Lately, I’ve been having these odd dreams in which I’m traveling to Quito, where I’m expected to take a ride in the space elevator. I’m oddly anxious about it, and I don’t know why, and I don’t board the silly thing. The End.
Anyway, this morning I discover that Angela Collier has a video about space elevators, and she dismantles the concept with math and engineering, which was very satisfying.
Very convenient. Next time this dream pops up in my subconscious playlist, I’ll just dismiss it and say it’s not possible, go away, and get back to that nice dream where I can talk to spiders.
Ms Collier certainly has a knack for the annoying. During another video of hers, she was playing a computer game (totally unrelated to the subject being discussed), her progress in which was displayed on part of the screen. I couldn’t stand more than a couple of minutes.
Here, she is writing totally illegible stuff on half the screen, while demonstrating she did no prep for her demo.
How does anyone get through her videos?
God’s death, what would anyone use a “space elevator” for anyway? Accessing and servicing whatever happens to be in a geosynchronous orbit right near the elevator’s terminal? Getting to a posh hotel in said orbit?
I guess it could hoist people and freight to/from vehicles going to/from other planets or asteroid colonies…but if it was really good for such a purpose, I suspect it would soon become a bottleneck.
And don’t even get me started on what happens if the main cable is broken at any point between the endpoints…
In Jungian lite parlance synchronicity is meaningful coincidence. You’re creating the meaning though, not the universe telling you something. When I was taking human anatomy I was also watching ER. Imagine how many meaningful coincidences I was having with that sort of experience!
Also why do actors like William Macy appear in other Chicago based shows? Happened with Jeremy Allen White too. What is the universe trying to say?
I always loved science fiction films depicting space elevators, go into space faster than in a skyscraper elevator going subbasement to penthouse.
Reality, were one to exist, figure it’ll take about a week.
The reality of a space elevator, well, they’re right up there with room temperature fusion. All one needs is a lot of unobtanium and they’re easy to make, as we’ve nothing that could support even a ribbon of that mass to orbital altitude of any type of orbit.
Anyone says they’re building a space elevator, I’ll happily ride my pet unicorn over, then fly my pegasus to inspect it.
Dammit! They’re off cavorting with my pink elephant again, I’ll never get any work out of them!
Oh, and what happens to satellites or space-stations in equatorial orbits below geosynchronous? Would they all have to change their courses to avoid hitting that damn cable? Or would the cable be made to wobble a bit, so those satellites and space stations could avoid hitting it — provided all their orbits were properly synchronized with the cable’s wobbling (per Kim Stanley Robinson’s speculation in his “Mars” trilogy)?
Ben Bova did a sci fi story many years ago (a quick goog suggests it was the book “Mercury”) where a space elevator collapsed. He calculated the kinetic energy of this thing falling to Earth would be as devastating as a nuclear war.
I never “got” space elevators either.
@ Rob Grigjanis
The stuff is math, and I enjoyed her working the problem instead of just giving the answer.
The whole video was engaging and entertaining.
Interesting video, definitely aimed at the mathematically challenged. Makes me want to go re-read Clarke’s The Fountains of Paradise to see how (and whether) he dealt with any of these questions, which are legitimate.
The one thing that jumped out at me was her use of the term ‘on’ rather than ‘over’ when expressing fractions, as in ‘m on 2’ rather than ‘m over 2’. Is this common? is it a regional thing? I have never heard anyone use that term before, so I’m curious.
Hi, PZ,
With all due respect to Dr. Collier, she doesn’t “dismantle” anything, and makes more than a few erroneous assumptions that she incorporates into both her critiques and her calculations, namely basing them on pop-science fluff-pieces rather than the actual design and feasibility studies that have been around for decades.
To say nothing of her crediting the idea to Heinlein when everyone knows it was Clarke.
Silentbob @6: I know the stuff is math, and it’s actually quite simple math (for a cable of constant mass per unit length) if you know calculus. I’m just amazed that anyone can see what she was writing. Did you actually follow what she said and wrote? Could you reproduce it?
I did not expect Collier to be so polarizing.
Just curious: is the disagreement about Collier’s style, or do you think space elevators are actually feasible?
I’d say she’s more annoying than polarizing. She’s smart and witty, but a bit too unrehearsed and rambling for my taste.
In “Old Man’s War” John Scalzi has the main character ride a space elevator to begin his journey. He points out how they are utterly pointless and quite possibly one of the worst, least efficient ways to get to orbit. He posits that the only reason to build one is as a flex to show the world just how rich and powerful you are.
A “skyhook” instead of en elevator would at least be feasible.
Reducing the necessary delta-v would be an immense game changer.
drken: the only orbit it will get you to at all is geosynchronous, which is about 24,000 miles up. For any lower orbit, you’d still be stuck with space shuttles or big dumb rockets.
PZ @11: I couldn’t get far enough into the video to get her take on feasibility. She’s just bloody annoying. Dunno about feasibility. Carbon nanotubes are supposedly in the right ballpark for tensile strength, but there are so many other issues. So if she concluded they were unfeasible (as you suggest), I’d tend to agree.
Silentbob: I dunno about all that, but if the cable were to break just below where it meets the terminal station, it would fall to Earth, and wrap itself around the Equator at least once. Basically what you’d see is a vertical sheet of fire cutting all the way around the planet, and leaving a straight line of scorched earth across Africa, South America and South Asia. That’s assuming the cable comes down straight and neat — it’ll look even weirder if it doesn’t.
And if the terminal station comes down with it, that’ll be even more destruction, depending on how big the structure or hollowed-out asteroid is…
Her “style” doesn’t bother me, although I do agree with Erlend Meyer that she rambles a tad. But it’s not like she’s the only science communicator on YouTube who does so.
As for space elevators being feasible, of course they aren’t. Yet. I mean, used to be a time when man-made heavier-than-air flight wasn’t feasible. Used to be a time when getting into space period wasn’t feasible. Lots of things weren’t feasible at one point in time, but later on became so. I do agree, though, that anyone claiming that technology will be up to the job inside of half a century is selling vaporware.
All Dr. Collier’s objections and warnings and many more she never touched on about have been addressed in various studies, along with possible solutions and the usual caveats of “more study is needed”. Overall, I would say that one should approach the above video with some constructive skepticism, and that like Dr. Hossenfelder, she would benefit from doing more research on a subject before opining on it.
Finally, on a personal level, I sure AF don’t want it to be a private venture. If space elevators will ever exist, I hope they’d be public infrastructure cooperatively built and maintained by multiple nations.
Following on to #17: Come to think of it, the cable is stationary relative to the Earth (as in, it’s circling the Earth at the same speed as the Earth’s surface is turning); so maybe it would fall straight down all on and around the ground terminal. So, short term, there should be a decent warning so everyone in the ground terminal could evacuate (not sure how far they’d have to go though). And as long as the elevator carriage has parachutes and heat-shielding, maybe everyone in it at the time might survive too…maybe…
I always loved science fiction films depicting space elevators, go into space faster than in a skyscraper elevator going subbasement to penthouse. Reality, were one to exist, figure it’ll take about a week.
Which means that, while a space elevator would save you the cost of launching massive rockets and fuel into space along with your payload, you’d still have to launch a small hotel, restaurant and maybe fitness-center complex into space. So, not sure how much you’d really save energy-wise…I’m sure OceanGate would offer to build it all out of the lightest possible composite materials — what could possibly go wrong?
Raging Bee @19:
Once it starts falling, the Coriolis force comes into effect, pushing it to the east, and increasing as it falls.
Rob: So, the spectacular round-the-world falling sheet of fire then, give or take the hotel-turned-meteor somewhere along the way. Got it.
While I have complaints about Dr. Collier (both in style and fact-checking), I would forgive her for the improvised math. I think it’s just really hard to present math to a mixed audience, including both physicists and people who find calculus scary.
The falling space elevator was fun in Halo ODST.
Actually, my biggest thing about the space elevator? Not even the physics. Just human nature:
It is ground zero for a terrorist attack.
That aspect of Sagan’s Contact was probably the most accurate thing in the entire film: that as soon as we truly have a scientific ‘marvel’ that represents the best of what science can offer, it’ll be attacked. Either it’ll be a grand statement by American religious luddites, or it’ll be a grand statement by the more known eastern terrorism against the West’s progress in general.
@25: “Really nice space elevator you got there. Be a shame if something happened to it…especially if it happened at some point that’s really hard to monitor in real time…which is to say, about 99% of the length of the cable…”
Also @25: A terrorist attack on a space-elevator system would threaten far more people than just the poor sods in the flying hotel complex at the time. Unless, of course, they break the cable far below where the flying hotel is, in which case everyone mostly has to worry about the flying hotel being dragged off into space, along with the upper terminal station. #Bone4Tuna intercepting and rescuing that lot.
