A Lunar Space Elevator Is Feasible and Inexpensive, Scientists Find | Hacker News Reader

A Lunar Space Elevator Is Feasible and Inexpensive, Scientists Find

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188 pointsevo_9198 comments

It is not the fault of the students who wrote the paper, but this is an exceptionally good example of terrible science reporting in popular media. Practically every sentence of the article is wrong including multiple errors in the title.

> Feasible & Inexpensive

The paper says "Building a spaceline would be a huge engineering challenge, stretching the limits of current human capacity - but not exceeding them" which is a far stretch from "feasible and inexpensive"

> Scientists Find

Second year graduate students

> In a paper published on the online research archive arXiv

An as yet un-peer-reviewed pre-publication draft that is being prepared for submisson --- it says that right on top of the first page.

> After doing the math, the researchers estimated that the simplest version of the lunar elevator would be a cable thinner than a pencil and weigh about 88,000 pounds

They they actually said: "Let’s say such a line was made of a cable with a0 =10−7m2: its total mass would then be around 40,000 kg.". So yes, they're right that it would be "thinner than a pencil". A cable with diameter section of 0.36mm is definitely "thinner than a pencil". (They are misquoting them saying thinner than a pencil lead... presumably referencing the lead in a 0.5mm mechanical pencil, not the lead in the regular pencil).

So yes, you could spend billions of dollars to build an elevator that could lift ~100kg off the surface of the moon to GEO, and take months to years to do it any any speed you're likely to achieve with a solar powered elevator tethered by a 0.36mm thick line.

Good points.

For anyone interested in skipping the superfluous and sensationalizing article, here is a direct link to the paper, avoiding the embedded tracking link: https://arxiv.org/abs/1908.09339

"stretching the limits of current human capacity - but not exceeding them"

That means "feasible".

As for inexpensive, it may be a poor word choice, but the alternative is launching rocket after rocket in to space, which is one of the most expensive things humans do.

This is not an alternative to a rocket. It goes from geosynchronous orbit height to the moon, so you still need a rocket from earth to the elevator.

It enables the use of smaller rockets rather than no rockets. With a probably optimistic ~billion dollar pricetag for something able to move 100kg. You would need to launch an awfully large number of rockets with tiny payloads to the moon for this to be a net cost savings.

> Practically every sentence of the article is wrong including multiple errors in the title.

>> But the Columbia study differs from previous proposal in an important way: instead of building the elevator from the Earth’s surface (which is impossible with today’s technology), it would be anchored on the moon and stretch some 200,000 miles toward Earth until hitting the geostationary orbit height (about 22,236 miles above sea level), at which objects move around Earth in lockstep with the planet’s own rotation.

I see. The difference is they proposed to build their Moon space elevator on the Moon, while others have proposed building the Moon space elevator on Earth... /s

Also, obviously, even if the Lunar tether reaches the _altitude_ of geosynchronous orbit, it will not have the _velocity_ of geosynchronous orbit (it will have the angular velocity of... the Moon). Since the Moon orbits once every 30-ish days and the orbital period of a geosynchronous orbit is 1 day, an object at the end of this tether would not be in geosynchronous orbit. I'll not bother to check and see if that orbit would have an apogee above 100, or 0, km.

In other news, Wet Streets Cause Rain.

We know the altitude of an object with an orbital period of a month: the moon is there. An object going the speed of the end of the tether would drop like a stone.

Objects in geosynchronous orbit are going 3000+ mph, where the end of the tether would be at about a standstill. Still, if you managed to grab on, on your way past, there would be plenty of swing and stretch. It would be hard to keep it from swinging all over, all the time, as the tides had their way with it.

This is actually a huge advantage. As I understand it, there would be no need to accelerated to GEO before docking with the station. The earth side of the teather would travel at about 10% of GEO. This is a large savings in energy expenditure because kinetic energy is m*v^2

True, presuming you could arrange to be at the same place as the tether at the same time. However, with it whipping around loose, how easy would that be?

Maybe a landing platform could be hung from it, helping to stabilize the end.

The key point:

the Columbia study differs from previous proposal in an important way: instead of building the elevator from the Earth’s surface (which is impossible with today’s technology), it would be anchored on the moon and stretch some 200,000 miles toward Earth until hitting the geostationary orbit height (about 22,236 miles above sea level), at which objects move around Earth in lockstep with the planet’s own rotation.

Dangling the space elevator at this height would eliminate the need to place a large counterweight near Earth’s orbit to balance out the planet’s massive gravitational pull if the elevator were to be built from ground up. This method would also prevent any relative motion between Earth’s surface and space below the geostationary orbit area from bending or twisting the elevator

>instead of building the elevator from the Earth’s surface (which is impossible with today’s technology), it would be anchored on the moon and stretch some 200,000 miles toward Earth until hitting the geostationary orbit height (about 22,236 miles above sea level)

Yeah, that's totally feasible and useful... /s

I hope those guys are not paid with public money...

> I hope those guys are not paid with public money...

I hope they are.

Engineering feats like this propel us as a species forward. Imagine the material science involved in simply achieving the correct tensile strength. Every thought that goes into this will find application in other areas.

