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The Spaceline: a space elevator alternative achievable with current technology (arxiv.org)
89 points by bookofjoe 11 days ago | hide | past | web | favorite | 131 comments





> By extending a line, anchored on the moon, to deep within Earth's gravity well, we can construct a stable, traversable cable allowing free movement from the vicinity of Earth to the Moon's surface. With current materials, it is feasible to build a cable extending to close to the height of geostationary orbit, allowing easy traversal and construction between the Earth and the Moon.

If we still need rockets to reach the bottom of the cable 36,000 km above Earth's surface, doesn't that negate the point of a "space elevator"?


You'd need to reach the height of 36,000 km, but not the velocity needed for geosynchronous orbit - only the velocity needed to match the angular motion of the moon, which I think would be 28 times less.

So it's more like "use the moon as part of a non-rotating skyhook."

https://en.wikipedia.org/wiki/Skyhook_(structure)


I’m not very good at physics but - intuitively- would not attaching a payload that have not reached geostationary velocity pull this space line thing to the side and down with the force for which it was not designed

Attaching the payload would indeed pull on the line, since the payload wouldn't have the velocity to maintain itself in that orbit, and so would need an upward force from the line to not fall down.

The line would of course need to be designed to handle that, as well as the increased load as the payload is hoisted up to the moon.


Why is it less energy expended to hoist a load on a line than to use rocket propulsion?

To attempt a generalized version of sibling comment from radford-neal, the energy that goes specifically into changing the position of the payload is the same, but how much of the total energy spent goes into that, depends on the method. For the rocket energy goes into so many things other than lifting the payload: kinetic energy of the exhaust, high temperature, sound, and - most importantly - lifting the fuel itself, quickly.

Or to think another way think of all the challenges with human powered helicopter where the record is reaching 3.3 meters in 64 second flight, whereas any moderately physically fit person can climb the same height on the rope much faster and hang out there indefinitely, because they did not have to lift the weight of the helicopter and send the big masses of swooshing air down from the blades just to combat the force of gravity.


You don't need to carry rocket fuel to do it. You can use solar energy.

Well, whether it takes less energy would obviously depend on how the hoisting is done. But one advantage is that you can hoist slowly if that's advantageous, whereas you can't do things slowly with a rocket, at least until you've achieved orbit.

Also, if we start with a rocket+fuel on earth, and end with a rocket on the moon, there's an inevitable input of energy to move the rocket to a location of higher gravitational potential, that would have to be provided with hoisting as well, but there's also be a lot of rocket exhaust (formerly fuel) that's now zipping around the solar system, carrying away energy.


Appendix B calculates a fuel savings of ~25% vs using only rockets to get to the moon.

The saturn 5 rocket weighed 3,000 tons and was 96% fuel. Even if we only save 10% fuel, that's an extra 300 tons we can get to the moon.


25% fuel savings doesn’t mean you convert the excess into payload. It just means you need less fuel. The Saturn V could get ~50 tons to the moon using ~3,000 tons of fuel. A 25% savings means you get 50 tons to the moon using ~2,250 tons of fuel.

A 10% reduction would mean you’d get approximately 5 more tons to the Moon (not 300.)

The Saturn V was capable of delivering 50 tons to lunar insertion. Reducing the fuel requirement improves the payload amount by that percentage. It’s not a direct trade off between the total mass of the vehicle and fuel for more payload.


25% fuel saving seems like a small saving compared to the cost of building such cable. And of course, the saving up to geostationary orbit would be zero...

I don't think you have your maths correct, either.


It's not about paying for the fuel, it's about not having to lift the fuel.

Going from a rocket that is 2880T fuel and 120T for everything else to a rocket that uses 25% less fuel gives you 2160T fuel and 840T everything else. Decreasing the fuel by 25% increased the rest of your budget by 7x!


That's not how I interpret "using the spaceline can save around 25% of the cost per launch in fuel" (c.f. paper).

If the starting point is 2,880t fuel for 120t payload, so 24t of fuel per ton of payload, then 25% less fuel gives 18t of fuel per ton of payload and thus your 3,000t rocket can now carry 158t of payload, or 32% more.

Even "we can see that the spaceline cuts the fuel cost of such a mission by approximately two thirds" is not a game changer compared to the cost of building the cable.

Space elevators are touted as game changers because although building one would be extraordinarily expensive it would slash the cost of sending payload to space by orders of magnitude, making it worthwhile.


