards that slashed to bloody ribbons the people who crowded the streets, screaming in terror. Older buildings were torn from their foundations as if a nuclear explosion had ripped through the city. The old cathedral's thick masonry walls cracked and its stained glass windows shattered, each and every one of them. Water pipes ruptured and gas mains broke. Fire and flood took up their deadly work where the sheer explosive impact of the collapse left off.
And still the tower fell.
Down the slope that led to the sea, villages and roads and farms and open fields and trees were smashed flat, pulverized, while the shock wave from the impact blew down woodlands and buildings for a hundred kilometers and more in either direction, as if a giant meteor had struck out of the sky. A fishing village fell under the shadow of sudden doom, its inhabitants looking up to see this immense arm of God swinging down on them like the mighty bludgeon of the angel of death.
And still the tower fell.
Its length splashed into the Pacific Ocean with a roar that broke eardrums and ruptured the innards of men, beasts, birds, and fish. Across the coastal shelf it plunged and out beyond into the abyssal depths. Whales migrating hundreds of kilometers out to sea were pulped to jelly by the shock wave that raced through the water. The tsunami it raised washed away shoreline settlements up and down the coast and rushed across the Pacific, flooding the Galapagos Islands, already half-drowned by the greenhouse warming. The Pacific coast of Central America was devastated. Hawaii and Japan were struck before their warning systems could get people to move inland. Samoa and Tahiti were hit by a wall of water nearly fifteen meters high that tore away villages and whole cities. People in Los Angeles and Sydney heard the mighty thunderclap and wondered if it was a sonic boom.
And still the tower fell, splashing all the way across the Pacific, groaning as part of its globe-girdling length sank slowly into the dark abyssal depths. When it hit the spiny tree-covered mountain backbone of Borneo it snapped in two, one part sliding down the rugged slopes, tearing away forests and villages and plantations as it slithered snake-like across the island.
The other part plunged across Sumatra and into the Indian Ocean, narrowly missing the long green finger of Malaysia but sending a tsunami washing across the drowned ruins of Singapore. Along the breadth of equatorial Africa it fell, smashing across Kenya, ploughed into the northern reaches of Lake Victoria, drowning the city of Kampala with a tidal wave, and continued westward, crushing cities and forests alike, igniting mammoth forest fires, driving vast herds of animals into panicked, screaming stampedes. Its upper end, still smoking from the titanic electrical discharge that had severed it, plunged hissing into the Atlantic, sinking deep down into the jagged rift where hot magma from the Earth's core embraced the man-made structure that had, mere minutes earlier, stood among the stars.
Across the world the once-proud skytower lay amidst a swath of death and desolation and smoking ruin, crushing the life from people, animals, plants, crushing human ambition, human dreams, crushing hope itself.

[Grey_Wolf_c]

Ben Bova's Solar System series (Mercury novel) depicted "falling space elevator" as extremely devastating - because, while it's low density, it's huge. The term "skytower" is used for it.

I'm sorry, I am sure he is a great writer, but Ben Bova is not an actual engineer that we could consider an authority on the subject. He's describing an object that I do not recognise as a realistic space elevator, which is a very thin, relatively wide tether that would burn up in the atmosphere. Anyone can write florid descriptions of destruction (I remember reading a similar one in one of the Mars novels - probably Red? - about the collapse of the Martian Space Elevator). That doesn't make them realistic.

As the elevator cable's thickness decreases, maintenance requirements and failure risks increase. Conversely, as thickness increases, maintenance requirements and failure risks decrease, but the catastrophic damage caused in the event of failure increases. It just doesn't seem worth it to build these on planetoids, especially if their Hill spheres become populated with exponentially more debris over time.

None of that in any way answer my question "why do you think that a falling space elevator "pose[s] a massive risk". Unless you meant financial, I suppose, but then so does all other space technology.

Yes, there is debris in space. But having a space elevator would make it a lot easier to clean, and a lot cheaper too, so I'd imagine that, if anything, the risk posed by it would be reduced if we did get a space elevator built. But even if it didn't, it doesn't actually address the concern of the dangers of a collapsing space elevator.