I can't imagine this will get built, especially not in our lifetimes, but I do hope that it is one of many such endeavors humans will embark upon.

>Engineering feats like this propel us as a species forward.

Note that there are no feats thus far...

What device did you type your comment on?

Probably specifically meant the purely theoretical (and devoid of any tangible engineering feat) paper

Not a space elevator or anything designed by the team in the article, I can assure you of that...

It's not like there can't be some research that's realistic and have tangible (even if remote) goals, like e.g. rocket research in the 20s and 30s, and some that's just theoretical filler -- and that we can never tell one from the other...

Without looking anything up, can you give an example of what material science advances have come out of NASA or other space programs, that have propelled us forward as a species?

> Without looking anything up

Not GP, but... what? Why are you filtering materials by the GP's in-brain knowledge?

There is the general-knowledge question I asked about space programs, and then the more personal question of whether someone has a familiarity with the subject or if they've adopted a talking point, perhaps as a side-effect of economic incentives much larger than themselves and their own lives. I think the personal question is at least as interesting as the general-knowledge one

This would enable getting back from the moon to be much easier.

I have only been following space elevator research cursorily, but it seems to me that this proposal lacks the most compelling reason for building a space elevator: breaking the bonds of Earth's gravity well. The most expensive part of space travel now is getting from the earth's surface to a stable orbit, and a space elevator from the moon does exactly nothing to reduce this cost.

It does still have some benefits, i.e. if I'm not mistaken you could still use this elevator to "slingshot" spacecrafts on interplanetary trips, and it would make it cheaper to get to and return from the moon. But it's unclear (to me) whether these benefits could actually be realized when the cost to get to the space elevator in the first place is still so prohibitive.

I guess what I'm saying is that this is just the cost analysis, not the full cost/benefit analysis. The costs may be a lot less than a space elevator from the surface of the earth, but the benefits are a lot smaller too.

> and a space elevator from the moon does nothing to reduce this cost.

How do you figure? My extensive Kerbal Space Program experience tells me that freeing oneself from the moon's gravity well is far cheaper than doing the same from the earth, and then you're approximately MGH of the way to freeing yourself from Earth's well.

Perhaps you jest but clearly we can’t launch anything from the moon until we bring it there, from Earth.

Unless it's something that was mined there, like Helium-3 or some other mineral resource that we may find it's cheaper to extract from the moon.

You need more dv to get to the moon than to leo. An elevator helps with this.

Or (I hope people have a sense of humor here), space Nazis [1]

[1] https://en.m.wikipedia.org/wiki/Iron_Sky

But once we have this space elevator it's much more feasible to manufacture on the moon.

The moon has negligible atmosphere, so you can use a relatively cheap electromagnetic gun to reach the moon’s escape velocity. If all your doing is sending raw material back that’s a cheaper option.

True. But if you're doing raw material extraction on the moon, you probably also want a cheap way to bring mining equipment down to the lunar surface.

Just run the rail gun in reverse! (Just no fragile humans please unless it's a very long barrel.)

If the equipment is G-force sensitive, an elevator could launch science equipment that previously couldn't be put into space at all.

> science equipment that previously couldn't be put into space at all.

The large amount of science equipment manufactured on the moon?

The large amount of science equipment that could be manufacturer or assembled on the moon.

The paper actually points out and hints some very interesting implications. It could also serve to stabilize a station at the L1 lagrange point. Such a station could be permanent as it would not need reaction mass to maintain orbit. If you couple this with cheap material from the lunar surface, you could create truly massive habitats from thin plastic sheeting from earth and regolith from the moon.

Launching from the moon would be easier and cheaper than launching from the Earth, unless you include the cost of reaching the moon. But according to the table in [1], going from the Earth's surface to geostationary orbit (to rendezvous with a lunar elevator) requires about 14km/s delta-v, and from there to the moon's surface only about 4km/s delta-v. Of the 18km/s total delta-v required, a lunar space elevator only eliminates the last 4km/s.

1: https://en.wikipedia.org/wiki/Delta-v_budget#Earth%E2%80%93M...

I haven't done the math, but are you sure that last 4km/s isn't way more significant than you're estimating due to the rocket equation?

A small mass savings near the end of the journey might translate to significant mass savings near the start.

Also I think (a series of) rotating elevators to get from LEO to GSO are also possible?

The end of the lunar space elevator wouldn't be traveling at geostationary orbital speed, it would be going 28x slower. A rocket could sync with the end of the elevator at this lower speed, which would significantly reduce the delta-v budget. [1]

Not sure how practical all of this, but that is at least the theory.

[1] Credit to mnw21cam: https://news.ycombinator.com/item?id=20996246

The really interesting part is that the elevator would not be in a true geostationary orbit traveling at ~3000m/s. instead it would be traveling at 100m/s at the same height.

The delta-V to reach GEO without the radial velocity to maintain it is closer to 10 km/s if you use the partial escape velocity equation. This is pretty huge as mass required to achieve delta-V is nonlinear.

Leaving moon's gravity well may be cheaper than getting to space from earth but it is far from free.

With a space elevator in the moon you can use raw material from the moon to assemble huge stations and ships in space.