But amusingly enough, a spaceline costs orders of magnitude less than a space elevator. It might make sense as an intermediate step, where we launch a Spaceline using ol' fashioned rockets, and then use it to haul up the materials we're going to build an actual space elevator from.

A space elevator needs to be anchored in geostationary orbit at about 36,000k altitude, which isn't possible with available materials.

This article talks about a cable from geostationary orbit to the moon.

That still leave the ground-to-geostationary part for conventional lifters.


I must have misunderstood - I thought the Spaceline was going to actually reach into the atmosphere?

Dang, I totally misunderstood, I thought this thing was going to go much lower.


It goes even further than that. Building bigger rocket engines is really hard because turbo-pumps and nozzles don't scale well. Smaller rockets are exponentially cheaper to design and produce. See the RD-170 program for example: it was easier to just use 4 nozzles instead of 1 to achieve the same output.

Sure, but imagine that 25% repeated over and over again.

This got downvoted, but it is correct. This cable won’t be a net gain if you want to go to the moon once, twice, or even ten times, but _if_ you want to, say, build a moon colony housing thousands, and you need to launch thousands of rockets, it likely will be. A concern would be how fast this line would wear down, and what would happen if it broke. Does it destroy your lunar colony, take out a zillion satellites in earth orbit, or just float down onto the moon, and burn down in the earth’s atmosphere?

If you build the cable out of moon it would probably be a nice big cost savings.

Of course, it still probably isn't worth doing, there's not huge dollar reasons for people to be on the moon, mostly just fun science reasons.


But aren't there huge dollar reasons for wanting to get into orbit in general?

I guess not particularly if you are comparing them to the cost of the largest structure ever built.

And this thing doesn't even help with the orbit part, it helps with getting to a different part of the solar system once you get to orbit.



The Saturn V could transport about 50 tons to the moon. If it could transport another 300 tons, as GP said, it would be able to move 7 times as much payload.

The GP's value is for 10% savings, the article says it saves 25% of the fuel, what means 750 tons extra, or 15 times as much payload.

At that point it's better to build stuff on the moon and move them into geostationary orbit on earth via the cable the other way around.

It's not like the moon lacks resources, we won't be needing to push 300 tons of anything to the moon if we have the capability to build the cable from it.


It still makes it easier to get raw materials to space; the difference is that in this case, the materials are mined on the Moon rather than Earth.

Couldn't we build another space elevator (or many) to orbit? Then we would just wait there, or move toward the moon's elevator?

I like Issac Author's "orbital ring" idea. He is on YouTube. If you like space and futurism you need to see this guy's ideas!

Also checkout my project: https://www.kickstarter.com/projects/poly-wants-a-cracker/po...


My favourite space elevator alternative is the space fountain ( https://en.wikipedia.org/wiki/Space_fountain ).

It's an active structure made of small parts that are held together with a stream of magnets.

If the fountain fails then each part can have its own parachute so that it can safely return to Earth. i.e. without destroying entire cities as would be the case for a traditional elevator design.


A space elevator wouldn't destroy cities if it fell. Most of it would be flung away from earth, the part that fell would mostly burn up in the atmosphere. The rest would flutter like leaves as it came down.

Wow, that is sick!! Thanks for sharing.

TL;DR we can stretch a really long piece of fishing line from the Moon's surface to geostationary orbit. And we can probably send payloads to the moon by docking with said cable in geostationary orbit too. The funny thing is I literally mean fishing line here, they call for using a constant cross section cable made of zylon or dyneema with a diameter of about 0.35 mm. At least initially. Both dyneema and zylon with this about this diameter are sold as fishing line. Initial payload to/from moon seems a bit low as the line at the Moon end can only support 100 kg. The dynamics of such a thin, flexible cable which passes through an unstable(!!!) lagrange point should be interesting. We know that very long space tethers can behave in weird ways[0]. This idea is crazy and attractive enough that it warrants further investigation to at the least find out what the problems are so other people don't make the same mistake. [0]https://pdfs.semanticscholar.org/6aff/70794d94b8979d0c5a67ff...

> Initial payload to/from moon seems a bit low as the line at the Moon end can only support 100 kg.

But on the Earth-side, it can support up to 2000kg! And launching from the Moon is cheaper, anyway, since it's a less massive object than the Earth, and there's no atmosphere to deal with.


That's interesting, I wonder if they meant 100 Earth kilograms or 100 moon kilograms.