Grey Wolf

[hamishspence]

I'm sorry, I am sure he is a great writer, but Ben Bova is not an actual engineer that we could consider an authority on the subject.

He does have something of a long career in "realistic sci-fi" though. His space elevator (millions of tonnes) is probably at the high end of the bulk scale, but he's not the only one to depict them that way.

[Lazymancer]

but this one was on the order of 100 or more metres in diameter. And that might have been at its thinnest point.

That's quite big for the thinnest point. It would make sense to build dozens (if not hundreds) of lesser space elevators instead. Moreover, space elevator should be primarily described as (a system of) cables going near each other, rather than a building.

Either way, the assumption is that there could be no potential response. But remember: megastructures don't work like regular buildings. Especially, if one is over 36 thousand km long. Middle sections could take hours to fall (I'm too lazy to calculate exact time, but g at 10.000 km is about 1.5 m/s^2), and that suggests a possibility of response.

For example, emergency cabins with rockets can ride up - or be already riding up, as every third cabin - and direct falling trajectory of cable, blow it up into pieces for better management, or even haul it upwards to be reconnected with the upper part (which will be lowered accordingly). There are many possible response scenarios.

As a "conspiracy theorist" (because acknowledging that science is heavily influenced by non-scientific factors is a definite sign of insanity) I'd like to say that Ben Bova's description presents a very biased point of view on space elevator.

[Grey_Wolf_c]

He does have something of a long career in "realistic sci-fi" though. His space elevator (millions of tonnes) is probably at the high end of the bulk scale, but he's not the only one to depict them that way.

Sure, but it is also not equivalent to the one being discussed. The only theoretically valid material for the elevator will not behave in the way described. Saying that a space elevator shouldn't be built because a novel once described one as being dangerous is not what I'd call conductive to the argument, especially if the one in the novel looks nothing like the one we theoretically could build.

From what I can see in wikipedia, the expected weight of the initial ribbon would be 750 tons, according to NASA's advanced concepts theoretical design. So Ben Nova's version is simply not comparable.

Grey Wolf

[Lord Torath]

Ben Bova's Solar System series (Mercury novel) depicted "falling space elevator" as extremely devastating - because, while it's low density, it's huge. The term "skytower" is used for it.

Some elevators might burn up:

https://en.wikipedia.org/wiki/Space_elevator_safety

but this one was on the order of 100 or more metres in diameter. And that might have been at its thinnest point.

The end coming down from nearly 25000 km up, would also have more time to accelerate to dangerous speeds, with air resistance not being able to mitigate those speeds enough. (The end above the geostationary platform, when the whole thing is severed, would go spinning off into space.)

Relevant bits from the novel:
The lower half of the skytower slowly, slowly tumbled like a majestic tree suddenly turned to putty. Its base, attached to the rotating Earth, was moving more than a thousand kilometers per hour from west to east. Its enormous length, unsupported now, collapsed westward in a long, long, long plunge to Earth.

The TOP end of the tower is moving even more quickly from west to east. If the break was right at the geostationary point, then bit just below the break would be doing about 10,800 kmph in the same direction as the base, and would have the very same angular speed as the base. Yes, you'll have gravity pulling the thing down, but it's going to be a very slow pull. It took over an hour for the bits of Challenger to fall back to Earth, and it was only a few dozen miles up. And the only thing pulling the cable to the side is atmospheric wind, which only extends up 100 km. The other 21,940 km will be pulled straight down to earth. The tower will collapse in a pile, not wrapping around the earth.

(Plus, a tower 100 m across? None of the semi-viable designs I've heard of are anywhere near that size).

[hamishspence]

I'd like to say that Ben Bova's description presents a very biased point of view on space elevator.

His one was only destroyed because it had been sabotaged.

And the only thing pulling the cable to the side is atmospheric wind, which only extends up 100 km.

I think the idea was that, once broken, the angular tension created by difference between the bottom (effectively moving at 1000 kph) and the top (effectively moving at geostationary orbit speed) was what was dragging it out in a "wrap-round".

Mercury was written back in 2005, but set 100+ years from now. Nanotech, in Bova's books, has progressed enormously.