If I have learned anything from sci fi that turned out to be true its that we shouldn't touch the moon at all.

That's a good point.

Not a snark, but how long before the is some sort of preservationist, nationalist, or other, declaration that the moon isn't they to be exploited.

I do not know how much is in place already with regards to regulation but you can be damn sure nations and people will be tripping over themselves once someone does find a means to make money using the moon for resources.

the fantasies of space elevators appeal the geek/nerd in many of us but as a world we are far from the need of one if not too far from being united to having one. throw in there are just enough parties with the means to damage or destroy the ground side of one if ever built

I don't think there are that many parties on the moon with the means to "destroy the ground side". In fact, as of right now there are actually zero. (Though that would probably change should a lunar space elevator be developed.)

If they can get up there to chain themselves to the bulldozers, they can protest all they want.

I just finished reading the "Mars Trilogy" by Kim Stanley Robson where there is a tension between the "red" and "green" parties (first group want to keep Mars pristine, the other want to terraform it). The reds even sabotaged Mars space elevator in order to slow down emigration.

I guess it will be like Antarctica, where several countries made territorial claims[1] over it, many of them overlapping.

The UN's 1984 Moon Treaty[2] is dead letter - it has never been defied but is defunct in practice as none of the most prominent space-faring nations have ratified it.

If someone settles in the Moon you can refute their claims over territory there but what else can you do? Set an embargo? Send a military force and try to kick them out? Nuke them? Someone with knowledge and resources to colonize that desolated rock in space is not an adversary to be underestimated...

[1] http://www.antarctica.gov.au/about-antarctica/people-in-anta... [2] http://disarmament.un.org/treaties/t/moon

Correct, the moon has a lot of aluminum and iron in its composition, once you can put a smelter and forger on the moon you have access to an endless stream of girders and other materials to make stuff out of. That stuff might not be strong enough for space elevator construction but there are plenty of other things you could use it for.

Whether any of that would be economical is another matter.

http://lunarpedia.org/w/Lunar_Aluminum_Production

What's the heat source for this smelter/forger?

The Sun.

Now that idea just makes me think of https://marvel.fandom.com/wiki/Nidavellir

I had this in mind:

https://www.alamy.com/solar-oven-huge-parabola-shaped-mirror...

Large sun focusing mirrors might be sufficient

That sweet Helium 3 nuclear fusion :) . Oh, wait, it would be just wasteful to use aneutronic fusion to smelt metals on the moon. You could probably sell the Helium 3 back on Earth and just use regular nuclear fusion to do that.

The moon has ideal conditions for solar energy harvesting: no atmosphere, no weather...

Since Project Apollo in the 1970s it is known that all the materials needed for manufacturing photovoltaic cells are present in lunar rocks and dust. Not saying it is an easy engineering feat, but the raw materials are there.

combine factorio with kerbel space program, and we'll have this sorted out no problem

you dont even need a smelter/forger if you want to build a massive space colony at the L1 lagrange point (which the moon elevator would pass through). Just pack the raw lunar regolith into prefab plastic sheeting similar. this is similar in concept to inflatable space habitats with the added benefit of radiation protection and thermal insulation.

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

That’s no moon.

"With a space elevator in the moon you can use raw material from the moon to assemble huge stations and ships in space."

Yeah, you really believe its trivial to work in space? Repairing the Hubble cost billions, I can't imagine the cost of building 1 ship in space (from stuff manufactured on the moon, which itself would be so costly I can't even imagine).

The space elevator itself would be something outrageously expensive and dangerous to build - but there will always be people willing to take the challenge.

And likely less expensive than the international space station.

The proposed mass of the lunar elevator is 40,000 kg, and does not include life support.

The mass of the ISS is 419,725 kg and cost ~150 billion.

For comparison, direct appropriations for the 2003-2010 war in Iraq (in addition to the defense budget) were 1.1 trillion.

> [T]his proposal lacks the most compelling reason for building a space elevator: breaking the bonds of Earth's gravity well.

This proposal actually addresses exactly that problem. The dangly end of this thing down around GSO will not actually be in orbit at GSO. Rather, it will be hanging, stationary, at that point. In other words, you don't need to get 'into geostationary orbit.' Rather, you just need to lob yourself up so that your apoapsis is as high as GSO. Once there, you hook onto the little eye loop, or whatever fancier attachment system they devise, and wait to be pulled up to the Moon, more or less.

The key thing here is that you don't have to expend all the delta-v to get into GSO, but rather just enough to lob you to height. This, in itself, is a tremendous savings, given that you're shaving velocity off of a very large delta-v, where the penalties from the rocket equation are most severe. However, even if you only got a free ride from actual GSO to the Moon, it would still be an important savings.

Sounds like pretty high consequences if you miss the hook!

The thing is moving at 300km/h up there. You'll have plenty of time to correct any miss.

It wouldn't exactly be stationary, it would be moving at about Mach 1 relative to the ground

https://what-if.xkcd.com/157/

We currently dock with the ISS which is traveling 10X faster.

Yes but we're discussing amount of energy needed

From a geostationary intercept, a payload can theoretically ride a cable climber (power only, no reaction mass) and gravity all the way to a circular lunar orbit. Per a sibling comment, this technically only requires a transfer orbit or less, not a GSO.