This is looking more and more like a great plot for a totally insane disaster movie…
And here’s yet another problem with a space-elevator: you can’t realistically test any part or subsystem of it in advance. The only way to know whether your design, and the materials you’ve chosen to use for it, will work, is to build the whole thing for real and start it up with freight and crash-test dummies. And find some way to monitor all 24,000 miles of that cable, at all levels of atmosphere and beyond, for signs of wear or damage. What’s the cable gonna be made of again? How will it react to oxygen, pollutants, ozone, clouds, unfiltered solar radiation, other particles…?
Let’s see now, geosynchronous orbit is already in use, why haven’t terrorist attacked the satellites up there or even the LEO ISS, which is at least a wee bit more conveniently closer?
So, let’s be more realistic then, maybe at an altitude that CERN was attacked and destroyed at. Oh wait, expensive high tech CERN, which many thought would eat the world still hasn’t been attacked.
Reality, a space elevator would be at risk from space debris (natural and manmade, Earthlink anyone?), aircraft and attack on the ground, where one’s talking about a rather robust sized building.
Raging Bee, some wrote of carbon nanotubes, which aren’t exceptionally reactive, but we’ve yet to make them of any kind of length needed to make a yardstick, let alone an orbital tether. So, the most likely construction would be out of unicorn fart derived unobtainium.
As for monitoring for condition and damage, that can be done by “cars” in transit easily enough, although damned if I know how this unobtainium thingie would get fixed in situ. Maybe with Harry Potter’s whammy stick?
More importantly, how could one, were one to get unicorns to fart enough to get enough unobtainium, put the damned thing in place – especially given the incessant launches of Starlink LEO birds?
@29: Why would anyone bother with a physical attack on satellites or CERN, when a cyber attack is so much simpler and cheaper? And certainly has happened.
Hackers attack CERN (fortunately not with much success)
US and its allies say Russia waged cyberattack that took out satellite network
This is why I’m building a space fountain instead.
I very much doubt that; if that’s the message that was taken from her video, either someone is not a good communicator or someone is not good at grasping communications.
The concept itself is sound enough; as noted above, popularised by Clarke.
The technology, of course, is not currently at hand, nor are the pragmatics.
So, the obstacles are geopolitical, financial, and technological — but not not conceptual.
In short, unless she has rewritten physics and/or math, all she can dismantle is the idea that it’s currently possible to actually do it.
—
Huh, just double-checked, and according to Wikipedia:
“In 1979, space elevators were introduced to a broader audience with the simultaneous publication of Arthur C. Clarke’s novel, The Fountains of Paradise, in which engineers construct a space elevator on top of a mountain peak in the fictional island country of “Taprobane” (loosely based on Sri Lanka, albeit moved south to the Equator), and Charles Sheffield’s first novel, The Web Between the Worlds, also featuring the building of a space elevator.”
So there we go, Charles Sheffield also deserves credit for the popularisation.
You didn’t expect a woman to be polarizing? I thought you had been around longer than that? :P
acroyear @25:
If one actually existed, it would be about as likely that terrorists could take it down as that they could take down a mountain. Most people just can’t grasp the scale or strength of such a thing.
dangerousbeans, nothing to do with being a woman, everything to do with the claim that she has “dismantle[d] the concept with math and engineering”.
If one actually existed, it would be about as likely that terrorists could take it down as that they could take down a mountain. Most people just can’t grasp the scale or strength of such a thing.
Really? You actually think a space-elevator would be as hard to destroy as a mountain? That’s silly even for you.
Most people just can’t grasp the scale or strength of such a thing.
Again, really? Read our comments on this subject and tell us what we failed to grasp about “the scale or strength of such a thing.”
Rob Grigjanis @1:”Ms Collier certainly has a knack for the annoying.”
It’s DR. Collier
I dunno, maybe I should dismantle the concept of a starship with math and engineering.
See how many think that I actually did anything, as there are just as many starships as there are space elevators.
Dangerousbeans #33: “You didn’t expect a woman to be polarizing? I thought you had been around longer than that? :P”
Indeed. Exhibit number one: Rob grigjanis calling her “Ms Collier” is the most belittling thing I have read here for a while.
Let’s see now, geosynchronous orbit is already in use, why haven’t terrorist attacked the satellites up there or even the LEO ISS, which is at least a wee bit more conveniently closer?
First, a space-elevator has parts on the ground, which would be more vulnerable to a non-state-terrorist attack than something that’s all up in high orbit.
And second, if a space-elevator ever comes into widespread use, then terrorists would have more chances to smuggle operatives or bombs into the freight car or flying-hotel-carriage, and thus bring destruction to the upper level as well as ground level.
(And speaking of freight vs. passengers, would a space-elevator be able to handle both passengers and freight?)
So, the most likely construction would be out of unicorn fart derived unobtainium.
Oh please — there’s no way we’d get enough unicorns to do enough farting to produce enough unobtanium for a space-elevator! That’s totally unrealistic! /s
(Funny thing, wzrd1…your spelling of “unobtanium” got redlined, but mine didn’t. Not sure what that means…)
scuba @37: I stand corrected. I find Dr Collier annoying. But when did ‘Ms’ become demeaning?
Vimes@8–
[1] Collier was directly quoting the textbook when she mentioned Heinlein. She even held the text up to the camera as she read it. She was not commenting on its historical accuracy.
[2] The textbook refers to the concept of a ‘skyhook’ satellite, which is not a space elevator, but the math for one concept is a good jumping point for talking about both.
[3] The textbook didn’t say Heinlein invented the ‘skyhook’ concept, just that he wrote about it. Which he did in Friday, 1982.
[4] Clarke didn’t invent the idea of a space elevator. That was Tsiolkovsky, in 1895. The counterweighted version popular today is from Artsutanov, 1959.
[4] In fiction, Clarke shares the honours with Charles Sheffield for writing the first fictional account of an engineered space elevator (Fountains of Paradise and The Web Between the Worlds published nearly simultaneously in 1979).
[5] Even that wasn’t the first fictional account. Brian Aldiss has a weird organic space elevator in Hothouse from 1962.
If you put most of the weight in the counterweight at the end, then you’ve reduced it to the problem of geosynchronous orbit at about 36000km. You could power it with electricity from the ground but you are pulling stuff up by pulling down on the weight on the end of it. This accelerates the weight so you have to have a rocket pushing it to keep the weight from moving faster then the earth turns, at the same time you have to speed up the cargo. You can’t compensate by using a bigger weight because you had to get the weight in space with a rocket. The point is the rotation speed may be constant along the length of the cable but the radius is growing as you go up and the speed of the cargo increases from about 1,000mph at the earth’s surface to 25,000mph at the end of the cable for the numbers Dr Collier gives or 9000mph if at geosynchronous height. At a minimum, you’re playing crack the whip at 9000mph.
Actually, what AC Clark is really remembered for is the invention of geosynchronous satellites.
Raging Bee @36:
As unlikely to be achieved is what I actually wrote, not as equivalently difficult which is how you interpreted it.
The very point: you clearly have no apprehension of the scale of one of those things should they actually exist and the sort of forces involved.
Many of the comments on this subject is what I wrote about: the misapprehension of the magnitudes of mass and forces at hand should this hitherto-imaginary structure actually exist, as evinced by the ignorant “terrorist attack” claim.
You reckon terrorists could destroy it, much as they destroyed the Pentagon.
How? With nukes? Rogue satellites colliding at orbital speed? Grey goo?
(People with angle grinders at the bottom? ;) )
I’ve never understood space elevators. I can understand how one might build one, but I can’t understand how you’d use it. I found a fairly good write up at:
https://users.wpi.edu/~paravind/Publications/PKASpace%20Elevators.pdf
but it still doesn’t explain what happens when I step into the elevator car. The paper does the standard calculus thing of looking at the forces at each point on the cable. There are up forces and down forces that balance out. I read through the whole thing, but I still have no clue why the whole space elevator doesn’t move down when I step into the elevator and add my weight to the downward forces. F=ma. What am I missing? Mass? Nope, got that.
Another thing to calculate is the surface area of the cable times 100mph-300mph winds in the atmosphere. Viscous drag makes for some really weird cable properties at these kind of lengths.
And oh god, can you imagine getting stuck in a space elevator with Elon Musk or even just some space elevator enthusiast?
As unlikely to be achieved is what I actually wrote…
And if terrorists have motive to target it, that makes it more likely that they will succeed in destroying it.
How? With nukes? Rogue satellites colliding at orbital speed? Grey goo?
All of those options, plus more, probably minus the grey goo, are quite plausible. Anything that could either break the cable or blow up or punch a hole in the passenger-carriage/hotel/restaurant would do.
…but I still have no clue why the whole space elevator doesn’t move down when I step into the elevator and add my weight to the downward forces.
That is a problem, so whatever counterweight is used to keep the terminal station from being dragged down, would have to somehow adjust on the fly to changes in the total weight of the carriage plus whatever/whoever is in it. And I have no idea how that would work, aside from continuous adjustment via rocket-bursts; which is kinda what a space-elevator is supposed to avoid.
And oh god, can you imagine getting stuck in a space elevator with Elon Musk or even just some space elevator enthusiast?