[Lord Torath]

I think the idea was that, once broken, the angular tension created by difference between the bottom (effectively moving at 1000 kph) and the top (effectively moving at geostationary orbit speed) was what was dragging it out in a "wrap-round".

Except that the top and the bottom have the same rotational velocity, which is also the same as the Earth itself. That's why it's called "Geostationary Orbit". There's no angular tension between the top and the bottom, just in-line tension, which will tend to pull the tower straight down.

[hamishspence]

Except that the top and the bottom have the same rotational velocity, which is also the same as the Earth itself. That's why it's called "Geostationary Orbit".

The angular tangential velocity is going to vary though, all along its length.


Angular tangential velocity of a Geostationary Satellite: approx 11068 kph or 6877.8 mph.

Angular tangential velocity of object sitting on Earth's surface: 1674.4 kph or 1040.4 mph

Hence, tension.

Here, there's several simulations of a breaking space elevator. There's some whipping back and forth, but the end, is a wrap-round:

http://gassend.net/spaceelevator/breaks/

[Lord Torath]

The angular velocity is going to vary though, all along its length.


AngularRadial velocity of a Geostationary Satellite: approx 11068 kph or 6877.8 mph.

AngularRadial velocity of object sitting on Earth's surface: 1674.4 kph or 1040.4 mph

Fixed that for you. Angular velocity is rotational speed. The platform, the tower, and the earth are all rotating at 1 revolution per 24 hours. They have the same rotational or angular velocity.

[hamishspence]

I'm sorry, I am sure he is a great writer, but Ben Bova is not an actual engineer that we could consider an authority on the subject. He's describing an object that I do not recognise as a realistic space elevator, which is a very thin, relatively wide tether that would burn up in the atmosphere.

Regarding the bulk of the elevator - a point is made in the original Arthur C. Clarke article:

http://spaceref.com/space-elevator/t...-c-clarke.html

that the weaker the material, the more massive the tether needs to be - and that it will taper outwards - being much wider at the top, than at the ground. It also talks about "megatons (millions of tons) of material"

The very minimum requirement for a space elevator is, obviously, a cable strong enough to support its own weight when hanging from geostationary orbit down to earth, 36000 km below. That is a very formidable challenge; luckily, things are not quite as bad as they look because only the lowest portion of the cable has to withstand one full gee.

As we go upwards, gravity falls off according to Newton's inverse square law. But the effective weight ofthe cable diminishes even more rapidly, owing to the centrifugal force* on the rotating system. At geostationary altitude the two balance and the net weight is zero; beyond that, weight appears to increase again -- but away from the Earth.

* My brief apologies to purists for invoking this fictitious entity.

So our cable has no need to be strong enough to hang 36000 km under sea-level gravity; allowing for the effects just mentioned, the figure turns out to be only one-seventh of this. In other words, if we could manufacture a cable with sufficient strength to support 5000 km (actually, 4960) of its own length at one gee, it would be strong enough to span the gap from geostationary orbit to Equator. Mathematically -- though not physically -- Jacob's ladder need be only 5OOO km long to reach Heaven.... This figure of 5000km I would like to call 'escape length', for reasons which will soon be obvious.

With a stepped, or tapered, cable it would be theoretically possible to construct the space elevator from any material, however weak. You could build it of chewing gum, though the total mass required would probably be larger than that of the entire universe. For the scheme to be practical we need materials with a breaking length a very substantial fraction of escape length. Even Kevlar 29's 200 km is a mere 25th of the 5000 km goal; to use that would be like fuelling the Apollo mission with damp gunpowder, and would require the same sort of astronomical ratio.

So, just as we were once always seeking exotic propellents, we must now search for super-strength materials. And, oddly enough, we will find them in the same place on the periodic table.

Carbon crystals have now been produced in the laboratory with breaking lengths of up to 3000 km -- that is, more than half of escape length. How happy the rocket engineers would be, if they had a propellant whose exhaust products emerged with 60% of escape velocity!

Whether this material can ever be produced in the megaton quantities needed is a question that only future technologies can answer; Pearson [8] has made the interesting suggestion that thezero gravity and vacuum conditions of an orbiting factory may assist their manufacture, while Sheffield [15] and I [10] havepointed out that essentially unlimited quantities of carbon are available on many of the asteroids.