This can be quite a good jumping off point to various places around the solar system, as many of these payloads will want a gravity assist from the moon anyway.

The American Delta IV Heavy has a payload to GTO of 14,220kg, and a payload to TLI of 10000kg. This means that getting to geostationary orbit is responsible for about 70% of the cost of reaching the moon. The ratio is a bit worse for the Chinese Long March 5. These are the two currently operational rockets for which my source lists a TLI payload.

70% is indeed the majority of the cost, but cutting 30% from your cost is still a pretty big deal.

source: https://en.wikipedia.org/wiki/Comparison_of_orbital_launch_s...

edit: misread tables

> The most expensive part of space travel now is getting from the earth's surface to a stable orbit, and a space elevator from the moon does exactly nothing to reduce this cost.

That's not the point, and not actually correct. [1] explains a lot of this. Getting to low earth orbit means you have to climb 100km, and go really fast. The climbing 100km part of this is easy - the going really fast bit is what makes going to space so expensive.

With this proposal, there will be a tether hanging down at the height of geostationary orbit which is travelling slowly around the earth - about 28 times slower than the orbital speed at that altitude. So, the elevator makes getting to the Moon cheaper by two mechanisms. Firstly, it means that a rocket just needs to get to geostationary orbit height, without having to do the speed bit as well. Secondly, you have a space elevator to get you the rest of the way.

Now, admittedly, geostationary height is much higher than low earth orbit, so the saving of not having to build up speed is not as extreme as if the tether hung lower. Geostationary orbit has a speed of 3.07km/s, so if you only have to go 1/28th of that, you are saving nearly 3km/s of delta-v on your rocket. This is not to be sniffed at.

The size of the rocket required to transport a set payload is exponential with the required delta-v, with a logarithmic base of the exhaust velocity of the rocket. So, if we can save 3km/s, and a decent rocket motor has an exhaust velocity near 3km/s, then the size of the rocket can go down by a factor of about e. This is a very nice saving.

One could extend the analogy, if one were to consider a slightly longer tether, and imagine we can get it to hang down to an altitude of 100km without hitting any satellites, then it would be travelling across the sky at around 60km/h. You could use a very small rocket indeed to climb up 100km, grab onto the end, and then go all the way to the moon. At a push, the X15 rocket-plane, plus some decent guidance system could manage it.

[1] https://what-if.xkcd.com/58/

The elevator is not positioned in an orbit, so one does not need to reach any orbit to get there. Up to a point, sending things up is easier than sending them into orbit.

Despite the really bad article that carries no information, I estimate the trip to the elevator takes ~9.5km/s (plus atmospheric losses). That isn't much more than LEO, and takes you all the way into the Moon.

Glanced through the paper quickly, couldn't find _any_ references to elasticity or strain, just the breaking stress. I would have thought that would be in the calculations somewhere, considering this thing would be a quarter of a million miles long! Maybe I'm just being a killjoy . . .

Think of the breaking stress as the harder problem to solve.

Breaking stress would cause catastrophic failure, for sure. Nonetheless when you're talking _hundreds of miles_ of cable, elasticity can make for some fairly profound failure modes too.

First sentence of the paper:

> 1.INTRODUCTION For a vehicle travelling in empty space it’s momentum, as well as it’s energy, comes from it’s fuel.

His, hers, its - are all possessives.

This is why one should have papers for publication, a CV, or a résumé proofread by someone who can spell correctly and understands grammar. Spell-checking software is fine, but it will not catch everything. If it is important, read it aloud to yourself, then get a friend to read it aloud to you. Yes, here the intended meaning is obvious, but this left me wondering about the accuracy of the rest of the paper, which is not a good start for any reader of anything written.

The comment[0] I made a previous time this was submitted[1]

> One major problem with the classical Earth based Space Elevator is the problem of security. It wouldn't take much (relatively speaking) for a terrorist organisation to create a credible threat.

> A Moon-based Space Elevator wouldn't have that problem.

[0] https://news.ycombinator.com/item?id=20977269

[1] https://news.ycombinator.com/item?id=20977142

The cable is held by tension from the Moon anchor point right, so I wonder about ramming a satellite a few hundred kilometers above the endpoint. If the cable is severed in this way, will the loose end fall straight to Earth causing damage? It's not itself in GEO orbit, it's moving at Moon orbital speed and pulled by Earth gravity. So I feel it should fall pretty much straight down.

Yes, clearly there are failure modes that can be provoked, and breaking the cable close to the Moon's surface will result in this thing falling to Earth from a great height.

But most terrorist organisations short of governments won't have the capability to provoke such damage, and it's likely that most of it will burn up on re-entry, and possibly can be designed explicitly to do so.

I think it would easily burn up in the atmosphere. With <1mm in diameter it has a huge surface area.

Was on HN 10 days ago: https://news.ycombinator.com/item?id=20895443

Without considering the fact that the orbit of the Moon is not circular, this only helps reduce the delta-v needed to go from the surface of the earth to the surface of the moon by ~3.2km/s, but we still need ~12-14km/s to go to GTO/GEO.

i think the bigger upside here is that getting humans into orbit and then to the moon via elevator is likely safer than repeated landings back and forth.