Edit: On 2nd thought they are probably using kg in terms of mass, not weight.


Grams are always mass, never weight.

I am not sure what's more exciting: passage through an unstable Lagrange point, or effects of a solar wind on the massive area of such cable.

> TL;DR we can stretch a really long piece of fishing line from the Moon's surface to geostationary orbit

To the height of geostationary orbit. The actual endpoint wouldn't be geostationary, as the moon orbits the Earth, it'd be pretty difficult to keep one end of a line in a stable spot above the earth's surface.


This is not a launch alternative. This is maybe a get-to-geostationary alternative. This is really an L1-base station-keeping alternative. The Moon-side cable anchors the base in the radial direction, and the Earth-side cable keeps the Moon-side cable in tension, and lowers the fuel cost of reaching orbits that the cable crosses.

The math suggests that putting a rendezvous base 41% of the way from the Earth to the Moon would be better than extending more cable down into Earth's gravity well. Meeting the cable closer to Earth would always cost more fuel. You could save fuel to GEO by meeting the cable further out, and then climbing back towards Earth. But every bit of extra cable you add on the Earth side increases the tension at the max-tension point at L1.

So just stop at the lowest-fuel rendezvous height, and add enough mass there to keep the whole cable in tension, but not enough to break the cable at L1.

(The L1 point is at about 84% the distance from Earth to Moon.)

The same type of material would likely be usable to anchor a halo relay satellite slightly beyond Earth-Moon L2, just by bolting it down to the darkest antipodal point on the dark side of the Moon.


> This is maybe a get-to-geostationary alternative.

How would you use it to reach geostationary orbit?


If you can source materials in situ, this might be a useful way to create GEO platforms at lower cost than Earth launch.

If the theory is correct, you could build satellites on the Moon and slowly walk them to GEO at zero fuel cost with solar power.

Any circular orbit with height between 5% and 41% of the Earth-Moon distance could be placed more cheaply by flying out to rendezvous at 41% with fuel, then walking it back in along the cable with solar power.


Well if we can get a cable to the Moon, we could attach it to a crank at the North or South poles to generate electric power.

Wouldn’t that slow down the speed of rotation of the earth and the revolution of the moon around earth? That would be catastrophic.

I'd imagine that the amount of energy stored in orbital momentum is a lot higher than anything we could use up in any reasonable amount of time.

Even if we built a space elevator, imagine that the payload travels up at 100km/h - really fast, basically a car on a highway. Even in that ludicrous mode, it would take 15 days for the payload to reach 36,000km. Probably unrealistic for human cargo.

You've got it... upside down. The line is dangling from the Moon, down to ~36000Km above us. That car would have to reach geostationary orbit, jump on that tiny cable, then travel... oops, the remaining ~350,000Km to reach the Moon. To do that trip in 15 days, it'd have to run at mach 1.0.

Edit: it gets even weirder, if you consider the Moon's distance from Earth goes from 363,300Km at perigee to 405,500Km at apogee... just how long should the line be? If it's 327,300Km, we might only be able to operate it near perigee, sure if it's a bit longer it'd be more usable, but it cannot extend past 363,300Km - or it would touch down here :) - that means there's no way we might use it near apogee, since it would always be too far away.


> That car would have to reach geostationary orbit

No, it would not have to reach orbit, it would have to reach the altitude but not the horizontal speed. Huge difference.


100 km/h is not particularly fast, not even for a highway (my German friends recently discussed their preferred Autobahn speeds, and that varied from 110 to 190 km/h).

A 747 cruises at 893 km/h.

Concorde cruised at 2140 km/h.

The ISS orbital speed is 27580 km/h — or 36 megameters in 1 hour 18 minutes.


We are talking about a thing attached to a cable and going up. How are those other speeds a reference?

Don't forget that for the vast majority of that trip the atmosphere is essentially negligible. A better comparison would be a vacuum tube train with a(n approximately) constant 1G handicap. From the perspective of the tether, any practical payload mass is negligible in comparison with the tether itself, and -- most likely -- it's under tension anyways, so pulling the payload against gravity would actually be releasing tension below it.

Space elevators are particularly strange to think about because they combine the weirdness of orbital mechanics with some, but not all, of the intuitions we have about terrestrial behavior -- and then add in the complications of being a megastructure on top of it, which humans have basically zero experience building. They defy easy reasoning.


It’s even better than that: by 6000 km the inverse-square law will have reduced that to only 0.25 gee gravity drag :)

I don’t understand why you think that’s a problem.

