It slows down the rotation of the Earth, so angular momentum is conserved overall. Wikipedia has a fine graphic showing the exchange:
While you’re lifting a payload, you are indeed pulling down on the counterweight. So you need a large enough counterweight, far enough out, to handle this.
So we first lift such a mighty counterweight up that it does not matter what we lift up, because what is already up makes what we want to pull up negligible? But putting that counterweight there is costlier that lifting what we need with rockets the moment we need it, instead of lifting it in advance for whenever we might need it. Where is the gain?
And the cable is designed to be so strong that it not only lifts the cargo we want upwards but that it can also slow down the rotation of the whole planet? The anchoring and the cable must be strong sideways indeed.
And the counterweight is probably not in geostationary orbit, but further away to be effective. Then the problem is not that the whole structure is slowing the angular momentum down when we lift cargo, but accelerating it when we don’t.
Those are demandind specifications. I have trouble visualizing it.
Flat-Earthers call it a theory, not understanding how that term is used in science.
How heavy the counterweight needs to be depends on how far out it is (go far enough out, and you don’t even need a counterweight, just more cable). It could also be a captured asteroid or cheaply-launched moon rocks, since it doesn’t need any particular properties besides mass. And most proposals have only a very small cable and counterweight launched initially, and then using that small cable to put up more cable and counterweight, to increase the capacity of the system by a few percent each time.
And while launching from the cable does slow down the rotation of the planet (and taking cargo down speeds it up), it’s by a truly tiny proportion. Giving any angular momentum to anything that leaves Earth (which we do with every launch) must necessarily slow down the Earth’s rotation, but it’s not by any remotely noticeable amount.
I don’t have a single comprehensive source for review but I’ve followed research in the journal of the Aerospace Medical Association (was Aviation, Space, and Environmental Medicine, now Aerospace Medicine and Human Performance) back when I had access to it as well as having a small bookshelf of books and collected articles on space physiology and medicine, as well as general biophysics and biomechanics. The best summaries of what physiologists in the field think to represent the issues and hazards of long duration hypogravity (fractional gravity) environments can be found in Kevin Fong’s Extreme Medicine (has an entire chapter on Mars) and Hanns-Christian Gunga’s Human Physiology in Extreme Environments, 2nd Edition pgs 315-9. While it isn’t specifically concerned with gravity on planets, Artificial Gravity, edited by Gilles Clément and Angie Bukley has extensive discussion in the various physiological issues in simulated gravity greater and less than that at Earth’s surface. And as a general reference to interpret the various effects from research papers I recommend Physics of the Human Body by Irving P. Herman. And while it isn’t specifically focused or gets into a lot of detail about the physiology, Erika Nesvold’s Off-Earth is a really good exploration of the ethical, social, legal, economic, and practical medical consequences of habitation in space and on other planets.
As you note, we have very little experience with fractional gravity on any mammals, and virtually none at all on humans (or primates) except for a few brief forays on the Moon. However, there is both a lot of work that has been done with bedrest studies, which while not a perfect analogue certainly give at least some idea of the impact upon muscular degeneration and effects on metabolism. I’ll note that mice are not a particularly good analogue for human physiology studies because they are anatomically not very similar to humans, and their metabolism runs much more energetically, so while they are a convenient test animal for small scale experiments I don’t think much can really be drawn from fractional gravity studies with them that is directly applicable to predicting effects on humans.
Most conclusions about human physiological issues in hypogravity come from inferences in freefall and return-to-Earth adaptation studies, and while these are also not perfect analogues there are a lot of reasonable inferences that can be made. Much of the popular interest is on the effects on the musculoskeletal system (loss of muscle, decrease in bond density and osteoporosis, impact on fine motor control and coordination), in part because these are so obvious and can also be somewhat mitigated through exercise and nutritional supplementation, but there are a whole host of other issues which occur in freefall from problems with the vestibular and ocular system to calcification, suppression of the immune response, hormone dysfunction, and even problems in intracellular signaling and metabolism. Being in some kind of a gravity field will mitigate these to some degree but the general consensus is that these problems will crop up in long term habitation in anything much less than a 0.5 g field, and that has become a pretty standard recommendation in consideration for any proposed centrifugal pseudogravity systems for habitats or spacecraft.
As others have stated, the cable would be tensioned via a counterweight that goes out past GSO, and the (immeasurably small) reduction in the angular momentum of Earth compensates for the effect of pulling the capsule up. In fact, if you draw a boundary around the Earth including the capsule, the angular momentum doesn’t change although the rotation rate will be slightly slower, just as with the basic physics example of an ice skater extending her arms while spinning will slow her down.
One of the concerns people often have is that a failure of the cable would cause it to collapse and destroy anything it falls upon, but in fact you would make the weakest point of the cable at the surface so that any failure would pull the cable up. In fact, it probably isn’t necessary or even prudent to anchor it at all, and leave the ground terminus loosely tethered and free to move radially. How you get people on and off a ferris wheel that is bobbing up and down will be left as an exercise for the creative innovator but I am confident some sufficiently inventive mind will come up with a Rube Goldberg contraption fit for purpose that will still be more mechanically feasible than the basic concept of a beanstalk.