Indeed this would avoid the tricky 'land to a place where nobody can help you' part.

Also would introduce the tricky no way to abort if something goes wrong. Unless you are willing to carry a fast abort rocket with you.

However since the moon period is not the same as a GEO period, maybe the fact that you could juste 'catch' the end of the cable before falling back to earth without having the speed to maintain at GEO could help you save some delta-v.

That's essentially what GTO is - an orbit going close and fast at one side and going high and almost stopped at high side.

Going directly up means over 1h of fighting earth gravity. So need Isp > 3600. Much cheaper to go for orbit.

An elliptical orbit that intersects the surface of the massive body at the periapsis side is fine, if you can change the orbit when you reach apoapsis.

But if you miss the cable, you're boned.

Useful for refueling GTO sattelites from the Moon however.

Just need to build a trans Lunar railway from the poles to supply the water.

Or attach the damn thing to one pole because no atmosphere.

You don't need to hold the orbit. Just jump to the elevator and grab on.

Would be nice to get the savings in delta-v, I've no idea how to compute this.

A ballistic approach is enough so the kinematics are simple, too.

The gravity is only 3% up there, so more tricky. And reducing with square of the distance.

Also every second you are accelerating straight up costs you gravity worth of Delta-v because gravity losses. So need to be quite short and intense burn to be worth it.

Eh?

You don't need to get sideways velocity at all. It's enough to only reach the distance. This is a major and huge saving in fuel.

My estimate is ~9.5km/s from the Equator to the hook. Add some more for atmospheric losses.

you can use the partial escape velocity equation. ~10 km/s to get to GEO without the radial velocity. delta-V to the moon is about 18 km/s (one way). Keep in mind that kinetic energy is proportional to velocity squared.

1. Build a space elevator anchored to the moon.

2. Mine raw materials on the moon.

3. Send them over the elevator piece by piece to earth orbit.

4. Assemble the counterweight for an earth-space elevator in orbit.

5. Have TWO space elevators, one to get you to orbit, another to take you to the moon.

6. Colonize the solar system or whatever.

The moon has a ton of deuterium which could be used for all sorts of things, nuclear power plants and possibly future propulsion tech for example.

Deuterium is abundant on earth as well. It's tritium and helium3 that's unusually abundant on the moon. But it's the result of solar wind deposition over copious amounts of time. And it's one of the dumbest reason to mine the moon.

You can run the math the flux of those elements from the solar wind is minuscule. The area you'd need to harvest for a reliable steady state is huge. You'd be better off putting up solar panels, energizing a rail gun and launching more solar panels into space made from lunar silica.

Okay, railgun it is.

Getting the counterweight to orbit is not the hard part of building an earth space elevator. The hard part is the 36,000 km cable from the surface of the earth to geostationary orbit that can support its own weight against earth's gravity.

There are a few experimental materials like carbon nanotubes that have the right tensile strength to weight ratio, but we aren't anywhere close to making them in more than microscopic lengths.

That's okay, we'll keep ourselves occupied with the moon mining part, you work on the cable.

7. Accidentally hit some debris with what is basically a giant golf club

8. Enjoy being stuck on Earth after the ensuing Kessler effect

1.5. Use a giant net to collect debris from earth orbit and send it to a landfill on the moon.

Or just weld the collected space junk together to build a counterweight ¯\_(ツ)_/¯

7. add a bunch of clones to the Moon station to keep things running smoothly

This a moon-GEO elevator which may have its advantages but it does not help you get out of the planetary gravity well.

Well, thanks to the bastard rocket equation, fuel savings have an exponential impact. If you can get to geostationary orbit and meet up with the elevator, then fuel up again at the moon, you save a considerable amount of weight in fuel.

Exactly. Overcoming Earth's gravity well is the reason building a space elevator is such an exciting prospect.

The geosynchronous orbit is about a 10th of the way to the moon, so this moon elevator would go 9/10ths of the way toward the Earth. That's significant but it's precisely that remaining 10th of the way where the vast majority of energy expenditure occurs. If you can make it from the surface of the Earth to geosynchronous, then making it to the moon is relatively cheap.

Not orbit, the Moon doesn't orbit the Earth in one day, but a month. The cable would be almost stationary compared to orbit. That needs less power.

Less, but not much less. Around 12 km/s instead of 14 km/s.

Escape is 11.2 so something must be off.

Edit: actually you don't need any specific speed. Reaching orbit does, around 9 km/s for low orbit, vertical uplift doesn't.

Rocket to Moon > rocket to orbit > rocket to reach orbit height vertically, so it should help, shouldn't it?

Of course pulling up using the cable would needs power, but it could be a solar powered elevator.

So if the elevator from the moon extends toward earth and is held in place by Earth's gravity... why stop at geostationary orbit height?[1] Wouldn't you want to extend the elevator as close to the earth as you could go? It seems to me that the closer to the earth the elevator goes, the more Earth's gravity keeps its location stable, and the easier it is to get from Earth's surface to the tip of the elevator.

It seems to me you'd want to stop the elevator just shy of the strength limitations of the cable material (with some margin of error).