The cars are limited by human reaction time, the aircraft are limited by air resistance, the ISS is a space station and all the constituent parts had to be brought up to that speed in the first place from the ground by rockets.

The main advantage of the category that space elevators are a part of is that energy and reaction mass doesn’t need to be carried with you as when you use a rocket, which makes them much easier.


Needing to apply force to a tiny strand at high speed is a difficult problem. So far even 40mph has been a major issue. https://en.m.wikipedia.org/wiki/Space_elevator_competitions

Several of these winners achieved less than 5mph and this is near earth so without the issues of supplying power over thousands of miles.


That is interesting, and also surprising.

Attaching to a very thin cable seems problematic, friction would wear on it at high speeds. I wonder whether a linear induction motor (also acting as contactless magnetic bearing) would be possible or fail due to the small surface area.

I believe this is way more limited by acceleration.

People still take ocean liners across the Atlantic. Those trips vary in length, but are at least a week long. If you told me I had to hang out in a cramped hotel for two weeks, but after that I would be on the moon, I'd sign up in a heartbeat.

An ocean liner is basically a party on the ocean. This is essentially doing a month-long sentence in a cell at ADX Florence supermax - with no yard time.

The view will be unlike any other.

Except... I can bring my Nintendo Switch, a pile of podcasts, maybe a book or two, some movies, maybe a laptop? I might not be super comfortable the whole time, but knowing that the moon was only days away would get me through it just fine.

This is an interesting idea, for sure.

But relative to the feasibility of this, how far are we from building automated systems to mine raw materials, refine and process them, and build stuff? Initially, of course, to build more such automated systems.


Forgive my ignorance, but if the moon orbits around the earth (ie not geostationary), how can the end of a line attached to the moon stay in one place above the earth?

The moon is tidally locked to earth, meaning it rotates at such a speed that the same side is always facing earth. (This is also why the moon can have a "dark side")

It usually takes millions of years of tidal forces from a planetary body acting upon its satellite for the latter to be tidally locked to the former.


Thank you. I understand that part, but is the line supposed to be at the same point above the earth?

No. It wanders around the earth every 28 days.

ohh - is there anyone here who can answer a question for me. I have been writing a science fiction story which features large rotating space stations like the one in 2001 and I wondered...

What is the force on a rotating structure in space. Is it possible to build a space station that rotates with 1G force on people at the rim, and if so what scale of structure could be built with current, or plausible technology? Where does a structure go from "imagined future" to "magic"?


The tricky bit isn't spinning to get to 1G, the tricky bit is building something where the axis is large enough that the Coriolis force isn't making your inhabitants sick. I've heard mixed reports about this; this, for instance https://www.nasa.gov/vision/space/livinginspace/23jul_spin.h... suggests that even a small room is something we can adapt to fairly quickly, but I've seen suggestions I can't seem to source right this second that we may need to go out to hundreds of meters to get it low enough for long-term habitation. But it was never clear to me whether those were numbers for what the human body can tolerate, or whether they were trying to get the force down to "imperceptible". Personally I wouldn't be all that worried about that; if we can develop "sea legs" I'm sure we can deal with Coriolus force, it's only if we inevitably get motion sick that it's a problem.

But merely getting up to 1G is trivial. I think the reason we don't have it today is that while getting "something" up to 1G is trivial, all the other engineering things that it interacts with get to be non-trivial, and right now, we're pretty tapped out with what we've got in space. If we continue to get more comfortable with space engineering and we continue to get better at putting stuff up there, I wouldn't be surprised we start seeing some spin-for-gravity relatively soon now.


There’s discussion on /r/spacex that two Starships could be teathered nose to nose and set in a rotation during their ballistic flight to Mars, in order to provide at least Martian (~.3g) level artificial gravity along the way.

This is important because even things like convection in the air stop working well at 0g, so for example, you can suffocate on your own CO2 if you fall asleep and there isn’t a fan running.

Funny enough the hardest part about this doesn’t even seem to be the tethering or rotation, but two things; 1) you have to make slight course corrections along the way because the initial burn is never precisely accurate enough to target something as far away as a planet, and 2) occasionally you have to put as much mass as possible between you and the sun due to solar storms spewing radiation, so you have to stop the rotation occasionally to point the engines sunward to use your fuel as a radiation shield, or to course correct.