Angular velocity is rotational speed. The platform, the tower, and the earth are all rotating at 1 revolution per 24 hours. They have the same rotational or angular velocity.

Angular speed was the term used in the Wikipedia article (since velocity is always on the same vector)

https://en.wikipedia.org/wiki/Geostationary_orbit

Every part of the tether has the same speed in radians per second - but the top is moving (in miles per hour) at a different tangential speed

https://en.wikipedia.org/wiki/Speed#Tangential_speed

from the bottom. Presumably that's why the "broken elevator" simulations depict the cable not collapsing vertically downward.

There's no angular tension between the top and the bottom, just in-line tension, which will tend to pull the tower straight down.

The point I'm trying to make, is that an elevator that breaks anywhere significantly above the ground, is not going to end up in a neat vertical heap, but trail out.

Think of a sling being spun around, with a heavy weight on the end. Then have it break just below the weight- the weight goes flying off - but the strap is still being spun. What is the strap going to do in the moments after the weight flies off?

The A. C. Clarke article above, also takes the "draped along the equator" interpretation:

But what if the elevator is severed?

Well, if the elevator is cut through at the Earth's surface, it would de exactly the opposite of a terrestrial building. It wouldn't fall down -- but would rise up into the sky! In theory, the loose end might be secured and fastened down again; but that would be, to say the least, a tricky operation. It might even be easier to build a new system....

If the break occurred at any altitude up to about 25000 km, the lower portion of the elevator would descend to Earth and drape itself along the equator while the now unbalanced upper portion would rise to a higher orbit.

[Lazymancer]

His one was only destroyed because it had been sabotaged.

That's not "only". Sabotage is considered to be one of - is not the - top reasons of potential space elevator collapse.

[hamishspence]

Sabotage is considered to be one of - is not the - top reasons of potential space elevator collapse.

True.

It would make sense to build dozens (if not hundreds) of lesser space elevators instead. Moreover, space elevator should be primarily described as (a system of) cables going near each other, rather than a building.

In Ian Stewart & Jack Cohen & Terry Pratchett's The Science of Discworld, humanity is shown as using a bunch of space elevators, all strung out along the equator, to get out into the Solar System, before eventually leaving it in inhabited asteroid ships, after the next Snowball Earth (caused by asteroid impact in this case?).

Possibly large-scale construction is easier than preventing massive climate change on this scale.

The base of the elevator the protagonists visit, is shown as coming out of a pyramid several km high - possibly because starting from a high building, an "artificial Everest", allows you to overcome many of the problems the atmosphere presents.

Going by the Arthur C. Clarke article - the less good your material, the bigger your collection of elevator cables needs to be if it is not to break.

[Rakaydos]

Humans will die whether or not we colonize space.

If all we are worried is about the death of the Sun, I think we can push of the issue for several orders of magnitude longer than humans have existed, and let technological innovation continue to make the problem easier.

Now, I am sympathetic to the reality that civilization-killing asteroids and other single-planet dangers do exist, but if one of those happens, and the Mars colony depends on Earth for survival, we haven't actually put our eggs in more than one basket. And the bottom line right now is that Mars does not sound independently viable.

Grey Wolf

The problem with "we have plenty of time" is "If not now, when?"
Looking at climate change as a comparison, will we decide to avoid a far off astronomical danger in time to develop all the technology needed to survive that danger?

[danzibr]

Interesting thread!

What living there would be like? It'd be a lot like life in the ISS. It's cramped, everything stinks and your health is steadily deteriorating, but you're surrounded by some of the most brilliant and motivated minds Earth has produced, and you get to have an experience few people will ever have.

This made me lol. Been there?

All I can really think of is John Carter (regarding this thread).

[LordEntrails]

Humans will die whether or not we colonize space.

If all we are worried is about the death of the Sun, I think we can push of the issue for several orders of magnitude longer than humans have existed, and let technological innovation continue to make the problem easier.

Now, I am sympathetic to the reality that civilization-killing asteroids and other single-planet dangers do exist, but if one of those happens, and the Mars colony depends on Earth for survival, we haven't actually put our eggs in more than one basket. And the bottom line right now is that Mars does not sound independently viable.