[1] I suspect the article may just be wrong about this, since as others have pointed out, the moon has an elliptical orbit that is not geostationary.

*Assume the ball and the man are point masses, the man is standing on frictionless ice, and there is no drag when the ball is thrown.

I imagine it's because stopping upper than GEO clears the plan from any thinking about debris.

A lower cable would cross GEO twice a month at a huge relative speed. There are a lot of things there to hit the cable.

Lot of talk about how

1. a Moon elevator is pointless without an Earth elevator, and

2. we can't build an Earth elevator with today's technology.

However we can build an orbital ring around Earth with today's technology, and it'd be much better than space elevators.

[Video] https://youtu.be/LMbI6sk-62E?t=254

[Paper] https://jenda.hrach.eu/f2/Low%20cost%20design%20of%20an%20or...

That paper doesn't strike me as being particularly credible.

If one end is tethered to the moon, and the other end is in geostationary orbit...you're gonna have a bad day. The moon is not in geostationary orbit.

Not an orbital scientist but doesn't seem that will work out very well.

I think it’s at geostationary orbit height.

I don't really understand how it can stay at geostationary orbit height without being at geostationary orbit speed. Can someone explain?

Well it's anchored on the moon at the other side

It's dangling down from the moon. Going at moons speed - 28 days per revolution instead of geostationary 1 day. So less centrifugal force form rotation that is compensated by the cable strain.

The end is at that height. Its center of mass is much higher, and it's rigid enough to hold together. It's also not orbiting—its other end is nailed to the lunar surface.

Well I started thinking about it more.

If it's orbiting at the speed of the moon, it's going slower than geostationary orbit speed. So its tendency will be to fall towards the earth. But since it's attached to the moon, this won't happen.

Not sure how much station keeping would be needed to hold this in place.

Also will have to deal with collision risks with objects in geostationary orbit. Though I don't think geostationary orbit is a particularly important factor. Could probably be a few thousand Km above geostationary orbit without any significant adjustment to their plan.

Crossing orbits full of satellites is going to be an even bigger problem with an Earth-based elevator. LEO is much more crowded than Geo.

But it would get really interesting if we were to first build this moon-based elevator, and later once the tech is good enough, also an Earth-based one. Because that one is not going to extend merely to geostationary orbit, but well past it; the center of gravity is going to be in geostationary orbit. So over a distance of 36,000 km, there will be two space elevators zipping past each other once per day.

The tether is thin. The orbit is kinda wide at that altitude. It's not going to hit anything, even if you tried.

Surely, for this to be useful, we need to be able to hit it if we try. Agreed that anything not trying to intercept should be able to avoid that by not trying.

Haha touche!

I meant even if you tried with the tether to swipe something. But we should be able to hit it with a rocket

It is not a point. Consider two masses connected by an rope, one in an higher orbit. The entire contraption will move along their common center of mass, and there will be tension on the rope. The same thing applies for an lunar elevator, one end is somewhere close to geostationary orbit, the other is the moon, and the entire system moves as a combination of internal tension and the movement of the center of mass.

The tether is hanging from the moon, not orbiting earth. There is tension in the tether. It's not in free fall.

because it's attached to the moon, which is preventing it from falling into the earth which it would otherwise do at that altitude going that slowly.

And the Moon orbit is not a perfect circle, the Earth-Moon distance varies quite a bit (about 40000km between perigee and apogee), so to actually keep the cable at the same earth altitude you will need to change its length quite a bit (seems like around 125km/hour).

Other end is not 'in geostationary orbit' it's at the same height from the earth as earth's geostationary orbit.

It would be an issue if you expect it to always end up to the same place, but if you don't care where the elevator ends on the Earth side, because you know it will be in your area at regular intervals, it can be dealt with.

I'm not sure why they picked geostationnary orbit distance if they don't look for locked location though, it seems the risk of colliding with objects would be greater at this height

Since the tether will be moving a lot slower than sattelites in geostationary orbit, and the risk of collision, I assume stopping just short of geostationary is the closest that will be deemed reasonably safe.

It will actually work out better than the alternative, because the moon is tidally locked. An elevator anchored to the moon will always be pointed toward the earth, an elevator anchored to the earth will rarely be pointed toward the moon.

What cable weighs less than half a pound per mile?

200,000 miles 88,000 lb cable

No friggin way.

Well since it's in pounds, that's an earth gravity weight measurement. Maybe they're talking about moon weight :)

Actually, not using a measurement of mass is a bit of a red flag.

EDIT: Perhaps I should read the paper and not rely on science-journalism. At first glance paper looks pretty good in terms of mass measurement and estimation.

Yes, that seems to have been misunderstood. The cable that weighs 88,000lb (in the paper, 40,000kg), has a cross-sectional area of 10⁻⁷m²! That won't stand up to much use.

If I'm reading (skimming) right, they say "this would only allow transport of weights up to 100kg".

100kg seems like a barely usable amount of lift. It means you need an in-orbit assembly system for most things you want to launch and I'm pretty sure you're not going to be able to launch a person with life support gear.

Yes, it's hard to imagine a cable-crawling machine and its power source being under that mass limit, let alone a payload.