Water supply as rad shield might be better, presuming you have a large water store. Designing a chamber within the water supply would put mass between you and radiation.

The formula you want is a=r*w^2 where r is the radius and w is the rotational rate. 1g is ~10m/s^2

The challenge in realizing it almost entirely an issue of getting materials into orbit, which is the problem space tethers are targeting. The cost of getting to geosynchronous orbit is around $30k per kilo, so at the moment economics is limiting us rather than tech when it comes to building crazy space stations. If we could bring the cost per kilo down 100x or so then so many more possibilities open up.


This is a straightforward calculation, with modest bookkeeping requirements, that is possible with undergraduate physics and calculus. As a sci-fi author, your craft will be improved by forcing yourself to do it. You'll be concerned with centripetal acceleration, yield stress, density, pressure, free body diagrams of pressure vessels, summation of forces on a differential element, and fairly basic integration (after taking symmetry into account).

It's trivially easy. You almost certainly have the stuff around your home. Take a piece of rope, spin it above your head. If the angle of the rope to the ground is less than 30 degrees you have at least 1g at the end of it.

There is a size limit, though--the required material strength scales linearly with the radius. Note that if you'll accept some fancy engineering this can be avoided: Instead of making it self-supporting you surround your station with a non-rotating ring. You can make it as thick as you need to provide the required strength. Wheels can't take the speeds involved, the connection will have to be magnetic levitation. Yes, this means maintenance but so long as you have an adequate safety margin you can power down and take apart any given piece for upkeep or even replacement. Just don't take apart too many at once.


If you use two balanced rotating masses with a single tether, the required material strength scales only with the total mass of the system; regardless of the radius, the acceleration of the two rotating masses is 1g. At some point the tether mass will become non-negligible and it will scale linearly, but that should be quite large for any reasonable tether material, particularly since only the very tips of the tether are accelerating at 1g.

http://www.artificial-gravity.com/sw/SpinCalc/

The math pencils out, but the station has to have a fairly large radius away from the axis of rotation. For example a station rotating once a minute with a radius of 500m produces 0.6g. This means that you are looking at a 1km diameter if the station is symmetrical, or possibly less if some sort of counterweight is setup.

Not at all implausible with current technology. Two habitats connected with a tether would do it.


There's nothing particularly difficult about building a spinning structure that has 1G of force at the outside (you see this all the time in carnival rides and the like -- if anything, it would be easier in a 0G environment, because you just have to start spinning and it'll keep spinning by itself forever). It's just not particularly useful -- astronaut comfort isn't really a priority, considering how expensive (and prone to problems) it would be.

This is far less about astronaut comfort and far more about health. Zero G turns out to be harmful in numerous ways, in addition to the other problems of space environments (radiation, iosolation, contamination).

My sense is that the most valuable information to come from ISS is of astronaut adaptation and long-term space effects on health.


If you haven't yet you should check out Isaac Arthur on youtube (https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g)

Specifically this video: https://youtu.be/86JAU3w9mB8


http://curious.astro.cornell.edu/about-us/150-people-in-astr...

tl;dr: it's trivial from a materials point of view, but hard from a "but you have to build it in space" point of view. We're still not great at building large structures in space.


Another interesting angle - is it possible that there is a continuous void between the earth and moon that never intersects any space debris already in orbit?

Considering geosynchronous orbits and that the moon progresses around that band of highest distribution of stuff to bump into, I would not bet on any window lasting very long without serious maintaince

I want to see an analysis of the different failure modes (catastrophic failure, hit by a satellite) and the consequences of the failure.

I believe this idea is just incompatible with reality.


I've suspected for a few years now that any long linear space structure will be prone to very high electrical potential deltas, particularly as it moves through different radiation and magnetic fields. How the structure behaves will depend on its own conductivity, with conductive structures creating sizeable currents, and insulating ones, massive charge differentials.

So free electricity?

Maybe, but more likely a bit of a problem.

Huge charges across long distances mean very large currents. That tends to create difficulties, such as massive heating.

Whether or not this will be the case I don't know, though experiments with simple long tethers (several km to 10s or 100s of km) should be relatively easy to test: try them in some empty space and instrument for charges and magnetic fields.

Might produce an interesting de-orbiting spectacle as well.


https://www-istp.gsfc.nasa.gov/Education/wtether.html

"The Space Tether Experiment"

> The space tether experiment, a joint venture of the US and Italy, called for a scientific payload--a large, spherical satellite--to be deployed from the US space shuttle at the end of a conducting cable (tether) 20 km (12.5 miles) long. The idea was to let the shuttle drag the tether across the Earth's magnetic field, producing one part of a dynamo circuit.