Look on the bright side: you can add more content to your “elevator speech.” (As for the space-elevator-enthusiast, just start talking about how thin and delicate the cable looks — they’ll shut up right quick. If the sight of airplanes flying right by them doesn’t shut them up first.)
Just a couple of comments, based on the whole discussion, and in no particular order:
Those talking about the cable falling… I haven’t done the math, but isn’t it in orbit? There was a movie recently where a plot point was satellites dropping things that then fell and created devastation like bombs, but people pointed out ‘no, the things inside the satellites have the same angular velocity as the satellites – they’re in orbit, and would need to be dramatically decelerated in order to fall out of orbit’. The same seems to apply to the space elevator. It’s in orbit and would need a velocity change to fall, not just to be cut. Perhaps cutting it near the centre of mass so that the part below that (closer to the Earth) now has an angular momentum wrong for the orbit of the new centre of mass… I dunno, I’d need to be convinced (with calculations) on the ‘falling’ point.
The conversational style in this video is a style. Some science communicators present like lecturers, others present like we’re sharing coffee and scribbling equations on the back of an envelope. I enjoy the latter activity in ‘real life’ and also enjoy that as a style of YouTube presentation. Your mileage may vary, and there are definitely people who offer a prepared lecture.
The materials science pieces is the toughest piece of this. The tensile strength of diamond is far from sufficient for the task. Perhaps the C-C bonds in carbon nanotubes are significantly stronger, but my chemistry degree isn’t offering convincing reasons they would be. If the material of the cable cannot withstand the forces, the whole argument is moot. Hence the references to ‘unobtainium’, I assume. This is also relevant to the ‘terrorist’ argument: if there is a thick cable – think ‘base of a skyscraper’ thick or thicker, not a rope – and it’s made of a material very significantly stronger than any known material we have now, what are you damaging it with?
Bugger: I numbered my 3 points but the software removed the numbers and didn’t leave paragraph spaces. I promise I wrote for easier reading!
Raging Bee:
Ah well, no point arguing about it, since it’s predicated on a nonexistent thing.
John Morales @46: The terrorist attack on the Pentagon on 9/11 failed to inflict significant damage because the plane didn’t impact it directly but hit the ground in front of the building. The Pentagon was also undergoing renovations at the time so it only had 800 out of a typical 4500 staff in it, resulting in much fewer casualties than would have occurred had it been fully staffed. Furthermore, it’s significantly easier to harden a building that’s only 5-6 stories tall against an attack (ironically, the plane struck on the only side of the Pentagon that had actually received that protection, which was what the previous renovations were for) than it is to harden a structure that’s 24 thousand miles tall. Especially if a hypothetical space elevator ends up being built by a low-bidding private company that skimps on redundant safety features to save money.
The mountain being indestructible argument does actually have weight. Remember the WTC bombing? Did some damage to the parking deck and one column, which was repaired rather quickly.
Compared to the jet airliner attack, which did cause the building to fail.
The cable, being under tension and kept aloft by the geosynchronous station overhead would then snap, causing the geosynchronous station to move away from earth. I believe Clarke himself covered that in one of his fiction books.
That below the break, well, that falls. At jet airliner altitude, no flaming reentry nonsense, but still a fair amount of mass falling from altitude and a very expensive device destroyed.
What movie was it that the protagonist jumped from a space elevator tower after a high power broadcast caused major damage again, parachuted to the ground once into the lower atmosphere, signal turned out to be his father sending it against the desire of earth? That’s actually spot on, as even at LEO altitude, he’s not moving at any velocity at all, orbitally speaking, so he’d basically be in the same velocity profile as the high altitude balloon jumper would be at.
As for being trapped inside of a space elevator with Musk or a space elevator maniac, well, that’s what they make airlocks for.
Oh, my spelling for the unobtainable substance, likely constructed with unicorn farts, well, remember, I am dyslexic and did it phonetically and my dictionary’s open source database is rather weak tea.
Passengers, freight, six of one, half dozen of the other. Never mentioned is powering the lifting device, one would also need a room temperature superconductor or a shit ton of fuel and oxidizer, plus life support. The mass just keeps increasing, the more one adds to what’s required to make a trip survivable. Station keeping might be accomplished by orbital tether gathering energy from shifting geomagnetic fields. While the overall field doesn’t change a great deal, field lines do move about in response to solar field flux interactions. Might be enough to run an ion thruster or ten. Or it’d be playing yo-yo…
So, we’d need at least three brand new technologies that we’re not even close to having yet, despite multiple generations working on them.
Oh, the cyberattack mentioned only wiped the software from the modems, one would think they’d be field upgradable by flashing them. One problem with the press is, a wiped computer is somehow destroyed. No, it’s temporarily inoperable, until it’s reimaged. With flash memory, once it’s reflashed, it’s operational again. BIOS flashing, that is just a bit more annoying, but still doable, as I’ve done it after a borked flash before, but typically requires physical access.
“Never mentioned is powering the lifting device, one would also need a room temperature superconductor or a shit ton of fuel and oxidizer, plus life support.”
Perhaps, though if we were harvesting materials from the asteroids or elsewhere and bringing them to Earth, sending some mass down the escalator could potentially be used to counterbalance sending some up…
@ 53, that’s true all around, as the only thing that actually exists currently are terrorists. No grey goo, no space elevators, no angels on heads of pins.
bravus @51: The upward terminal would be the only part of the system in a stable orbit. The cable would go down from there all the way to the ground, and would not be “in orbit” — it would be moving too slowly to stay in orbit anywhere in that range of altitude below the terminal.
wzrd1 @55: Actually, your spelling might be considered “correct,” because you’d spelled the “obtain” part of “unobtainium” correctly (with the “I”). I guess the final authority on this question would be the “Avatar” script…maybe it’s spelled my way there, and the spellchecker was working from that script?
@ 53, that’s true all around, as the only thing that actually exists currently are terrorists.
Whenever anyone thinks that way, THE TERRORISTS WIN!!!
Pullquotes from https://www.scientificamerican.com/article/space-elevators-are-less-sci-fi-than-you-think/
(By Stephen Cohen on November 25, 2022)
bravus@51–
The elevator is rotating at the same angular velocity as the Earth, i.e. 1 revolution per day. That means the only part of the tower ‘in orbit’ is at geosync height. Anything launched outside geosync height range requires additional thrust to achieve orbit.
An easy way to look at it is that the tether starts attached to the Earth surface. Clearly it is no more in orbit than we are now. (If you are posting from Tiangon or the ISS, I apologise.)
If you like math: the rotational velocity is given by v = 2.pi.r/T, where r is the radius from Earth centre and T is the period. Orbital velocity is given by v = sqrt(GM/r), where G is the gravitational constant and M is the mass of the Earth. If we’re sticking to Earth orbit, then all the variables except r can be treated as constants, and you can see that rotational velocity changes linearly with height, while orbital velocity changes with the inverse square root of height. Obviously the two velocities can only match at one place.
I’ll leave it an exercise for the reader, but if you’re interested you can do a little algebra to find the value of r where orbital velocity equals rotational velocity, plug in the necessary values of G, M, and T, and I bet you can guess the answer that comes out… (hint: these equations use r as height from Earth’s centre not from Earth’s surface).
Not especially, though I was adequate at it.
So, isn’t a velocity a vector and your formula a scalar?
We’re using velocity as a scalar here. You only need to worry about vectorising when you start thinking about direction, which doesn’t matter for calculating simple orbit heights.
I don’t have the time or inclination to watch the video right now, but the requirement that the cable be left untethered at the ground end seems nuts. Why wouldn’t you anchor it to a mountain or somthing?
Gotta think about direction.
“A geostationary orbit, also referred to as a geosynchronous equatorial orbit[a] (GEO), is a circular geosynchronous orbit 35,786 km (22,236 mi) in altitude above Earth’s equator, 42,164 km (26,199 mi) in radius from Earth’s center, and following the direction of Earth’s rotation.”
BTW
The climax of “Provenance” by Ann Leckie (in the narrative universe of the “Ancillary” novels) takes place at an orbital station far up on a space elevator.
John, like I said, you need to vectorise if you’re plotting trajectories. When you’re just calculating the heights of orbits, which was the question asked, then it doesn’t matter.
chrislawson, sure. The difference between geosynchronous and geostationary orbits is not the height.
@2. Raging Bee :
God’s death, what would anyone use a “space elevator” for anyway? Accessing and servicing whatever happens to be in a geosynchronous orbit right near the elevator’s terminal? Getting to a posh hotel in said orbit?
Yes and yes and more – and don’t you read any SF where there’s a lot of fictional space elevators used as mentioned by others already here – sometimes as the main theme of the story and sometimes as more incidental bits of worldbuilding?
Like the already mentioned Clarke classic ‘The Fountains of Paradise’ :
https://en.wikipedia.org/wiki/The_Fountains_of_Paradise
Plus the ones (pretty sure there were a few? Unreliable memory maybe serving?) in Kim Stanley Robinson’s Mars trilogy / series ( https://en.wikipedia.org/wiki/Mars_trilogy ) where at least one was destroyed in a rebellion / war with planetary consequences and in Kevin J. Anderson’s SF novel Blindfold (https://www.goodreads.com/book/show/18604409-blindfold ) where one features and we get a tough fictional journey along it that two of our main characters barely survive (spoilers, sorry) although its not the books main focus or macguffin. They’ve also been shown in a Star Trek ep or two..
