🚀 Faster, Please! — The Podcast #23
↕️ A conversation with physicist Stephen Cohen on the engineering challenges of building a space elevator
Thanks to SpaceX, it’s getting cheaper and cheaper to launch stuff into orbit. But just imagine if instead of using rockets, we could send cargo and people to space on an incredibly tall elevator. This may sound like a total sci-fi idea, but it has some grounding in real-world physics. In theory, we could build a space elevator by putting a counterweight in geostationary orbit and attaching a cable between the satellite and Earth. An elevator could then climb the cable, delivering payloads to space at a fraction of the cost of propulsive rockets. As you can imagine, it isn't quite that easy, which is why I'm joined today by Stephen Cohen.
Stephen teaches physics at Vanier College in Montreal and has been working on space elevator concepts for almost 20 years. Recently, he wrote “Space Elevators Are Less Sci-Fi Than You Think” for Scientific American. Stephen also has a new book, Getting Physics: Nature's Laws as a Guide to Life, which was released earlier this year.
In This Episode
Space elevators 101 (1:42)
The engineering challenges (7:14)
The economics of space elevators (11:07)
Space elevators in sci-fi (19:21)
Below is an edited transcript of our conversation.
Space elevators 101
James Pethokoukis: In the intro, I tried to do my best at explaining what a space elevator is. But the simple version is we have something big and heavy in orbit, a cable extends down from that thing, attaches somewhere on the Earth, and we run an elevator up and down it. That's a space elevator. Am I right?
Stephen Cohen: Sure.
Now that we have a picture in our heads, why is it something more than just an interesting engineering thought experiment? What attracts you to it, other than sort of a technical problem that would be interesting to solve on paper?
Well, it's space infrastructure, which is something we don't currently have and never have had. Right now, and for all time we've accessed space, going to space is like a one-off each time. Sometimes you have some reusable parts, but basically what a space elevator is, is a bridge instead of just a bunch of boats.
And the advantage of a bridge over boats is what?
Access. Right now, each time you want to plan a mission, to simply put something into orbit requires a lot of planning. The weather has to be right. And then you want to plan another mission, you sort of have to begin again. With a space elevator, you can just days in advance say, “Okay, we're going to send something up to a desired orbit.” And just hours later after that one would be sent, you could send something else. And you basically have a housing — that's what the climber is, effectively — that you put the payload inside and up it goes. That's the transformative part. But we haven't talked about really the cost savings, the energy savings, and that's just basic physics.
The way you get around in general is by applying forces. And that's something you do without thinking. When you walk, you push on the ground. When you fly through the air, you're basically pushing on air molecules and they push back. But in space, you have none of that. And so what rockets do is they literally are the medium. The fuel you bring is the medium you're pushing against — rather, you're throwing it out the back. It's a hugely wasteful, inefficient way to get around. It's preposterous when you think about it. But it's the only way we can get things to the speeds we need to get them to. Just as a mode of getting things into this is extremely practical. You can't compare the efficiencies. It’s orders of magnitude of difference.
It really strikes people. When they hear the general concept, they really think it's something big and it sounds like it's amazing. It's something that is science fictional, but maybe we could turn into science fact. There's something else about it, I think, that just grabs people's attention.
Yeah, for sure, because it's a physical connection to space. It's like, if you could just touch the cord at the Earth port, then you're in contact with something that's reaching out all the way into space, which is wild. But I think there's an element missing. People don't realize tethers in space are not a new thing. We've had missions since the ‘70s that are effectively two bodies orbiting earth connected by a long tether, sometimes kilometers long. Now, that's not in the ballpark of 100,000 kilometers long, which is a common number thrown out there for what the eventual space elevator might be. But a lot of the same technologies are involved. The biggest difference is of course, instead of two bodies connected by a tether, like a big spacecraft to a small spacecraft, say, this is a big structure connected all the way to Earth. The amount of tension is tremendous. That's the big difference. That's what effectively becomes the big engineering challenge about it all.
To be clear, the cable would be connected to something large in orbit, and that could be something we build, but I've also heard maybe it could be a small asteroid? Am I confusing two different things there?
It doesn't have to be something we build, but likely it won't be an asteroid. The way at least the first space elevator will likely be constructed would be you send the cable up in a spacecraft and you drop it, you sort of spool down the cable over time. And it would be a lengthy unspooling. The dynamics of that are super interesting. But the point is, at the end of it you can now connect that cable to the ground. It's good to have something functional at the other end, not just some mass. Of course, the mass you're going to have at the far end, the particular value of that mass, that depends on how long the cable will be. So to achieve an equilibrium, you can't just choose any random mass. It would have to be planned.
The engineering challenges
Let's get to some of the challenges. And as you answer those questions, we may also find out why you think this is something that can be done. You mentioned that cable. That seems to be the chief engineering issue, as you mentioned: finding an ultra-strong lightweight material to make up that cable. Is that something that needs to be invented? Are we talking an innovation? Do we need radical new science, or can you see how that cable could be manufactured in a decade if we got serious about funding that kind of research?