The experiment failed, for a fascinating reason...


Thanks!

I was vaguely aware that the experiment had been tried. I'd either forgotten or never known the specific reasons for the failure. Though it rather closely matches the scenario I'd suggested above.


Your description reminded me of it. Cheers.

I was wondering the same thing - does it burn up in the atmosphere, or destroy all life on the planet?

Ask Russell @ xkcd

Let's ignore the physical catastrophe of a cable at 380,000+ km long dislodging and becoming either locked in earth orbit or crashing into earth.

Let's ignore that.

What if the cable attached to the moon and then locking the other end to earth orbit alters the month's orbit? There was some stuff I read that the moon doesn't neccesarily orbit the earth. It's the earth and moon orbit each other. Essentially our orbit is significantly affected by our moon compared to other planets and their moons. We have a ton of wildlife that depends on the current orbit pattern. Wouldn't this cable greatly alter said orbit?

Plus, 380,000km worth of material? Along with they just hand wave that all failure will happen on the moon side and never affect earth... um okay? Because of testing?

This seems more like a joke paper in all honesty. Kind of like the paper based on Silicon valley where they dev an algorithm to find out how long it would take to give an audience handjobs. d2f ratio


> they just hand wave that all failure will happen on the moon side and never affect earth... um okay? Because of testing?

They don't handwave it, they calculate it. It's significantly less handwavy than your back-of-napkin objections.


Can you point to the specific section of the paper where this is calculated. I admittedly only skim read, but I did go through the whole thing, and I'm convinced I missed this kind of analysis somewhere.

A cable in geosynchronous orbit isn't a problem. Of course the problem is it won't stay there: the dynamics are likely to be interesting.

Now let's do some basic arithmetic. Moon is about 10^22 kg. Saying it will move as a result is like worrying that a semi will stop because of a fly.


If you hang a cable from your home ceiling with a little weight at the free end, what position do you expect the cable to stay? Why do you not expect it to stay down if you hang it on the Moon?

The cable in not in geosynchronous orbit, it's in geosynchronous altitude. If it is cut, it will fall into Earth.


Just to check, are you asking about a tiny cable on the surface of the Moon, or a 1-light-second-long cable where both ends are “the bottom” in local gravity even though they’re on opposite ends of the straight cable?

Oh, thanks, I see the problem now.

I imagine it would be nailed down at the Moon, even though it 's going "up" on that end too, the Earth end will very likely weight much more than the Moon and pull the entire cable. But the fact that it must be much fatter near the Moon does indeed complicate things.


> Wouldn't this cable greatly alter said orbit?

Only if it's extremely massive.


So a cable at 300,000km at, oh, I dont know, an average of 1 meter wide, is not massive?

I think scrapping together enough material on earth to build this will be the first problem.

2.36 x10^8m for rough cylinder volume.

Steel weight would be 1,864,400,000,000 kg. Yes itd be a hybrid material. So, half of that? Maybe a quarter? That's not an insignificant amount of material at a 1m diameter cable. We haven't even gotten to labor costs and energy required.

I get astronomically, its insignificant. But I doubt it's going to have no effect.


"Let’s say such a line was made of a cable witha0=10−7m2: its total mass would then be around 40,000 kg. This is about twice the mass of the original lunar lander,and would make transporting and constructing such a cable completely plausible" -- From the linked paper.

The moon weighs 70,000,000,000,000,000,000,000 kg.

That's 37,545,591,074 times your projected weight of the cable.


Is it valid to say this is the "weight" of the moon?

I would have assumed such measurements would deal exclusively with the moon's mass.

Are the two terms used interchangeably in this context? (honest question, I really have no idea)


I'm using them interchangeably here, but you're right, it was sloppy - I should have said mass. (They're only equivalent on Earth, and even there a given mass's weight changes based on where you are!)

> So a cable at 300,000km at, oh, I dont know, an average of 1 meter wide, is not massive

Massive to a man, not a moon...


No it would actually have no effect... Again your numbers are vastly out of scale.

> What if the cable attached to the moon and then locking the other end to earth orbit alters the month's orbit?

Sorry that's functionally just not possible. More so the moon is already moving away from the Earth at a couple centimeters per year. You're not even going to slow that down.