Grey Wolf

The problem with "we have plenty of time" is "If not now, when?"
Looking at climate change as a comparison, will we decide to avoid a far off astronomical danger in time to develop all the technology needed to survive that danger?

Yes. How do you (Grey Wolf) expect "technological innovation" that would help the human race live on places other than the Earth to be invented if we don't start "small" with things like space stations and Mars colonization?

As Rakaydos says, we have to start somewhere.

My point is not pro or con Mars colonization. My point is to shed a tiny light on the need to develop space fairing technologies. One can argue a thousand points, but in the end it's a biological need if we chose to survive.

[Lvl 2 Expert]

Interesting thread!

This made me lol. Been there?

I wish.

Not even so much for the whole having been in space thing, but because that would mean I had been both a world class student and a pretty extreme badass. I'd brave a few months of astronaut sock smell that saturated the filters a long time ago for that. Instead I'm just a sort of okay at times student and a more regular badass.

I have visited the place on Google Streetview, which is pretty cool in and of itself, but not the same thing. (Plus it seems that that is/might be(?) a mock-up that only looks like the real thing but sits on Earth.)

As the elevator cable's thickness decreases, maintenance requirements and failure risks increase. Conversely, as thickness increases, maintenance requirements and failure risks decrease, but the catastrophic damage caused in the event of failure increases. It just doesn't seem worth it to build these on planetoids, especially if their Hill spheres become populated with exponentially more debris over time.

On the other hand, there is a potential case to be made for space elevators on planetoids and small planets/moons involving velocity. The top of a space elevator moves faster than the bottom, so a ship docked to the top of an elevator will still have a bunch of velocity compared to one landing on the planet. It doesn't slow down as much when docking, and doesn't need to be sped up again as much afterwards. So the elevator might be worth it in energy conservation. You can increase the effect by spinning the planetoid up, while simultaneously creating artificial gravity for any people on the planet, as long as they have a roof that won't mind becoming a floor. This costs kind of a lot of energy though...

The main problem with this is that I don't think it works at all with anything even sort of resembling the current generation of spacecraft. Flying from Earth out to Mars is not a matter of flying up in a straight line. It's about escaping Earths gravity well to get to an independent orbit around the sun. From there on you speed up to reach a wider orbit, which will make you fall behind on Earth in rotation. Mars is in a wider orbit still, so you're gaining on it, and you aim to reach the same orbit as Mars by the time your place around the sun matches up. If a vessel like this undocks from Ceres at a high speed relative to the planet it probably goes into some weird elliptical orbit it can't really get out of. But it could be a beneficial thing for far future craft or objects aimed at leaving the (inner) solar system...

Here, there's several simulations of a breaking space elevator. There's some whipping back and forth, but the end, is a wrap-round:

http://gassend.net/spaceelevator/breaks/

Those are pretty cool. That's sort of what I'd expect happening too (although I'm glad someone did the calculations because I would not be sure by a long shot): the cable gets pulled down by its own weight, at which point its higher absolute velocity starts translating to a higher angular velocity, which makes it wrap around the earth in the direction it was moving in.

It's kind of counter intuitive, at first glance I'd somehow be expecting the elevator cable to slow down and wrap in the opposite direction, but that makes no sense and is based on experience with small objects in an environment full of air that has not matched velocity with the object.

[factotum]

Every part of the tether has the same speed in radians per second - but the top is moving (in miles per hour) at a different tangential speed

https://en.wikipedia.org/wiki/Speed#Tangential_speed

from the bottom. Presumably that's why the "broken elevator" simulations depict the cable not collapsing vertically downward.

Wouldn't orbital mechanics have something to do with it as well? Let's treat each part of the tether as a separate object. In that case, all the pieces between the ground station and the geostationary satellite are travelling at less than orbital speed for their altitude, so once the tether breaks they will start falling into lower orbits. Now, as an object drops down into a lower orbit its speed increases, so wouldn't that cause a differential speed that would cause the tether to bend?

I really need to get a copy of Universe Sandbox so I could simulate that...