The key property is called specific strength. It's not just strength, but it's the strength-to-density ratio. And that property in the material existed since the mid-‘90s. But it's very costly to produce. Time consuming as well. Now, on both fronts, there have been big improvements since then.
Are these carbon nanotubes? I always hear about carbon nanotubes. Is that what you're talking about?
The two candidates that are talked about these days are carbon nanotubes and just graphene. These options, there are some issues with repeatability. So the process, you think you're doing it the same twice, but you don't get exactly the same properties each time. It still needs to mature, but the basic science is there. It's become a materials engineering kind of problem.
Is it an engineering problem that we just sort of have to work the problem, and it'd be great if we had funding, but it doesn't require a radical breakthrough? We think we know how to get there. It's just sort of resources and effort and time.
Yeah. Yeah. There are probably solutions to every problem that stand in its way. I would say as the material problem is getting solved and as time is going on, a new problem is entering that is on the same level as the material problem. And that's our very, very crowded space environment. That is only becoming a bigger issue. That problem is only going to get worse with time. And the equator is a fairly busy area. It's very likely that the space elevator will be situated slightly off-equator, and the mechanics of that are sound. That's not a big issue.
On land or in the ocean?
Probably in the ocean, is the proposals I've seen. Those are sort of the details, I would say. And it will come down to economics, won't it? We're still at the stages of design, but there's really no company that is clearly in charge and no administration, institution is pulling all the strings. What we have right now is a big project with a bunch of academics scattered around the world that are, I would say, dabbling in it. A lot of work has taken place. I would say low-intensity work. That is, you get 10 very useful studies done in the course of a year. That's peanuts for something on this scale. There needs to be probably a champion or several on the business side, I guess. But also governments need to get involved for this to really take off.
The economics of space elevators
It must be annoying that you can't find a super billionaire — they seem to be very interested in rockets. You need to find one who's interested in a space elevator. That would seem to be an important piece to the puzzle when you look at how things are going in space and rocketry.
Yeah, on the economic side of things, if you want return on investment, you probably need to work on steps to get there. So partial space elevator, that's something which is basically a larger space tether. Space tether on the order of thousands of kilometers. So it's an easier challenge, but the payoff isn't nearly as high. There need to be small aspects that are worked on that have return on investment that get you there. There are several that could be listed. If I could speak about the big investor of which you just mentioned, there's another project that really reminds me of the space elevator: something called Breakthrough Starshot that you haven't heard of it. It's an attempt to send something interstellar. To send to another star system a very small payload, on the order of grams, that we could then once we get there take a picture of, say, an exoplanet and send it back. And we’d get something way cooler than what our best satellites can do. That project also has a few major engineering challenges, but I wouldn't say science challenges. We're now at the point where there's a road to it. It's also probably decades away. It has spinoff technologies. They're really very similar. And the interesting thing is, there seem to be investors putting more money into that one than space elevators. That's my impression. Not boat loads of money.
Isn't that a Mark Zuckerberg thing? Hasn't he put money in that?
I don't think he's the only one.
I'm not fully aware of all the happening surrounding Breakthrough Starshot, but it's worth mentioning that the space elevator is completely transformative for life in our solar system, really. We talk about colonizing the Moon and Mars, and that would be really neat. But it's sort of a pipe dream if you can't support it. Sending a single person or several to Mars, that's a big, big undertaking. But now for them to live there in a supported way? The amount of mass you have to get there is tremendous. And you can't do this in a sustainable way without infrastructure. The point I'm making is, a space elevator [is] really transformative for the solar system. And I don't want to speak down on Breakthrough Starshot. I don't want to speak ill of that project. Totally cool. I'm on board. But that one, I would say, is transformative in the sense that you can actually send something to another star. We've never done that before. But it wouldn't change life as we know it, unless our picture happens to show something living on an exoplanet.
Someone else's space elevator, perhaps!.
It's really the economics and efficiency of getting something off the ground, into orbit. Has that economic potential calculus been changed, or would it be changed, by reusable rockets? I mean, when you first got interested it was probably either pre-SpaceX or maybe SpaceX’s early days, and those costs have come down and are expected to continue come down. At some point, does that make a space elevator irrelevant?
Before we get to the cheaper chemical rockets, there are other changes that have taken place. For example, nuclear rocketry. There's also the idea of solar sails and things like that. But of course, none of those can address the primary reason why a space elevator is useful, and that's to get out of the Earth's gravity well. That's where you need chemical rockets or, well, nothing else. Nothing else will do it, because you need a tremendous amount of power in order to reach those speeds, unless you can just climb along a cable. Of course, those chemical rockets get cheaper. It doesn't mean they necessarily become routine, in the sense that weather will always be an issue, safety always a concern. They're not green, and if you intend to get really serious about space in the way people are talking about it, we are talking about such wasteful practices there. It's just unconscionable in a way. That's not the economic side, I realize. But an economic study needs to probably be repeated regularly to see whether this is the best way forward, purely based on economics. Access, environmental considerations: Those are other elements that also need consideration. But the economic story, I'd say, is evolving. Chemical rockets will always have a certain ceiling that you just can't beat, and we're maybe getting close to it.