Anyhow, toanswer your quetsion – they ar eused for transporting things from planetary surfaces to usually low planetary orbits both passengers and freight and are, of course, speculative science fiction. Written by some pretty smart and scuientifically knowledgable people.
That they are unlikely to be built soon is as true as saying we’re unlikely to build interstellar craft soon and yet that doesn’t mena they’re not theoretically possible or desirable for some stage of our future thinking optimistically here. So, whilst I’ll have to see the video here (I’ve not done yet) before judging I’llbe surprised if anything she says rule sthem out a s atleats theoroetically posisble concepts.
Sigh. I’m sure I had Raging bee’s quote in blockquotes. Guess yáll know where..
Yes and yes and more – and don’t you read any SF ..
Okay. That worked that time.
Clarifying typos :
So, whilst I’ll have to see the video here (which I’ve not done yet) before judging I’ll be surprised if anything she says rules them out as at least theoretically possible concepts.
@ bcw bcw : “And oh god, can you imagine getting stuck in a space elevator with Elon Musk or even just some space elevator enthusiast?”
Three words : Space. Elevator. Music! ;-)
@ wzrd1 :
“All one needs is a lot of unobtanium and they’re easy to make, as we’ve nothing that could support even a ribbon of that mass to orbital altitude of any type of orbit.”
From their wikipage :
Source : https://en.wikipedia.org/wiki/Space_elevator
That last part making a lot of sense to me in terms of at least starting to build some on smaller scales on lower gravity and less populated and vulnerable places where obvs any harm deliberately done or catastrophic failures won’t be as destructive in terms of lives and damage. Whether you could call a “space elevator” built on something as small as a large asteroid a real “space elevator” is another issue but our Moon, Jovian and other gas giant moons, maybe even ones on Pluto & Charon? Obvs thinking far future and when we have much more advanced tech but that kinda goes without saying?
Actually there’s alreasy been this serious (~ish?) idea :
Source : https://www.technologyreview.com/2019/09/12/102622/a-space-elevator-is-possible-with-todays-technology-researchers-say-we-just-need-to-dangle/
Indeed, another article found online notes :
Source : https://www.businessinsider.com/space-elevator-on-earth-moon-2019-9
@ 52. bravus :
Yeah, it seems to happen here annoyingly – & asterisks disappear completely too. A workaround I’ve found / been told is putting a dot (full stop, period) symbol first atleats for asterisks.. Lessee :
.1) Testing
.2) testing
.3) testing
Uh.. Only going to comment on, “Wouldn’t my weight pull it down?” The principle idea here is that the size, weight, and position of the “orbiting” end would be pulled on by the Earth’s rotation, so as long as it was itself heavy enough angular momentum would keep it “pulling” against the earth much more strongly than your weight could effect it. This however presents its own problems, in as much as you have to worry about it not being so much that it just rips loose from the foundation. If anything, too much cargo/people at the orbiting ends would risk ripping it loose from the base, and given how certain people tend to treat basic safety concerns when pushing crazy ideas, or tourist destinations, etc., I don’t see a lot of hope that such people wouldn’t cut corners, and f it up, in many different ways, all of them disastrous.
But, short term, its quite correct that the biggest issue is, “Do we have, or can we even make, something strong enough to survive its own weight at that length, when wiping around a golf ball at the end of it, never mind a space station?” And, so far, the answer is, “Nope.” And this is a major problem, because even of the frame was far more stable, solid, and building like, the cables….
@ StevoR
Watch the video. The first bit is just solving a physics problem, but then she goes into a long discussion of why space elevators – are least on Earth – are batshit.
chrislawson @61:
If you mean orbit around Earth, then no. The period is also variable. You’re describing circular orbits, which can occur for any value of r greater than Earth’s radius. You can use those equations to solve for r, given T, or for T, given r.
If the space elevator is extended a bit beyond the altitude of geosynchronous orbit, the mass of the extra bit will pull in the opposite direction to the weight of the lower elevator so it will ‘anchor’ it in space.
Plus, you can not reach escape velocity at the further end.
NB-you must keep the traffic up and down balanced to avoid oscillations.
.
I am aware the problems are horrific, which is why I would favor a rotating skyhook instead.
Erratum: you can *now” reach escape velocity at the other end.
Spell check sabotaged me again.
Several people have said the space elevator isn’t in orbit. I disagree.
Consider a cylindrical satellite in geosynchronous orbit, aligned radially (one end toward Earth, the other away). How long does the cylinder have to be to cease being a satellite?
Remember, although the end can be tethered to the surface, it doesn’t have to be, because the tension at the bottom of the cable is zero by definition. So it can be seen as a very elongated geosynchronous satellite.
BTW, that also answers the question of why you can yank on the cable (i.e. attach weights at the bottom) without destabilizing the whole thing. The orbit is energetically stable; an attempt to deform it (as long as it’s not too strong) it results in a restorative force.
Anyhow, toanswer your quetsion – they ar eused for transporting things from planetary surfaces to usually low planetary orbits…
Wait, what? Not on Earth they aren’t — the only orbit a space-elevator can get you to is geosynchronous orbit, which isn’t “low” by any stretch. Unless you intend to have other stops hanging on the cable at lower altitudes. But such stops would be useless, because they’re stationary so nothing moving at orbital speeds can dock with them; and if the cable broke above them, they’d fall straight down to Earth with little hope of rescuing anyone on board.
…So Penoyre and Sandford have taken a different approach. Instead of anchoring the cable on Earth, they propose anchoring it on the moon and dangling it toward Earth.
That doesn’t solve the problem, because when you have your payload on the Earth-atmosphere end, it’s being pulled down by Earth gravity. And you’d need a counterweight on the Earth end, otherwise your payload would just fall (and take the cable) down toward the Moon when gets to the Moon end.
Rob: Who says “the tension at the bottom of the cable is zero by definition?” There would have to be tension, for a variety of reasons already mentioned above, so there would be tension all along the cable.
Also, a space-elevator consists of several parts, one of which — the orbital terminal — would be “in orbit,” even it’s partially held there by conflicting forces. And other vehicles would be in stable orbits to dock with the terminal.
@72. I don’t think “we just make the counter weight heavy enough that pulling down to raise the cargo doesn’t matter” is a valid argument. Look at it this way: conservation of angular momentum means that when I haul cargo dm up the the weight D, the initial angular momentum P=wM is the same as the final angular momentum P=w'(M+dM) so w/w’=(1+dM/M). If dM is 1% of the mass of M, the angular velocity of the end of the cable has still changed by 1%, for something going 25000mph, it has just started going off station at 250mph. The cable tension will eventually do something but at the very least you’ve just made a giant oscillator – there’s no obvious dissipation so you won’t return to a stable point. It gets worse with each additional cargo. The suggestion of sending asteroids down has the problem that “catching” something going as fast as asteroids and synching them to the cable end is a huge energy cost.
@80. This is what I mean when I said “playing crack-the-whip” at 25000mph.
@80 just noticed I wrote “D” instead of “M”. Should be:
Look at it this way: conservation of angular momentum means that when I haul cargo dM up the the weight M, the initial angular momentum P=wM is the same as the final angular momentum P=w'(M+dM) so w/w’=(1+dM/M).
@82 And duh, there should be an R in there everywhere. I flunk notation.
Look at it this way: conservation of angular momentum means that when I haul cargo dM up the the weight M, the initial angular momentum P=wMR is the same as the final angular momentum P=w'(M+dM)R so w/w’=(1+dM/M).
@83 and if we are using w as omega, angular velocity, you have some 2 pi’s also.
@84 I’m just going to claim I’m using units where 2piL =1
and L=R
Raging Bee @79: T = 0 at the bottom of the cable is part of the spec for the cable. Someone above linked to this paper;
https://users.wpi.edu/~paravind/Publications/PKASpace%20Elevators.pdf
Consider a person holding on to a high bar. Another person is holding on to the first person’s feet, another on to their’s and so on. So there’s a chain of people hanging suspended. Each of them feels the force of gravity, plus the weight of all the people below them. It’s that extra weight which defines the tension. It is maximum for the person holding on to the bar. But it is zero for the last person in the chain.
I suppose you could shorten the cable radius by a percentage equal to the accumulated weight you are sending up so you compensate for the decreasing speed of the weight as you move mass up with a smaller R so angular velocity stays the same. However the distances you have to pull the weight in are pretty big.
Again, my point is that sending weight up the cable changes the radius of that weight and it’s the same thing as the figure skater extending his arms- the speed of rotation changes from conservation of momentum. At %1 change in weight, the speed changes by 1% and something that goes around the earth once per day is now going around the earth 1% slower than the earth is turning. That is not good for your cable.