Stranger
I’m not sure I fully understand this question, but note that you only have to put the counterweight in place once. After it’s there, it stays there and allows the elevator to be used an unlimited number of times to lift further payloads (ignoring any finite lifetime the elevator materials might have due to wear and tear).
Thanks for all the references.
I am a little skeptical of the existing conclusions just by virtue of the lack of real experimental data. Actually, I’m a little skeptical even of our data on microgravity, despite everything coming out of the ISS. The reason is that the ISS is not much different than JAXA’s little mouse cage. It’s too cramped to really let people exercise properly. They have their little exercise bike, but the corridors are too small and covered with fragile equipment to really cover the envelope of microgravity exercise.
What the ISS “should” have is something like a 20-meter diameter recreation module. It should have a track for running, rings that people can race through, a “tree” for gymnastics, a variety of balls and other sports equipment, enough padding so that people can use full muscular force without injuring themselves too badly, and so on. In short, enough space and equipment so that people can actually have fun doing exercise in microgravity and use all of the muscle groups available to them.
Similar things will apply to mesogravity. An actual proper habitat will have a recreation room where people discover that they can, for example, easily jump a meter or two in the air when they use as much force as they would on Earth. And so they’ll exert those muscles in similar fashion. It will require some mental rewiring to figure out the best way to move, but our brains are pretty good at that. It does require a habitat larger than the inside of a submarine, though.
Well, that’s a nice fantasy but there is no way that “a 20-meter diameter recreation module” could be shipped up to orbit; even proposed inflatable modules aren’t nearly that large. Skylab actually had a large open interior volume but in freefall you can’t run on a track, or do gymnastic bodyweight training, and so forth; for obvious reasons the only strength training that can be done is spring/elastic resistance training. And when astronauts are exerting themselves big globules of sweat accumulate so a bunch of free moving exertion is going to rapidly slather the module (and indeed, the entire station) is going to coat every free surface with a slimy layer of sweat residue. The exercise equipment on the ISS, while not providing a varied and interesting fitness experience, is purpose-designed to provide the maximum possible mitigation to muscular and bone density loss in freefall and astronauts use the equipment daily as scheduled. There is absolutely no reason to believe that people playing zero-gee Quidditch will somehow get much better health outcomes notwithstanding the potential for traumatic injury that could be problematic to treat in freefall conditions (in the case of an open wound or anything requiring surgical intervention) and astronauts are actually cautioned against being too frenetic as there are neither medical personnel nor procedures for dealing with a serious trauma in space other than basic first aid and returning the injured crewperson to Earth for treatment.
But this is all basically irrelevant because again, musculoskeletal degradation is basically the least of an astronaut’s physiological issues; problems with vision and vestibular system are universal and without effective mitigations, and suppression of immune function, cellular metabolism, and even genomic damage (not just due to radiation; telomeres have actually been observed to shorten in freefall conditions) are problems that we have absolutely no idea how to counteract. Even the basic lack of Earth gravity creates issues that infectious particles that would be surface-bound fomites on Earth can become airborne in low gravity and create much greater sanitation problems. Former astronauts in general have a higher rate of early onset chronic conditions including cancer, cardiovascular issues, dementia, and so forth than the general population even though they were selected for being in excellent fitness and general health, and hypothetical future denizens of Mars would likely be more representative of the general population, selected for their skills rather than fitness or longevity.
I know how much people want to believe that space physiological hazards are just trivial issues that can be fixed with exercise and a weighted vest, or that we can plunk glass domes on Mars and start growing tomatoes and making wine, or that the only thing preventing us from colonizing Titan is a lack of go-getter spirit but the reality is that everything you do in space is orders of magnitude more difficult than you would imagine from terrestrial experience, and even experimental mission and habitation analogues on Earth frequently fail for the most prosaic of reasons. Physiologically, humans are highly adapted to living in the delicate balance of Earth surface conditions and have difficulty adapting to even moderate variations within that. Living in freefall or significantly hypogravity conditions will have substantial impact upon health to a degree we can only make informed guesses right now but the idea that some gymnastics practice will be a cure-all is just hopefully pleading.
Stranger
The lack of proper superheavy lift rockets has been a bit of a factor in that. Perhaps that will change soon. A 20 m ball has a surface area of 1250 m^2. A 150 ton payload allows a generous 120 kg/m^2. Surely an inflatable can easily fit in that mass constraint, even with all the required extras (not to mention a 1000 m^3 volume).
Sure you can. There are videos of astronauts doing literally just that in Skylab. Not to mention zero-g gymnastics and other things. And that was only 7ish meters.
Adequate ventilation, hydrophobic coatings, capillary liquid transport, dehumidifiers, and any number of other things can solve these problems. Possibly difficult to incorporate into the ISS as it exists. Less so in a dedicated room.