[hamishspence]

Wouldn't orbital mechanics have something to do with it as well? Let's treat each part of the tether as a separate object. In that case, all the pieces between the ground station and the geostationary satellite are travelling at less than orbital speed for their altitude, so once the tether breaks they will start falling into lower orbits. Now, as an object drops down into a lower orbit its speed increases, so wouldn't that cause a differential speed that would cause the tether to bend?

That might work as well. Differential speed, going by the modelling shown, would also rip the thing apart, at least toward the end of the process.

Regarding the size of these cables - I think the point is that they're freight elevators - designed to make it easy to transport thousands of tonnes of material into space (and thousands of tonnes of lunar or asteroid mined material back down to Earth)? They are a tool in the "industrialization of space".

[Mechalich]

Yes. How do you (Grey Wolf) expect "technological innovation" that would help the human race live on places other than the Earth to be invented if we don't start "small" with things like space stations and Mars colonization?

Simple answer: transhumanism.

If at some point in the future the technology is developed to make uploading the human mind into a quantum computer or other substrate viable then suddenly all of the space travel problems that are related to maintaining homeostasis in 50-80 kg of living mammal disappear. Transporting robot bodies or just server boxes around the universe is orders of magnitude less complicated than hauling actual people from place to place. The technical challenges of building a starwisp to go to Alpha Centauri are significantly less daunting than those of getting humans to any point in the solar system beyond Mars, and if you had digital uploading you could actually put humans in the starwisp.

In terms of making human life beyond the earth viable you cannot avoid considering changing the 'human life' part of the equation and it may in fact be the part the most amenable to change.

[Grey_Wolf_c]

The problem with "we have plenty of time" is "If not now, when?"
Looking at climate change as a comparison, will we decide to avoid a far off astronomical danger in time to develop all the technology needed to survive that danger?

I was answering a post who claimed that we had to leave Earth before the death of the Sun. With a seven billion year deadline, I really don't think that leaving "right now" is at all a concern.

Yes. How do you (Grey Wolf) expect "technological innovation" that would help the human race live on places other than the Earth to be invented if we don't start "small" with things like space stations and Mars colonization?

As Rakaydos says, we have to start somewhere.

My point is not pro or con Mars colonization. My point is to shed a tiny light on the need to develop space fairing technologies. One can argue a thousand points, but in the end it's a biological need if we chose to survive.

You are engaging in a fallacy that the only way to develop the technology needed is colonize other planets. Such argument is, in a word, nonsense. Like multiple people have pointed out, we'd be better served colonizing the deserts/poles/sea bottoms of the Earth first. It will teach us far more, far safely, far cheaper, with far more immediate benefits.

Regarding the bulk of the elevator - a point is made in the original Arthur C. Clarke article:

http://spaceref.com/space-elevator/t...-c-clarke.html

that the weaker the material, the more massive the tether needs to be - and that it will taper outwards - being much wider at the top, than at the ground. It also talks about "megatons (millions of tons) of material"

Seriously, stop relying on sci fi writers, and maybe start looking at the actual engineering designs by NASA and others. The current design for the space elevator tapers at both end, not just the ground, with the widest point midway.

If he talks about "megatons" of material, then he too, like Nova, is wrong. NASA designs calls for a 750 ton cable. Now, taking actual engineer's word for it, it would have less density than a feather. The terminal velocity of a feather in Earth atmosphere is not sufficient for it to cause any kind of damage, no matter how many feathers you dump at once from high orbit, nor how it wraps around the Earth.

The elevator's utility is partially based on its operational lifetime and maintenance costs, which may encourage designers to use thicker cables. Destruction won't necessarily be limited to the surface, especially for objects in low orbits that're unfortunate enough to get hit. My math may be off, but for a km-long section of a cm-wide Martian elevator with a mass of ~125 kg, terminal velocity is ~36 km/s. Since aerosynchronous orbit is ~20 megameters above the surface, and gravity drops as altitude increases, it seems that no single section will reach that downwards velocity before impact. While I can't speak on the cable's resistance to atmospheric friction, at least multiple megagrams to gigagrams of cable would hit the Martian surface at hypersonic speed if it doesn't burn up.