A free standing tower is one for which the tension drops to zero at both ends, requiring no restraint at either end to keep the tower in place
Yes, but a space-elevator cable/tower will NEED to be restrained at both ends, otherwise it will all fall to earth because most of it is moving too slowly to stay in orbit. And that’s not even counting the passenger or freight carriage(s), which will move the cable every second they’re accelerating or decelerating on it.
Raging Bee: That’s a fundamental misunderstanding. As I noted earlier, the whole cable is in orbit. You can’t isolate a segment of it and say “it’s moving too slowly”. That segment is connected to the rest of the object. The ends don’t need tethering any more than a satellite needs tethering to stay in orbit.
I can use exactly the same equations as in the paper I referenced to describe the cylindrical satellite I mentioned in #77.
Yea, they would need tethering, because all the parts would be moving in sync with the orbiting terminal. Which means everything below the terminal — meaning the cable and the carriages moving on it between the ends — would be moving too slowly to stay in their positions (because required orbital speed goes up as distance from Earth goes down), and would fall to Earth if not tethered.
Also, what’s the point of having a cable that’s not tethered at either end? The whole point of a space-elevator would be to take people and freight to and from the end terminals. A cable that isn’t tethered to either terminal would be as stoopid as a railroad track that stops one inch short of a station platform.
It’s that extra weight which defines the tension. It is maximum for the person holding on to the bar. But it is zero for the last person in the chain.
No, it’s not zero, because gravity is pulling him downward, and his muscles are being stretched, to some degree, by the opposite forces of gravitational pull and his hand holding onto something.
One last shot, then maybe one of the other physicists can chime in:
Another way to picture this stuff.
You have a space station in geosynchronous orbit. On board are two very long cables.
If one cable is lowered towards Earth, it will eventually destabilize the station. But the direction away from Earth is also ‘lower’; the centrifugal force is larger than the gravitational force in that direction.
So you can balance the station’s orbit by lowering a cable outwards at the same time as you lower one towards the Earth. The different rates of lowering are determined by the forces acting on the cables.
For the near cable to reach Earth’s surface, the balancing outward cable would have to be about 100,000 km long. Bingo, you’ve got your space elevator.
No tethering required, except from the space station.
Experts say a space elevator could be possible with current technology, especially if we were to build it on the moon. A lunar elevator would not have to contend with the Earth’s gravitational force or space debris.
Given that the Moon has only about one-sixth of Earth’s gravity; and given that the Moon rotates a LOT more slowly than the Earth (as in, 28 Earth-days as opposed to 1); I suspect the only good place for a lunar space-elevator would be exactly opposite the Earth. Not sure how high it would have to go…IIRC there was a Lagrange point not that far above that point on the Moon…?
That would be okay for the Moon’s human population. Not sure how much good it would do for Earth…
So you can balance the station’s orbit by lowering a cable outwards at the same time as you lower one towards the Earth.
True. And as I said before, you’d need a big honkin’ counterweight at the end of the “upper”/”far” cable, whose inertia would counter the drag of the “lower” cable plus the carriages moving up and down on it.
For the near cable to reach Earth’s surface, the balancing outward cable would have to be about 100,000 km long.
That would be very unwieldy (and where do you get that particular number?); a counterweight would be much better.
No tethering required…
Yes, tethering would be required. All the parts have to be firmly attached to each other, or none of it works at all. Since the counterweight would be pulling the orbital terminal, plus cable and carriage, “up” away from Earth, the lower end of the cable would have to be attached to something to counter that pull — at least when the carriage isn’t so near the ground, and thus isn’t pulling downward so much.
birger @75: how would a “rotating skyhook” work, exactly?
In the referenced paper. And there goes my last spoon…
Multiple views of the Halo ODST space elevator collapse. Now I’m imagining it strewn across the planet.
https://youtu.be/ew_axJAMr7E?si=MdCMPZtlD1IE47TV
Thanks, Brony…love the insurance ad that preceded it…
@78. Raging Bee :
Fair point. Still getting stuff into orbit and space generally and off the Earth’s surface.
@73. Silentbob : “@ StevoR
Watch the video. The first bit is just solving a physics problem, but then she goes into a long discussion of why space elevators – are least on Earth – are batshit.”
I’m certainly planning to watch that video soon. Started last night but just too tired then. I am intrested to see what Arthur C Clarke missed in this concept given he’s pretty good generally when it comes to doing the maths and working out the science in his stories. Among other decent SF writers who are pretty bright and mathematically & physics-cally capable. I understand the issue of not having sufficiently strong material for Space Elevators (SE’s) but not with the actual physics here so again, be surprised if they previous (& current e.g. noted in #71) writers & engineers missed something obvs that makes the whole idea “batshit” but, yeah. Will see it & see..
Is it possible to use the effect from magnet suspension with a series of bars to extend a cargo into orbit? I know nothing about the physics involved except that of you could use it to grab and lift sections maybe nature and the forces allow (and if acceptable conductors are found).
It seemed a bit safer in principle with my limited knowledge. The opposite end might be a tether to the moon.
@ 102 StevoR
It’s the physical limitations, the relative superiority of rockets, and how absurdly dangerous it would be in practice.
Clarke was not wrong that it’s hypothetically possible.
This thing has to be 36,000 km long, pass through tons of debris in equatorial orbit with a relative velocity many times faster than a rifle bullet, and if it breaks it will catastrophically collapse in a way that will make the World Trade Center look like a child’s game of Jenga.
way late to this thread…
I’m too lazy to try looking up the numbers, but for now, I’m just going to accept the claims that more knowledgeable people have made about even theoretical materials merely being unable to support their own weight, let alone shielding, infrastructure, payloads, etc.
As to WHY? – the importance always struck me as getting mass/objects out of Earth’s (or whatever’s) gravity well, via just energy, and not by chucking mass (reminded of what one of the ?robots? said in Interstellar, something about how humans can’t seem to go anywhere without throwing something away…), especially since it gets exponential, with having to carry more mass to propel the mass that is propelling the payload, etc. In all the SciFi I’ve read of orbital elevators, it was always for supporting/facillitating a massive off-Earth presence of people, with frequent exchange of people/materials on/off planet.
As to how payloads traverse the elevator, I would suspect many authors/etc. have thought far more about this than me, but laziest is energy production at either end, routed along the elevator, that the cargo pod (or whatever) accesses via the elevator, and uses to power whatever climbing mechanism.
More efficient/balanced, but greatly increasing the elevator mass (and needing even more unobtanium magic…), would be to have multiple ‘conveyor belts, so that cargo pods going up can be assisted by opposite cargo pods going down. Would take a long damned time, but that might be less of an issue for transporting stable, regular cargo, like fuel, O2, water, etc. Maybe have separate, powered, very small, powered passenger pods, that pretty much constantly accelerate/decelerate at a comfortable rate to minimize travel time for humans/etc.
Not trying to argue for them, just trying to explain why build them, as I have encountered. It’s pretty much reliant on a significant, constant, human presence outside of Earth, that is also constantly exchanging material with Earth’s surface, and I also don’t really see a reason for that. Space Operas are neat and entertaining and all, but I don’t think they make much sense…
@ StevoR
As she says in the video, the only known material sufficiently light and strong is carbon nanotubes, and the record for the longest carbon nanotube is…
1 meter.
But do yourself a favour and watch the video. You can skip the math and just watch the discussion part.
[interjection]
Silentob to StevoR:
He doesn’t need to, given what he wrote.
He understands nothing currently available is strong enough, but says he can’t see how otherwise it’s … hey, I can just quote! My emphasis.
“
I understand the issue of not having sufficiently strong material for Space Elevators (SE’s) but not with the actual physics hereso again, be surprised if they previous (& current e.g. noted in #71) writers & engineers missed something obvs that makes the whole idea “batshit” but, yeah. ”Right? He doesn’t see how there’s some supposed problem with the physics (the idea), other than the pragmatic technological one.
And he was indirectly implying what I myself wrote directly earlier, @32:
“In short, unless she has rewritten physics and/or math, all she can dismantle is the idea that it’s currently possible to actually do it.”
Hm. The math is not the important part, is it? Just the discussion.
Right. I reckon you’re trying to be crudely condesdending towards StevoR, here.
(You, of course, did not skip the math. That much is supposedly implicit)
PS StevoR, earlier: “So, whilst I’ll have to see the video here (I’ve not done yet) before judging I’llbe surprised if anything she says rule sthem out a s atleats theoroetically posisble concepts.”
I think he already thinks of doing himself the favour, Silentob.
@ StevoR
Discussion part of video starts at 17m:19s
It’s pretty much reliant on a significant, constant, human presence outside of Earth, that is also constantly exchanging material with Earth’s surface, and I also don’t really see a reason for that.
The constant, human presence outside of Earth I can easily imagine. I cannot, however, imagine it ever being profitable to ship significant quantities of raw materials from the surface of one planet to the surface of another — with or without a shiny unobtanium space-elevator (with only a few exceptions, like food from Earth or metals to planets that don’t have them). Maybe from asteroids down to Earth-surface — but even then, it might just be cheaper and easier (and safer) to drop heat-shielded freight-containers with huge parachutes into coastal waters for pickup.