Humans are adapted to an astonishing range of temperature and pressure variation. It doesn’t seem at all moderate to me.
At any rate, whether the health problems are actually solvable or not isn’t the point. My point is that we haven’t actually done the experiment yet. We have a lot of inference and supposition but the data we have is only valid within an extremely limited environment.
This is probably a stupid question, and it comes from a deep well of ignorance.
Sailing ships can sail into the wind by a technique called “tacking”, instead of trying to sail staight, they take a zig-zag path that roughly averages out to the straight line.
So, can solar sails be made to tack in a similar way?
No.
Simply put, the tacking of sailboats is possible because of the interaction of the force of the wind on the sails and the force of the water on the keel. A solar sail has no equivalent of a keel and can only sail “downwind”.
My deep pit of ignorance just got a little less deep, thank you.
Yes, solar sails can tack, in an indirect way. Any object in orbit around the Sun has a particular orbital velocity; if you add velocity to a craft by orienting your solar sail at the right angle you will make your orbit larger, and if you angle the sail differently, you can reduce your orbital velocity and make your orbit smaller.
Basically you are using gravity to tack towards the Sun; if you want to go towards the Sun, you create thrust with your sail in a ‘forward’ direction, to reduce your velocity and this allows gravity to pull you downwards towards the centre of the Solar System.
The satellite IKAROS used solar sailing to get to near Venus; it is a long process, but it works.#
That’s an excellent point, thank you, although it exploits an entirely different principle than sailboat tacking, namely the basic principles of orbital mechanics. You cannot, in the general case, tack directly “against the flow of photons” the way a sailboat tacks against the wind, but yes, in a stable orbit you can use a solar sail to speed up or slow down orbital velocity and adjust the orbit accordingly.
Downwind, but not directly downwind. For a reflective sail, the force vector will be perpendicular to the sail–not parallel to the light rays. Consider a photon bouncing off a mirror at a glancing angle. There won’t be any change in momentum in the vector component parallel to the mirror. But in the vector component perpendicular to the mirror changes sign after the bounce–so it transfers double that amount.
The actual amount will be less at glancing angles, but it will still always be in the perpendicular direction. So at a right angle to the light in the limit.
All that said, this kind of “tacking” isn’t useful for maximizing the thrust, but it might be for going into a particular kind of orbit.
While that description of how solar sails work is correct, it isn’t really physically analogous to how a sailboat sails upwind, which is essentially by producing lift on the keel laterally, and you wouldn’t tack (come about, bringing the bow across the wind repeatedly to average a straight course upwind) a solar sail back and forth.
People also have exaggerated notions of how effective solar sails are; they really produce very low thrust-to-mass impulse, providing a tiny but constant acceleration which is suitable for very low mass payloads that can take months or years to maneuver even down near the Sun but will never be suitable for crewed spaceflight or sending large cargos, and is essentially worthless at the orbit of Jupiter (at 4% of solar insolation compared to that at Earth orbit) or beyond.
Stranger
1973 - during an MIT introductory physics lecture on scaling, Professor Philip Morrison flung cherries at a chocolate pudding pie at varying angles to demonstrate how the earth’s crust reacted to meteorite impacts.
This is one on my pet peeves about science popularizers (in general, not you Chronos)
The hand-waving away of incomprehensibly difficult feats of engineering as solutions to their pet theories/.projects.
“Oh, we’ll just capture an asteroid”
The average asteroid is about the size of a small mountain and travels around 40,000 MPH. Discounting the fact that you don’t have any control over the size of passing asteroids, there are none in the area of geosynchronous orbit. How far away would you have to go to get one and how would you slow down a mountain going that fast? I’m not saying that it couldn’t be done, but the effort, energy and resources that would go into such a feat (and this doesn’t take into account the same investments into to cable system itself) would easily cancel out any benefits that could be gained from the space elevator, imho.
All the questions you are asking are answered through simple delta-V equations.
If we want a big counterweight that is X tons in a geostationary orbit, we can either lift that weight from Earth, or we can find an asteroid that heavy and push it into position.
Both problems are calculable. We can figure out exactly how big of a rocket and how much fuel it would take to nudge any given asteroid to the right position - it is rocket science, but quite simple rocket science, of the sort that NASA is quite good at. Certainly, planning that maneuver is orders of magnitude simpler than planning a multiple gravity assist flyby of a half dozen bodies across the solar system. We can then figure out how big of a rocket and how many launches we would need to get an asteroid tug into orbit; and we can figure out how many launches of what rockets it would take to put up X tons of dumb mass into orbit; and then we can decide which is easier, and do that.
Both of these tasks are beyond our current capacity, and I am not trying to trivialize the difficulty of accomplishing either task. And obviously, the counterweight has got to be the single easiest part of a space elevator, since it’s just mass in orbit, and we know how to put mass into orbit or move it around; it’s just a matter of scaling that up.
But I don’t think we need to overplay how difficult it is either. None of the questions you asked are actually major obstacles to building a space elevator - not compared to the monumental improvements in materials science that would need to occur before we could even think about it.