I had assumed the cost of the crawler was already included in that 100kg figure, but if not then yeah, that's even more marginal. It really can't bring its own fuel source along, it needs to work on beamed power or maybe a current between two parallel tethers.

So, uh...if it can transport 0.25% of its weight, then that kind of implies that the safety margin is of the same approximate order, doesn't it?

The paper uses some pretty exotic materials. The one with the lowest density has a strength-to-weight ratio only 8 times that of steel...

I have questions about stuff like, how they stop it icing up...

how this magic material handles the temp gradients from the cold of space to the relative warmth of of earth and then when it gets hit by unfiltered sunlight.

Zephyr Penoyre (one of the paper's authors, then at the IoA in Cambridge) gave a talk about his proposal for a Moon-tethered spaceline earlier this year, and is available here: https://upload.sms.cam.ac.uk/media/2921572

The talk starts with a polemic about the way research is done, but the physics starts about 16:40.

> Future moon travelers will still have to ride a rocket, though, to fly up to the elevator’s dangling point, and then transfer to a robotic vehicle, which would climb up the cable all the way up to the moon.

And... how long is that going to take? A few months? A year? 200,000 miles is like going the circumference of the earth 10 times and I can't imagine a machine attached to a pencil-thin cable could go very fast

If you assume the cable car is moving at a constant 60 miles per hour, about 4 and 1/2 months (138 days). The current fastest elevator in the world (in the Burj Khalifa) moves at 22 mph; taking that up would take you about a year. However, I'm told by a friend who used to work at Otis (the company that made the Burj Khalifa elevator) that the limiting factor on elevator speed is the comfort factor - there's a hard cap on the acceleration a standing person can comfortably take, and the course of the elevator is generally pretty short, so you don't have time to build up a decent head of speed.

Presumably we can do a little better than that

As a side note, usually cables used in lifts/cablecars/cranes/etc. have a 5x (or even in some cases higher) factor of safety, i.e. the cable needs to resist at least 5,000 Kg of traction in order to be used for a 1,000 Kg weight.

Part of this high factor is because cables wear down during use (and they are usually inspected for all their length every 6-12 months), particularly when relatively high travel speed is used.

https://what-if.xkcd.com/157/

One disappointment with space elevators is the speed at which you can safely travel along them. For example, if you were to travel at 1000 km/h it would take 15 days to get to the moon and just under 2 days to get to geostationary orbit (if there was also an elevator for that)

That might be disappointing for human travel, but for cargo its great. As long as throughput is high the latency is no big deal.

But you can't get high throughput unless you have a large capacity in terms of mass. It's easy to say "cargo", but if the only cargo it can carry is a flea, not so much.

Disappointing in one way, but in another, it seems like it would be extraordinarily pleasant to be able to enjoy such a journey for a while, perhaps like old trans-oceanic cruise ships...

you don't need to hold on to the tether all the time.

Linear motors on the car and embedded metal strips every 1 km and you have a Lunar rail gun :)

In practice the rotational speed difference at different altitudes will make it a bit more complex. But nothing that cannot be solved.

Non scientific observation: The Earth and Moon are not a consistent distance apart. XKCD What-If 157 (https://what-if.xkcd.com/157/) neatly illustrates this with the idea of connecting a pole from the earth to the moon, noting that "it's enough that the bottom 50,000 km of your fire station pole would be squished against the Earth once a month".

I realize XKCD isn't precisely accurate, but even if 50,000 KM is a /rough/ estimate Geostationary Orbit Height is still around 35,785 km, significantly less. As the moon and earth move closer this Lunar Space Elevator would need a winch capable of taking in 50,000 KM (or perhaps less, but not much or the earth's gravity would become much stronger on the 'station' end and the rope's strength calculation would be off) of "rope" or we're all gonna have a bad time.

However wonderfully grandiose these elevator pitches sound, I'm always left wondering: what happens when some space debris hits the thing? Instant Kessler syndrome?

I believe the traditional answer is "lasers" or "someone else will figure that out"

No, dual cosmic lashing. One end hitting the moon the other one earth.

On earth side the atmosphere will take the worst of it, moon will hurt...

Also there is hardly any space debris beyond geosynchronous.

Interesting, that makes the case for a moon -> geostationary stronger...

A part falls on Earth, another part falls on the Moon. Or a part falls on Earth and the other part is kept hanging at the same place, depends on where you cut it.

This guy did some simulations on cable breakage: http://gassend.net/spaceelevator/breaks/

This is fantastic, thanks!

If that cable resist interaction with the atmosphere, some people on earth are going to be royally fucked

http://gassend.net/spaceelevator/breaks/break100.gif

Interestingly most debris on the tail end appears to be ejected outward rather violently

I've heard them described as a very thin ribbon, in which case it would be (thousands of tons of) material wafting down like paper, not a heavy cable slamming into the ground and crushing buildings. I don't know if that describes the standard design, or just one particular one.

From the paper [0]:

> Values taken straight from Wikipedia.

[0] https://arxiv.org/pdf/1908.09339.pdf

Let's not call a 200,000 mile, multi billion dollar cable "easy", but let's say it's doable, important, and absolutely necessary!