And the people on the planets/asteroids from which said materials are mined, would almost surely want to build their own shops and factories, to make finished goods to sell at higher prices (and of course create lots more jobs locally) and with much lower shipping costs. No population, on or off Earth, would want to remain just a mining-town forever.
Raging Bee@111> nah, when it’s looked at too critically, I don’t think any of it makes much sense. Just that a hypothetical space elevator is something with a huge investment cost, but also a greatly reduced per-use cost for getting mass in/out of gravity wells, so only makes sense in a economies of scale situation, where there are tons of material shuffling on and off Earth on a daily(?) basis or so.
I think that even if stuff were orbital-dropped with heat-shielding, even that heat-sheilding is wasted mass and energy, so eventually, just using solar sails or whatever to gradually steer the cargo around the solar system, and eventually to the counterweight station, and then be gradually trundled down the gravity well, would eventually be more efficient if it’s a frequent/repeatable event. A whole lot of “if”s in there doing a lot of heavy lifting…
Aside from that, just want to mention that I appreciate that you are one of the few regular commenters I recognize (aside from the blog-owners) that I feel are willing to engage in actual discussion and exchange and questioning of ideas. Far too many of the (non blog-owning) regular commenters just flood the comments with contrarian nit-picking.
Thanks, @lochaber. In all honesty, it’s kinda fun nitpicking contrarywise about this space-elevator fantasy. It’s rare that one gets to dig into a single concept or idea and find so many layers of sheer batshit ridiculousness.
And I haven’t even got to Chapter LXXXVII, “How to Save a Carriage Full Of Passengers If the Cable Breaks Above Their Location.” Get your popcorn and strap in for a whole new sub-genre of insane disaster movies! Think “Towering Inferno” meets “Airplane”… (Shorter version: a space-elevator will never make sense for passenger transport.)
Raging Bee @94: Sorry, missed that. My explanation of tension was crap, and I could have made it much simpler.
Suppose a string is hanging from the ceiling. The tension at any point in the string is equal to the weight beneath it. As you approach the bottom of the string, that goes to zero.
In the example I gave, the tension is not zero where the last person is hanging on. It’s zero at the lowest point of their body.
It’s a little more complex than that. There would be tension, but also weight bearing of the lower segment, shearing loads from winds, increased weight bearing on load velocity changes in torsion, compressive loading and tension. One way to help balance the whole hot mess would be with a belt instead of single cable, but then one has to figure out howinhell to maintain and replace bearings and roller surfaces at both ends.
While remembering, one’s loads for a human compartment are basically entire self-contained hotels for several days at a minimum in either direction.
As for orbit below geosynchronous, detach a load with an old fashioned rocket at a higher altitude than the desired orbit, trade altitude for velocity leveraged by the rocket accelerating the load, lower the ellipse as one gains the average of the desired orbit to more circularize it. Far less propellant needed than boosting it out of a sea level gravity well.
Of course, we still don’t have anything with anywhere near the strength needed for reaching up that high. If one finally does acquire that substance, one seized bearing can turn the whole thing into a really expensive broken fishing line, with the one that got away potentially being the anchoring station and the one you didn’t want, the load falling from altitude. Won’t be likely to get reentry flames, as one isn’t coming in at orbital velocities, but instead under normal gravitational acceleration, braked by the atmosphere, keeping it at effectively its terminal velocity at that altitude. Nobody burned up jumping from a balloon at over 18 – 28 miles, despite being supersonic for part of the way and slowing to around 110 mph as they approached the ground. With the second highest jumper missing a suit glove for the entire trip.
While remembering, one’s loads for a human compartment are basically entire self-contained hotels for several days at a minimum in either direction.
Plus a few rocket motors and enough fuel for same, to either boost it into a stable orbit or break the fall to Earth, if the cable breaks above it (depending on its altitude when the break occurs).
And since we’d have to be carrying rockets plus fuel — LOTS of fuel, which would be really dangerous and flammable — that would defeat the whole point of a space-elevator, which was to AVOID carrying all that extra weight up into orbit.
wzrd1 @115:
Depends what height it falls from. A quick calculation based on conservation of energy tells me that an object dropped from a height equal to the radius of the Earth (so, about 6,400 km up the cable) would hit the atmosphere going about 8,000 m/s. I think that’s enough to burn up.
Raging Bee:
https://en.wikisource.org/wiki/Popular_Science_Monthly/Volume_34/November_1888/The_Problem_of_a_Flying-Machine
(Kinda fun, indeed)
Rob Grigjanis @ 117, like the satellite whose nuclear reactor scattered across Canada or Skylab, where NASA got a littering bill for components strewn across a small region?
Remember, atmospheric drag will decelerate the object and most of the fireball of entry is due to higher velocity and adiabatic heating of the air itself. More likely, some surface melting, tons of outer skin being ripped off, but the core of the object would likely remain intact.
Although, construction could be lightened, given less acceleration given by elevator vs rocket racing fuel exhaustion against gaining orbital velocity. Lighter could mitigate against a large massive object impacting, but then one faces many smaller objects falling, so designing the device to fragment into lighter objects would come into play – much as is now done with anything else placed in orbit.
wzrd1 @119: I don’t know what height that satellite was orbiting at. In the example I gave, the initial velocity of the object hardly matters.
Most of the final kinetic energy comes from the difference in gravitational potential energy. The initial potential energy per unit mass is −GM/2R (R is the radius of the Earth, M its mass). The final potential energy per unit mass (just before it hits the atmosphere) is −GM/R.
The difference is −GM/2R. So the kinetic energy as it hits the atmosphere is approximately the negative of that. Plug in the numbers, multiply by 2, and take the square root. That will be the speed as it hits the atmosphere.
PS Skylab orbited at about 400 km, or less than a tenth of the Earth’s radius above ground. The difference in gravitational potential energy is far less than my example.
Re #120:
Rob Grigjanis @ 122, you neglected the fact that the fluid of the atmosphere is of variable density, hence it hitting the atmosphere isn’t a single impulse event, but an increasing density and decreasing velocity protracted event.
There’d be a whale of a difference in stresses upon an ICBM warhead vs an object falling from an orbital tether, all due to a lower velocity on the part of the object falling from the tethered object vs the already nearly orbital warhead.
wzrd1 @123: Yes, I completely neglected lots of factors. I only calculated the speed of the incoming object before it hits the atmosphere. I also neglected its size, shape and composition, which would all affect its fate. Not sure what your point is.
A quick look says an ICBM reaches a max of about 7,000 m/s. The object falling from a point 6,400 km up the cable reaches a speed of 8,000 m/s before it hits the atmosphere, which is low Earth orbital speed (see my #117). Sure, the ICBM has all kinds of stresses on it as it travels through the atmosphere, but how is 8,000 less than 7,000?
BTW, an object falling from near the top of the cable (near geosynchronous orbit) will hit the atmosphere at about 10,000 m/s.
An object falling from near the top of the cable is in orbit, just not geosynchronous orbit, it’d end up in a highly elliptical orbit. It doesn’t suddenly go orbital only at the top. It’s from LEO and below that one can consider it simply as a dropped object.
As I’ve frequently said, orbital dynamics are highly counterintuitive. If one is in orbit and loses a tool, where is that tool going to go? Straight down? Into a shared orbit, to potentially strike the station or ship that you’re in? Magically disappear?
Shared orbit, so one stands a chance that the damned thing might just eventually find itself striking one’s vessel. As altitude shifts the frame of reference toward orbital velocity, one also would, with a released object, find it on a highly elliptical orbit, resulting in a rapidly decaying orbit, the closer to geosynchronous, the more circularized.
wzrd1 @125: Thanks! You raise an important point which I did indeed neglect.
If the object is released from the cable below GEO, it will be, at that moment, at the apogee of an elliptical orbit. Released close enough to GEO, its orbit will not intersect with Earth’s atmosphere, and it will simply keep orbiting.
It’s only at some critical distance below GEO that the orbit of the object released from the cable will be eccentric enough to graze the atmosphere at perigee. The calculation done by Midtek here looks OK, but I’m going to check it at some point;
https://www.reddit.com/r/askscience/comments/5johuj/would_an_object_dropped_straight_down_from_a/?rdt=33804
According to Midtek, the critical distance from Earth’s centre is at 29,800 km, corresponding to height up the cable of about 23,400 km. In terms of Earth’s radius R, we have (as heights above Earth surface) GEO at 5.62R and the critical height at 3.68R.
An object dropped from the critical height will graze the atmosphere at about 9,500 m/s. Below that height, it will enter the atmosphere.
Rob Grigjanis @ 126, thanks! I really didn’t feel like doing the math and you found someone that already wrangled it. Below 29820 km, one grazes increasingly the atmosphere and ruins one’s weekend plans.
