> 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.
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
>> 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.
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.
Maybe a landing platform could be hung from it, helping to stabilize the end.
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.
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.
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
Yeah, that's totally feasible and useful... /s
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.
Note that there are no feats thus far...
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...
Not GP, but... what? Why are you filtering materials by the GP's in-brain knowledge?
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.
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.
edit: misread tables
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.
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.
The large amount of science equipment manufactured on the moon?
A small mass savings near the end of the journey might translate to significant mass savings near the start.
Not sure how practical all of this, but that is at least the theory.
 Credit to mnw21cam: https://news.ycombinator.com/item?id=20996246
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.
With a space elevator in the moon you can use raw material from the moon to assemble huge stations and ships in space.
Whether any of that would be economical is another matter.
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.
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 guess it will be like Antarctica, where several countries made territorial claims over it, many of them overlapping.
The UN's 1984 Moon Treaty 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...
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 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.
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.
That's not the point, and not actually correct.  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.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.
> 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.
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.
Going directly up means over 1h of fighting earth gravity. So need Isp > 3600. Much cheaper to go for orbit.
But if you miss the cable, you're boned.
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.
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.
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.
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.
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.
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.
8. Enjoy being stuck on Earth after the ensuing Kessler effect
Or just weld the collected space junk together to build a counterweight ¯\_(ツ)_/¯
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.
Edit: actually you don't need any specific speed. Reaching orbit does, around 9 km/s for low orbit, vertical uplift doesn't.
Of course pulling up using the cable would needs power, but it could be a solar powered 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).
 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.
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.
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.
Not an orbital scientist but doesn't seem that will work out very well.
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
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.
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.
I meant even if you tried with the tether to swipe something. But we should be able to hit it with a rocket
88,000 lb cable
No friggin way.
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.
If I'm reading (skimming) right, they say "this would only allow transport of weights up to 100kg".
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.
The talk starts with a polemic about the way research is done, but the physics starts about 16:40.
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
Presumably we can do a little better than that
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.
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.
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.
On earth side the atmosphere will take the worst of it, moon will hurt...
Also there is hardly any space debris beyond geosynchronous.
If that cable resist interaction with the atmosphere, some people on earth are going to be royally fucked
Interestingly most debris on the tail end appears to be ejected outward rather violently
> Values taken straight from Wikipedia.
I've tried looking this up, but from previous comments on this paper, I infer that I am misunderstanding something.
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...
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.
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!
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.
That's using an unpowered transfer orbit. Burning the whole time would be considerably quicker (and be extremely inefficient).
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.
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.
No that's absolutely not what a space elevator is