Your intuition is out of proportion, because astronomical objects are big. Really big. A spider riding on the left side mirror of a 18-wheeler has a better chance of drastically changing its course.

Relevant and entertaining take on this: https://what-if.xkcd.com/157/ (A direct moon - earth elevator, not really what's proposed here though)

The earth-moon cable is just a construction fixture. The space elevator extends to geosynchronous orbit, as usual.

Ok, I didn't read the paper...

But everyone knows the moon orbits the earth with a cycle sort of similar to what we call a month.

How is an earth-moon cable feasible again?


Suggestion - read the paper.

"The Spaceline. By extending a line, anchored on the moon, to deep within Earth’s gravity well"

That's the concept. They could have fit that within in the title of the post.


A pointer to where they compute this in the paper would be helpful. There are a lot of comments to just read the paper because they calculate this or that, but I quickly read the thing through and either missed all these points being covered, or it's not actually considered.

I assume it's not anchored to the earth

Wouldn't be much use if it wasn't.

well… you should read the paper, or maybe a summary like this one[0], before jumping to comment.

[0] https://news.ycombinator.com/item?id=20897565


"The Spaceline. By extending a line, anchored on the moon, to deep within Earth’s gravity well""

I'm excited for a space elevator almost exclusively just so we can get rid of nuclear waste. Nuclear power is the only current viable alternative to fossil fuels at scale, from my understanding. However, people are squeamish about the spent fuel...if we could take it up to space and launch it into the sun (without having to deal with the perils of escaping gravity) it would be a lot easier to get rid of.

That sounds way more dangerous than just burying it in the ground. An accident in space spreads radioactive materials over some part of the Earth almost at random. An accident in a buried site stays mostly contained, with the biggest danger being contaminated groundwater.

OH yeah?!

Launching stuff into the sun actually takes quite alot of energy. In fact its even harder than leaving the solar system entirely.

Nuclear waste isn't a big deal. After reprocessing what's left will decay to ambient in 10,000 years. Every major proposal for how to deal with it will have no problem storing that.

If you're scared about plutonium from reprocessing there are two things:

1) Reactor fuel has been in the reactor for too long. It's going to have far too much Pu-240. Reactors don't care, bombs do. You can't make a reliable bomb if you have too much Pu-240.

2) Deliberately leave hot stuff in the reprocessed fuel. So long as it's not a neutron absorber the reactor won't care. However, it will leave the fuel deadly hot. The industry has the equipment to handle it (after all, it's no hotter than the spent fuel they are removing), the terrorist does not. Nobody's smuggling it out because they won't live to accomplish it, not to mention that it will set off alarms from far away.


That's a lot of optimistic handwaving. If it's so easy why haven't we managed to do it in the last 70 years?

This sounds like a pretty bad idea. Putting radioactive material into space orbiting the earth would be pretty dangerous, and getting something from earth's orbit to the sun is not easy. I think the risk of an accident under this plan would be way higher than our current plans.

But don't you still have the problem of getting the waste from the Earth's surface to the elevator and thus still have all the danger involved?

So they found a material which won't degrade in 24 hours due to radiation from the sun, or a way to shield it? And what about the mismatch between the rotation of the moon around the earth and geostationary orbit? And they figured out how to go around the van Allen belts to prevent frying sensitive electronics in the payload? And when the payload gets close to the moon, they have a way to stop it plunging to the surface of the moon for unscheduled disassembly?

Of course the cheapest part of getting to the moon is the bit between the edge of space and moon orbit. Approximate current cost: $0. Approximate cost after placing 40,000kg fishing line in the way....

I'm also pretty sure crawling along a fishing line will be prohibitively slow compared to letting a rocket coast with its engines off.


> Of course the cheapest part of getting to the moon is the bit between the edge of space and moon orbit. Approximate current cost: $0.

No, it costs 4.1km/s of delta-v. https://en.wikipedia.org/wiki/Delta-v


I meant to compare getting to GEO (as proposed in the paper) as opposed to Lunar orbit.

Of course the way I worded this is a bit lose, and it's not completely free, but according to the figure in that Wikipedia article, the difference between getting into geosynchronous orbit and getting to lunar orbit from earth seems to be only about 0.1km/s, which is negligible compared to the total cost. So my basic point still stands.

It's actually really expensive to get to GEO.


"letting a rocket coast with it's engines off"

you should play some Kerbal Space Program to get an intuition for delta-Vs and the cost of getting to the Mun.




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