Ran into a referenced in your link’s Wikipedia article mentioning terminal velocity of a 30-06 round fired straight up and allowed to fall to terminal velocity (or tower dropped). Around 200 mph. Never got around to calculating that one either, but figured it was in that broad range.
Way back when I was in high school, one local brain trust would go out on Independence Day and fire a .44 magnum into the air. When challenged on his creating a hazard, he proclaimed, “That’s OK, it burns up on reentry”. Explaining that pistols and rifles do not launch suborbital rockets had no effect. So, the police learned where to monitor the following year.
Even back in the ice age, I had zero tolerance for irresponsible firearm usage.
Oh, remembered the movie for a tower based space jump, Ad Astra.
Stayed faithful to physics, then wandered off and got lost.
Other similar, if dubious endeavors:
https://en.wikipedia.org/wiki/Space_diving
From 120000 feet, achieved Mach 1.25. Kittinger did his jump, despite forgetting one glove from 102000 feet. Hand was swollen to three times normal size and was useless, resolved to normal size and function two hours after landing, with no deleterious effects.
I’m willing to bet he never forgot to wear both gloves for anything for the remainder of his life. I sure wouldn’t.
wzrd1 @125:
No kidding. The orbital dynamics of extended bodies (of which the space elevator is an extreme example) are a fucking nightmare, but also really cool.
Way late for this, but I only just watched Dr Collier’s calculation (I’d only got a couple minutes in before).
I’d give her a zero for physics, and a missed opportunity.
She says that the forces acting on a segment dr of the cable at radius r are just gravity and centrifugal, and that they must sum to zero since the segment is stationary (in a rotating frame). So,
ρdr [ ω²r − GM/r²] = 0 (eq 1)
She then integrates this from R to R+L, to solve for L. But the equation is wrong; each segment is also acted on by tension forces above and below, the difference between which we denote as dT. The correct equation is
ρdr [ ω²r − GM/r²] + dT = 0 (eq 2)
The reason her calculation works is that
∫dT = T(R+L) − T(R) = 0
because T(R+L) and T(R) are each zero, since both ends of the cable are hanging free. But T(r) in general is certainly not zero.
At the very least, she should have mentioned tension before saying we can ignore it for the purpose of the calculation, which is only to find L.
But then she waves her hands (literally; she does that a lot) and says we have no materials strong enough. With very little more math, she could have demonstrated that by calculating the maximum tension, at GEO.
@110.Silentbob : “@ StevoR Discussion part of video starts at 17m:19s.”
Thanks for that – watching now.
Angela Collier at the 19 minutes 5 seconds mark :
Hmmm.. I’m sure Arthur C Clarke spent a lot more than ten minutes thinking about Space Elevators before going ahead and writing The Fountains of Paradise in 1979.
The lack of unobtainium material – yet – yeah, we know that’s an issue, and there are people still working on potential possibilities.
Space junk – yes, that’s an issue and not just for Space Eevators. Humanity does have to do something about that. Yeah, its something that needs to be taken into account and if one ever gets built that would happen. We already track space junk of course. So how might that SE’s be protected from SJ? Well,
Source : https://worldbuilding.stackexchange.com/questions/100051/can-you-protect-a-space-elevator-from-space-junk
There’s a paper linked there under the “Delving Deeper” answer there too.
The oscillations issue? See this online paper for the technical details there :
Source : https://arc.aiaa.org/doi/10.2514/1.29010
Powering the elevator?
Source : https://www.colorado.edu/faculty/kantha/sites/default/files/attached-files/25753-58722_-tyson_sparks–may_3_2014_1128_am-_sparks_final_paper.pdf
Yes, another factor that is considered and addressed by SE proponents and designs.
So far, I’m wondering how much research Angela Collier did into SE’s and whether she’s actually checked to see if her obvious questions have been addressed by the SE planners / proposers before making this clip. Maybe she has and will address these points and their proposed solutions later on here? I’am only at the 23 minutes mark so far in viewing so willhave tosee if it improves and that happens later on..
To Be Continued..
Broken link for the Analysis of Space Elevator Technology by University of Colorado take II :
https://www.colorado.edu/faculty/kantha/sites/default/files/attached-files/25753-58722_-tyson_sparks–may_3_2014_1128_am-_sparks_final_paper.pdf
By Tyson Sparks Spring 2014 Final prohject. FWIW
Oh, c’mon man. It’s not that nobody can imagine hypothetical solutions, it’s that rockets are a far more sensible solution to GEO access by any metric.
Off world… maybe? On Earth space elevator is crazypants.
Heh. No. Point is that PZ was unduly impressed.
As I noted @32, “The technology, of course, is not currently at hand, nor are the pragmatics.
So, the obstacles are geopolitical, financial, and technological — but not not conceptual.
In short, unless she has rewritten physics and/or math, all she can dismantle is the idea that it’s currently possible to actually do it.”
Heh. We haven’t even began offworld construction, so Earth is it.
And if you seriously imagine that pushing mass around via exothermic chemical reactions is the go, you’re sure stuck in the mindset of a primitive.
(The famous rocket equation)
The whole idea is to decrease cost to space by a couple orders of magnitude, so bootstrapping actual expansion of productive materials.
John @135:
But she has rewritten it. She says that for a segment of the rope, Fc = Fg. Dividing by the segment’s mass on both sides;
ω²r = GM/r²
That is screamingly, embarrassingly wrong. Mathematically, it should be obvious that, since ω, M and G are constant, there can only be one value of r which satisfies this (and that is r=Rg, the orbit radius of GEO). For example, near the Earth’s surface, in SI units, the left side is about 0.03, while the right side is about 9.8.
Collier was obviously winging it. I’d cut her some slack if this was an informal discussion, but it’s presented as an educational video.
Just for shits and giggles, and cuz I have nothing better to do this morning, let’s calculate the maximum tension in the cable.
The force balance equation, eq 2 in #130, can be recast as (with M the Earth’s mass)
dT/dr = ρ[ GM/r² − ω²r ]
The rhs is zero ar r=Rg, so we can write ω² = GM/Rg³. Also, the square bracket is positive for r<Rg, and negative for r>Rg. That means the max of T occurs at r=Rg. Integrating both sides from R to Rg;
T(Rg) − T(R) = ρGM ∫ [ 1/r² − r/Rg³ ]dr
T(R)=0 because tension is zero at the free end of a string. The integral on the rhs is the same as the one Collier does, but with upper limit Rg rather than R+L. Doing the integration and gathering terms, we end up with
T(Rg) = (ρGM/R)[ 1 − (3/2)(R/Rg) + (1/2)(R/Rg)³ ]
Looking up the values of G, M, R and Rg (all values using SI units), we get (N is Newtons)
T(Rg) = ρ(4.8e7) N
A bit more: The tension can be written T=ATs, where Ts is the required tensile strength and A is the cable’s cross-sectional area.. The linear density ρ can be written ρ=Aρ’, where ρ’ is the volume density.
The specific strength is defined as Ts/ρ’, so we have
Ts/ρ’ = T(Rg)/ρ = 4.8e7 N-m/kg = 4.8e4 kN-m/kg = 48000 kN-m/kg
For comparison, the specific strength of stainless steel is 63 kN-m/kg
Resumed watching the Angela Collier clip now ..
“Helicopters crash all the time .. I won’t get in a helicopter.
Ohh-kaay. Her choice and no issue with it as a personal thing but helicopters do famously work and lots of people use them. Certainly we have accidents with helicopters (& planes & cars & hell, youcan trip over walking even) occassionally but saying they crash “all the time” is just silly and obvs wrong.
<.i>”We don’t live in a Star Trek World”
No, not yet. It is however something to aspire to and work on creating maybe? I mean we don’t have SE’s now is stating the obvs & says nothing about why we’ll never have them.
“I’ve done some reading.”
Well, good, that’s promising and I’m looking forward to her discussing what she’s read and why he thinks the peices she’s read don’t cover the issues she’s raised & answer her questions so far..
Now at 38 mins mark. Will have to break this again..
https://slate.com/news-and-politics/2011/06/are-helicopters-safer-than-cars.html
Also, seriously dude? I watched the thing in one sitting like a fortnight ago?
Continuing .. 25 minutes 5 secs mark she shows an excerpt from this Scientific American article :
https://www.scientificamerican.com/article/space-elevators-are-less-sci-fi-than-you-think/
Without it being immedately clear where it was from by naming it on screen or having a link in her “show more” section meaning I had to google & find it (ditto all the other things linked here.) :
Over which she’s scrawled in red text “more practical?!?!?! (sic) I don’t believe you. without (sic) fuel?!?!?! (sic – yes 6 alternating question marks and exclaimation marks each time.) Which, well, rude and unhelpful and has me wondering if she knows how lifts (elevators) work and certainly doesn’t convince me she knows better than the actual engineer who wrote the article. Its like a creationist saying “I dont believe you!” to a science text and about as worthwhile.
30 minutes mark Angela Collier (AC) paraphrasing : “We don’t know where the Earth and other stars will be in 400 or, wait, 800 years.”