Why is it absolutely necessary?

In what way it's any more "doable" than e.g. digging a tunnel through the Rockies with a spoon (or worse, a spork), given our current technology?

Does someone know what the difference in deltav would be to get to geostationary orbit from earth vs getting to lunar orbit from earth? A citation would be helpful.

I've tried looking this up, but from previous comments on this paper, I infer that I am misunderstanding something.

GTO: ~11.5 km/s

GEO: ~14 km/s (but you don't need that speed since the end of the cable is much slower, orbit period of ~28days instead of 24 hours)

GEO <-> moon surface ~3.2km/s

superbe reddit source: https://www.reddit.com/r/space/comments/1ktjfi/deltav_map_of...

Thanks but that's not what I asked.

I asked what is the difference in delta-v between getting from the surface of the earth to GEO vs getting from the surface of the earth to Lunar orbit.

The answer is, there is almost no difference.

Would the Earth end still be in geostationary orbit even with a variation of the Moon's orbit by 43,000 km?

And if the Moon end snapped would that mean the entire 380,000km cable would crash to Earth? How much weight and what velocity would it be? Crazy!

It's 40,000kg, so about 400 heavy people or a handful of elephants. Might not be fun if you're under it, but not particularly damaging to the planet.

But what velocity would it be traveling?

Project Thor aka "the Rods from God" concept is a satellite with 20'x1' 76,000 lb (34,000kg) tungsten rods. No explosives just pure mass and kinetic energy.

From low Earth orbit they would travel at Mach 10. Supposedly at impact each rod would release the same energy as a small nuclear bomb.

totally unrelated but is there an opportunity or reason to tow a load to space via cable behind a "rocket tractor-trailer" instead of rockets pushing from behind? Maybe the tractor is a quadcopter or octocopter but rockets instead of electric props. Rocket engines would need to aim their exhaust plumes outward slightly to avoid melting the tow cable, but it would seem like a tow cable could provide some level of suspension (stretch) to the load, reduce structural requirements since materials dont require stiffness in tension. Maybe there is no benefit compared to just pushing the load as is traditional.

https://what-if.xkcd.com/157/ A fire pole would be slightly cooler.

I think given the lack of precision in this article it is better to replace the sensational news article with the actual arxiv paper.

how to get to moon without a rocket: 1. go to moon and install thin wire on moon and dangle it toward earth 2. use a rocket to reach wire

Wonder how fast the 'elevator cart' would be able to move. Considering that it takes a rocket several days to reach the moon.

Most of the time the rocket isn't firing - you have a few minutes of burn to change your delta v, then you just gotta sit and wait before decelerating into orbit (or landing). Assuming the cart was attached to a powered cable you could in theory constantly accelerate until half way, then constantly decelerate. Possibly this would allow you to make the trip faster (unless you had humans on board, then you're probably stuck with short accelerating bursts).

I'm pretty sure humans can take 1 g continuous.

I'm not sure if the cable can take it: 1 g yields pretty large speeds quickly. Somebody already made the math at [1]

[1] https://space.stackexchange.com/questions/840/how-fast-will-...

> it takes a rocket several days to reach the moon

That's using an unpowered transfer orbit. Burning the whole time would be considerably quicker (and be extremely inefficient).

How about moving stuff _from_ the Moon back to Earth. Harvesting the Moon for rare materials is going to be relatively cheap.

What rare materials would these be?

The only thing that comes to mind at the surface would be Titanium but even there the economics would probably not make sense if your goal is to use the material on Earth.

I have no idea. Just trying to make a point for the utility of the project. Maybe rare earth elements or something else yet to be discovered?

> What rare materials would these be?

Every material is rare outside of gravity wells. Even plain steel, aluminium or magnesium cost >$1000/kg if you want them delivered to geostationary orbit. So this material would not be for earthbased construction, it would be for space factories, space-based solar microwave powerplants, spaceships etc.

Absolutely, but GP said to import those materials back to earth.

good point

Is there a sweet spot where both gravity's will pull it towards eachother?

The 'sweet spot' is called the Lagrange point.

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

Ok could be good but the real issue is leaving earths atmosphere.

if the tether would be visible in sun light, and because of eccentricity of moon orbit not always pointed directly to earth, it could look interesting from earth.

Wouldn't a skyhook work better on the Moon?

Now that's a zip line!

So inexpensive: Only a few billion dollars not going to people and communities who actually need it.

They'd just blow it on booze and drugs right?

"And you know lately, I've been thinking about how I love Jesus / Because Jesus was a dirty homeless, hippie peace activist / And he said, 'Drop out and find God' to anybody who would listen / While turning water into space bags..."

I'll believe it when I see it.

> proposed the idea of a “lunar space elevator,” which is exactly what it sounds like—a very long elevator connecting the moon and our planet.

No that's absolutely not what a space elevator is

A space elevator is a device that lifts things to space along a static cable. A space elevator on Earth lifts things to outside of Earth's orbit. A space elevator on Mars lifts things to outside of Martian orbit. And a space elevator on the moon lifts things outside of lunar orbit. This definitely is a space elevator by any definition of said technology.

> connecting the moon and our planet

Reply on news.ycombinator.com