If I got into a space elevator capsule on Earth, how long would it actually take to get up to a space station?
In all likelihood, there will be a station at geosynchronous that's 36,000 kilometers high — so about three Earths away — and it will probably take a week to get there if you could go in the area of the high-speed trains we've become accustomed to on Earth. That would be beautiful views for a week. What's cool is as you go up, the weight you feel goes down gradually until you reach this geo place. And then you are indeed weightless, just floating there like they do in the ISS. However, you'll have passed the ISS a long, long time ago, because that's only 300 kilometers off the surface of Earth. You couldn't put a station there on the space elevator. Physically that just wouldn't work. Geostationary is the ideal place for a space station because it imposes no new tension on the cable. In any case, it would take a week, is the short answer to that question.
But that week would be a far more relaxing experience than taking a rocket.
And let's be clear, this would be way cheaper once you've got it. Operating one of these, you wouldn't pay millions of dollars a person. Not even close. I can't know exactly what the number would be, but it could be 100 times less for one person once this thing's really up and running. Plus you don't have to spend a week going to geo and a week coming back. If we're trying to recreate the experience of going up to 300 kilometers, it could be an hour up and down and you've achieved a nice view of Earth.
Space elevators in sci-fi
It's an interesting concept, but one which is probably used more in scientific literature than in movies and TV shows. I think the first time I ever saw one on a screen was in the recent TV series based on Foundation by Isaac Asimov where they had a space elevator. Now, of course, the space elevator — spoilers — the space elevator in that show, there's a terrorist attack and it falls down and just kills…
Is there a portrayal of this technology in science fiction that you're aware of or that you think is interesting?
There's some artistic license, perhaps, going on there. What would happen if it's severed, if that's the conversation we're having, the portion beyond the severance likely is gone never to be seen again. And then the portion below, its future really depends on where the severance happens, exactly what that looks like. There was a study done when I was doing my master's — in like 2005, 2006, I think — [by] someone named Paul Williams, if I remember right. He did animations on exactly this question. It flies down to Earth, the lower portion below severance. And it would, like, paint a line on the equator —whatever didn't burn up in the atmosphere on the way down. But we're talking about a cable that's like one meter wide and very thin. So don't imagine a building collapsing that's wrapping around the equator. It's a rubber band, if you want to imagine something.
The piece you wrote in Scientific American, have you gotten any feedback on that from other scientists, astrophysicists, engineers? What kind of response have you gotten, if any?
Oh, I've gotten letters from high school students. “Can you tell me this? Can you tell me that?”
It was a completely honest piece. I am not what I would even call a space elevator advocate. But the moment I start talking about it, I get excited. To be clear, a quick perusal of some of the online message boards reveals a lot of, well, trolling where some people who may be informed, some people who aren't, just write a thousand reasons why this will never happen, X, Y, Z. But most of the feedback I've gotten in the circles I would ask through are just: “That was delightful to read.”
I think it approached it with the appropriate level of seriousness for something that's interesting, it's not tomorrow, but it's possible. And let's give it some thought. That seems like a very reasonable approach to the issue.
I'm a college teacher at this point. I've worked in the space industry. But my goal is to capture people's imagination when I'm in the classroom. That's at least a big part of it. The space elevator ticks a lot of boxes in that department. Exactly where it'll go in terms of economics and all that, I don't really know. And in my day-to-day life, space elevator is something I dabble in when I have free time and when I feel like it. It is something I write about in a small part of the book that I published recently, but it's mostly a general physics book, for example. It's not the focus of my life.
Let's say we elected an American president who said, “This is something we can do. We're going devote resources. This is a new Apollo.” With enough effort, could you say within a decade we could have a space elevator, if we had that kind of enthusiasm and allocation of resources?
I think in a decade we could have a design that is pretty mature, and I think a decade after that it could be built. But again, that would take the kind of backing that is associated with serious projects. And you’d talk about many countries coming together. To go on a little tangent, there was a film that had a space elevator recently released in China. I cannot recall what it was, but a lot of the recent conversations I've had because of that Scientific American article were from that. Journalists in China wanted to know more about space elevators. Their question for me was along the lines of what you just asked me, is this realistic? And I said it’s probably true that the engineering challenge becomes a bit smaller than the challenge of getting all the groups to do this thing together. The scale of the teamwork, cooperation for a project on this scale, this is a lot bigger than the International Space Station. Not just in terms of its physical size, in terms of things like space law that come into play, all kinds of areas, some of which we haven't even considered yet.
That may sound like a bug, but maybe that's actually a feature. Get everybody together working on something.