Actually we kinda really do given science – stellar kinematics is a thing (https://en.wikipedia.org/wiki/Stellar_kinematics ) as is stellar dynamics and then there’s Milankovitch cycles predicting orbital changes for Earth well beyond a mere 800 years and this whole argument looks like one of absolute ignorance and from someone who I’d have expected better of. Whilst yes, chaos theory and the fact that we don’t know everything means there are some uncertainties, this certainly doesn’t mean we don’t know to within a reasonable degree and in any case has very little to do with SEs.
AC : “We launched JWST with rockets” Oh for pities sake. We rode to X on horseback so we can’t build a car and drive there in half the time or catch a train etc.. . the SE is more efficient and cheaper than using rockets. We could launch – and access the JWST and more cheaper and more efficiently if we had a Space Elevator.
31 mins AC red text : This is a made up number .. how .. write a paper on hypothetical cost savings of hypthetical object that won’t exist? (Bradley C Edwards referenced)
Estimates how do they work? Maybe she could contact the author or google and read that paper and find out? Someone performs hypoethical calculations on, yes, something we don’t yet have and that’s ..bad.. somehow? Seriously?
Superfluous Titan reference with no relevance to SEs. Billionaire tourism – not the main reason people suggest SEs.Then a rant about fridges and data and charging for stuff.. coz.. huh? Why? Oh. Becoz AC don’t understand the utility of having a space elevator. Argument for incredulity. Oh and then she claims we didn’t have a reason to go tot he Moon? Sheesh. Yeah we did. “Capitalism cynicism, monthly fees” – AC, relevance to SE nil.
37 mins AC – Liftport an SE company (https://en.wikipedia.org/wiki/LiftPort_Group ) is a “scam” in her view becoz they are long running? They work on stuff make progress and that’s ..bad somehow? They illegally sell shares – ok, illegal and is bad – and that’s ..somehow proof SE’s won’t work? Huh?
39 mins AC – Are you going to harvest helium 3 from the Moon? Well, yeah, that is an idea that’s been seriously proposed and could do a lot of good. $8,000 is useless and can’t do anything with. FFS..So she thinks its all “people living in StarTrek being scammed by scammers” Her personal & if you ask me very short-sighted and Luddite-Ish opinion, nothing to do with the physics of SEs.Then AC oh I suck at playing computer games so I won’t play one of the ones where you build fictional SEs becoz that’s somehow relevant? Nope. (But finally I’m at the 41 mins mark! Just 5 mins to go.)
41 mins Japanese company Obiyoshi predicts that by 2050 we’ll have a working SE, wrote paper she can’t access or read. Calls them “Liars” because there isn’t one yet. Ok. First time I’ve heard of them, seems like she doesn’t know much about them at all either but is happy to call them “liars” because .. they’re ambitious and have made an extremely bold prediction that they are working to make a reality? No SE and its 2050 now already. Wait, we’re in 2023 last I looked.
We havern’t yet done X does NOT mean we can’t or won’t do X or doing X is impossible. AC repeating “Why!?!?” again as if deliberately not reading the listed theoretical benefits (cheaper, more efficient access to space basically) is somehow a case or worth grating repetition.
43 mins Google X , nanotubes aren’t good enough yet.. Ok. Nothing new learnt or shown. 2018 Japan tests mini SE satellite thing – she’s bad at finding stuff in Nihongo & the first article I find again yet again answers her incessant “Whhhhyyyyyy? question in its first paragraph :
Source : https://www.smithsonianmag.com/smart-news/researchers-take-tiny-first-step-toward-space-elevator-180970212/
Oh well, I guess she’s gonna seriously address the physics and give us some reaosn toactually think she’s right .. nope. End of video, list of names scrolling by for three minutes or so. Yay?
So from what I can tell its one big long annoying we have’nt done it yet so it can’t be done and we don’t yet have the material to build it and mocking people who think its a good idea becuase, well, they haven’t done it yet and we don’t live in a Trek world yet which is somehow proof that the physics of SE’s are wrong?
Others here were actually impressed by this and entertained by it? I sure wasn’t.
@Silentbob : Also, seriously dude? I watched the thing in one sitting like a fortnight ago?”
Guess I have a busier life and less free time than you do then? I admit I found that a tough slog to get through – you might have guessed I grew increasingly annoyed by Angela Collier and frustrated by her lack of actual arguments. Getting through that 45 minute clip was a chore for me. But I watched it in full, carefully, taking my time and deconstructing it as I went becoz I said I would and it was recommended to me by people I respect here.
Okay & thanks for the link but that ‘s not the same as helicopters crashing “all the time” (AC) and being excessively dangerous given we do see helicopters being flown and working and delivering as advertised most for the time. Or that we aren’t improving the safety of helicopters and building better ones all the time. Random side tangeant but yeah. No one says AC or you or I have to fly choppers & no one says anyone has to use or develop or be fans of Space Elevators but those who don’t want them just mocking them without having substance beyond the, yeah, we have’t yet developed them yet and don’t yet have th material able to build them seems pretty petty, unimaginative and mean-spirited and backwards to me. What’s wrong with having bold, ambitious ideas and thinking that SF idea X, Y, Z, might actually be worth trying tocreate in reality and then working to do so?
@132.
AC doesn’t.
What’s wrong with having bold, ambitious ideas and thinking that SF idea X, Y, Z, might actually be worth trying tocreate in reality and then working to do so?
Nothing at all. But sooner or later someone’s gonna have to sort out which bold, ambitious ideas are workable and useful, and which aren’t.
So from what I can tell its one big long annoying we have’nt done it yet so it can’t be done…
If you’d actually read and processed all the objections stated here, you’d know that’s not even close to what we’re saying.
Raging Bee, um, StevoR has just finished watching the video and is relating what the video claimed, as he understood it.
Not the objections here, but the source material. The video.
To be fair, Silentbob strenously advocated to StevoR to watch it, kept putting in timestamps, and kvetched when StevoR was insufficiently prompt in his review.
So it’s a bit rich to complain he hasn’t addressed “what we’re saying”, when one of the things “we” have said is to waste time watching the video.
Ah well, peer pressure. Works on some people.
It was one, Juan Ramón! I gave one timestamp because I thought he’d fall asleep during the calculus, so I labeled the discussion bit. I was being nice.
Okay fine. Stevo has investigated the space elevator and found no flaws.
Space Elevators are Go!
Heh. You were trying so very hard to be patronising.
(Not everyone was as impressed by the calculus as you were, SnotBag)
—
Oh yeah, yet again: Care to share your natal name, so that I may return the favour? After all, you very much prefer to do that than to actually use the nym.
@147. Silentbob : The time stamp was useful thanks again.
Well, not quite. I did watch that AC video and find itwa sfullof flaws and lacking insubstantive science.
Space Elevators may or may not be go one day. Currently w edon’t have the materal capable of builduing them 0- but that doesn’t mean we’ll never have the material capable of doing so or that w eshouldn’t work onhow we might actuallymake them possible.
@ 144. Raging Bee :
Do I think Space Elevators are possible and something we’ll have one day?
I don’t know but I think they are a fascinating idea that might work so why not work to try to find out?
Smarter people than me have certainly put in considerably more than a few hours work of designing, writing about – both SF and non-fictional – and planning SEs.
Another -15 mins long Space Elevator video here Anton Petrov with bonus spider silk & artificial spider silk references. Plus it turns out early SE designs and concepts were discussed in Soviet media publication Pravda – truly!
Fixes for clarity from #149 : I did watch that AC video and find it was lacking in substantive science and didn’t (in my view anyhow) disprove the idea of the SE. Space Elevators may or may not be go one day. Currently we don’t have the material capable of building them – but that doesn’t mean we’ll never have the material capable of doing so or that we shouldn’t work on how we might actually make them possible. Some companies as AC noted are working on the SE idea and trying to make them happen and if they can’t, well, I guess they can’t. But meanwhile saying just we don’t yet have the materials whilst true isn’t overly helpful and noting that we don’t live in a Star Trek world is both tritely obvious and boringly unhelpful.
@ 149 StevoR
You’re welcome. I honestly just went to the trouble of finding the timestamp to save you having to slog through the math part.
No idea why grandpa Morales is having a meltdown about it. X-D
In your dreams, maybe. Your fantasy.
FWIW, I judge StevoR as smarter than you. Like, intellectually.
(No offense intended)
Just by chance I came across the video Rob Grigjanis must have been referring to at #1 – and it’s brilliant!
Collier literally plays a video game (and smashes it) while coherently describing the entire history of string theory and her objections to the public presentation thereof. I’d be lucky if could coherently recite the alphabet while playing a game that fast paced, but she barely misses a beat. Astonishing stuff.
So I’ll ask the opposite of the Grigjanis question: “How does anyone not find her videos thoroughly charming”? (It’s rhetorical. I know the answer. There are old blokes who are offended by smart women. How dare this uppity woman be so capable?! Grrr.)
@ 152 John Morales
Just seen this. I bet you’re so stupid you don’t even realize this is insulting Stevo, not me.
I’m right aren’t I, brainiac? :-)