Your math is wrong, because you don't account for the atmospheric friction, which is crucial when dealing with an object with the density and air resistance of a feather.

As to your pet idea, given that MEXTs are just space elevators but with massive hooks that you need to attach to, I don't see how they are no less subject to sabotage, and therefore would cause the same or more damage (since they'd have to have much higher density materials) if attacked. And given NASA's record of trying to capture fast moving objects with hooks, I find the actual practical problem of MEXT to be significantly higher than that of a Space Elevator. Not that I can really compare, since I'm having trouble even finding realistic assessments of the likely issues of MEXTs.

Grey Wolf

[hamishspence]

Seriously, stop relying on sci fi writers, and maybe start looking at the actual engineering designs by NASA and others. The current design for the space elevator tapers at both end, not just the ground, with the widest point midway.

If he talks about "megatons" of material, then he too, like Nova, is wrong. NASA designs calls for a 750 ton cable.

That would be the prototype space elevator - the "Redstone rocket" to the serious industrial space elevator's "Saturn V". Just because the first space elevator might be that big, doesn't mean that all future space elevators are constrained to that size.

Bova was probably using A.C. Clarke's concept, as the basis for his own version.

Now, taking actual engineer's word for it, it would have less density than a feather. The terminal velocity of a feather in Earth atmosphere is not sufficient for it to cause any kind of damage, no matter how many feathers you dump at once from high orbit, nor how it wraps around the Earth.

A feather-density object that weighs many tons, and is hitting target at many kph, is going to do damage. Low density, does not mean harmless.

[Grey_Wolf_c]

That would be the prototype space elevator - the "Redstone rocket" to the serious industrial space elevator's "Saturn V". Just because the first space elevator might be that big, doesn't mean that all future space elevators are constrained to that size.

Bova was probably using A.C. Clarke's concept, as the basis for his own version.

There is no certainty there exists a material that could create that design. NASA's 750 ton design is the final design. There isn't a bigger one after it, AFAICT, just more of the same.

A feather-density object that weighs many tons, and is hitting target at many kph, is going to do damage. Low density, does not mean harmless.

It also doesn't mean it's harmful. I've put forth my numbers. You have not actually made your case, other than quoting sci fi books as if that would be authoritative. But they are not.

GW

[hamishspence]

You have not actually made your case, other than quoting sci fi books as if that would be authoritative. But they are not.

A. C. Clarke knows more about what would be needed than most sci-fi writers, at least - being virtually "the father of the space elevator".

[Bohandas]

Ben Bova's Solar System series (Mercury novel) depicted "falling space elevator" as extremely devastating - because, while it's low density, it's huge. The term "skytower" is used for it.

Some elevators might burn up:

https://en.wikipedia.org/wiki/Space_elevator_safety

but this one was on the order of 100 or more metres in diameter. And that might have been at its thinnest point.

The end coming down from nearly 25000 km up, would also have more time to accelerate to dangerous speeds, with air resistance not being able to mitigate those speeds enough. (The end above the geostationary platform, when the whole thing is severed, would go spinning off into space.)

Relevant bits from the novel:

Although buckyball fibers are lighter in weight than any material that is even half their tensile strength, a structure of more than thirty-five thousand kilometers' length weighs millions of metric tons.
The skytower wavered as it tore loose from the geostationary platform, disconnected from the centrifugal force that had pulled it taut. One end suddenly free of its mooring, its other end still tethered to the ground, the lower half of the tower staggered like a prizefighter suddenly struck by a knockout blow, then began its long, slow-motion catastrophic collapse.
...
The lower half of the skytower slowly, slowly tumbled like a majestic tree suddenly turned to putty. Its base, attached to the rotating Earth, was moving more than a thousand kilometers per hour from west to east. Its enormous length, unsupported now, collapsed westward in a long, long, long plunge to Earth.
...
Slowly at first, but then with ever-increasing speed, the skytower's lower half collapsed to the Earth. Its immense bulk smashed into Ciudad de Cielo, the tethers at its base snapping like strings, the shock wave from its impact blowing down those buildings it did not hit directly. The thunder of its fall shattered the air like the blast of every volcano on Earth exploding at once. Seconds later the falling tower smashed down on the northern suburbs of Quito like a gigantic tree crushing an ant hill. The city's modern high-rise glass and steel towers, built to withstand earthquakes, wavered and shuddered. Their safety-glass facades blew out in showers of pellets. Ordinary windows shattered into razor-sharp shards that slashed to bloody ribbons the people who crowded the streets, screaming in terror. Older buildings were torn from their foundations as if a nuclear explosion had ripped through the city. The old cathedral's thick masonry walls cracked and its stained glass windows shattered, each and every one of them. Water pipes ruptured and gas mains broke. Fire and flood took up their deadly work where the sheer explosive impact of the collapse left off.
And still the tower fell.
Down the slope that led to the sea, villages and roads and farms and open fields and trees were smashed flat, pulverized, while the shock wave from the impact blew down woodlands and buildings for a hundred kilometers and more in either direction, as if a giant meteor had struck out of the sky. A fishing village fell under the shadow of sudden doom, its inhabitants looking up to see this immense arm of God swinging down on them like the mighty bludgeon of the angel of death.
And still the tower fell.
Its length splashed into the Pacific Ocean with a roar that broke eardrums and ruptured the innards of men, beasts, birds, and fish. Across the coastal shelf it plunged and out beyond into the abyssal depths. Whales migrating hundreds of kilometers out to sea were pulped to jelly by the shock wave that raced through the water. The tsunami it raised washed away shoreline settlements up and down the coast and rushed across the Pacific, flooding the Galapagos Islands, already half-drowned by the greenhouse warming. The Pacific coast of Central America was devastated. Hawaii and Japan were struck before their warning systems could get people to move inland. Samoa and Tahiti were hit by a wall of water nearly fifteen meters high that tore away villages and whole cities. People in Los Angeles and Sydney heard the mighty thunderclap and wondered if it was a sonic boom.
And still the tower fell, splashing all the way across the Pacific, groaning as part of its globe-girdling length sank slowly into the dark abyssal depths. When it hit the spiny tree-covered mountain backbone of Borneo it snapped in two, one part sliding down the rugged slopes, tearing away forests and villages and plantations as it slithered snake-like across the island.
The other part plunged across Sumatra and into the Indian Ocean, narrowly missing the long green finger of Malaysia but sending a tsunami washing across the drowned ruins of Singapore. Along the breadth of equatorial Africa it fell, smashing across Kenya, ploughed into the northern reaches of Lake Victoria, drowning the city of Kampala with a tidal wave, and continued westward, crushing cities and forests alike, igniting mammoth forest fires, driving vast herds of animals into panicked, screaming stampedes. Its upper end, still smoking from the titanic electrical discharge that had severed it, plunged hissing into the Atlantic, sinking deep down into the jagged rift where hot magma from the Earth's core embraced the man-made structure that had, mere minutes earlier, stood among the stars.
Across the world the once-proud skytower lay amidst a swath of death and desolation and smoking ruin, crushing the life from people, animals, plants, crushing human ambition, human dreams, crushing hope itself.

http://lesswrong.com/lw/k9/the_logic...lization_from/

[Grey_Wolf_c]

A. C. Clarke knows more about what would be needed than most sci-fi writers, at least - being virtually "the father of the space elevator".

Unless he also somehow knew more about it 50 years ago than NASA engineers today, I'm still not going to take his word over theirs.

GW

[hamishspence]

While it's possible that the fictionalized versions are impossibly big - I was under the impression that, eventually, they would need to be big, to support a thriving space industry.

[Grey_Wolf_c]

While it's possible that the fictionalized versions are impossibly big - I was under the impression that, eventually, they would need to be big, to support a thriving space industry.

This is no different from saying "Isaac Asimov had FTL drives. Therefore, eventually, they would need to exist, to support a thriving space industry."

Clarke, Asimov et al are not constrained by physical realities. Clarke was able to assume a material capable of holding up its own weight even when it weighted megatons. But since we don't know of any such material, saying "if the Space Elevator was made of unobtainium and then fell over it would destroy Earth, therefore all Space Elevators are dangerous" is a non-sequitor.

Grey Wolf