Nope. It’s just a theory.
From Wikipedia (and numerous other sources):
a force causing any two bodies to be attracted toward each other, with magnitude proportional to the product of their masses and inversely proportional to the square of the distance between them.
The erth weighs a hell of a lot more than your chair. The sun weighs a hell of a lot more than the earth.
If you and your chair were next to each other, there’d be a very, very small attraction, because the masses are small.
From Gravitational Force Calculator (omnicalculator.com)
If a 30 pound chair is next to a 150 pound person, the force is 7.39e-8 newtons.
The 150 pound person next to the earth would have a force of 668 newtons. Which is why we stick to the floor and not to the chair. And since our legs are strong enough to lift us off the floor, adding the chair to the picture is pretty trivial.
The sun’s force on you would be 0.4032 newtons. Still quite a bit less than the earth’s force, which is why we don’t go spinning off to a toasty end.
The sun’s force on Earth would be 3.5 × 10^^31 newtons. I assume that if the earth stopped revolving around the sun, we’d ALL wind up a molten blob on the surface of the nearby star.
You could come up with some rough figures for the mass of a comet, and average distance, and have fun with how those gravitational forces change as it gets further or closer.
I’ve learned to rate futuristic technology on the “If” scale.
For space elevators, it’s “If” carbon nanofibers can be made in bulk without defects and “If” they can be strung together for miles and “If” they can be constructed in space and “If” room-temperature superconductors can be created, and probably several more “Ifs” that you can supply.
Anything at four “Ifs” and over is magic in today’s world. (If you don’t like the word “magic”, then substitute “wishful thinking” or “wildly overoptimistic.”)
Two or three “Ifs” may, with luck and money, appear in this century.
One “If” has a good chance of being investigated today, although it may not ever get past the lab stage.
I started reading adult science fiction more than 60 years ago, around the time of the Mercury program. I naturally bought into the hype coming at me from both sides. As the decades went by I grew increasingly jaded by purported images of the future. Our real advances (computers and medicines alone!) don’t necessarily transfer to other forms of technologies. Fortunately, they have also not yet translated into dystopias. I’m keeping my eye out, though.
We could, it would just take us a LOT more energy to do so. Look at those huge rockets, filled with fuel, that it takes to get a little space capsule off the planet. The “gravitational well” would be that much deeper.
We might also have problems with it being even harder to adjust to low / zero G, if we were adapted to a higher gravity.
Interesting discussion of space elevator issues. I have a mental image of how we have much smaller-scale things in place now - conveyor belts, regular elevators etc., that move things via mechanical cogs or cables. There’d be some obvious technical issues making an elevator that long (a geosynchronous orbit would be on the range of 26,000 miles. At that point, the force on a 150 pound mass is only about 11 newtons - so any such device would need to compensate for the lesser forced required as the object ascends.
Pions are “Bosons” (though they are not elementary particles in the quantum chromodynamics sense). Yukawa indeed invented them in 1935 to describe interactions between nucleons, like protons and neutrons. Charged (and, later, neutral) pions were later artificially produced and detected in particle accelerators, and Yukawa picked up his Nobel prize.
That’s only one if: If they can be made in bulk. If we can make them in bulk, then we can string them together, and make them in space. There’s no “if” to either of those. And you don’t need room-temperature superconductors.
I think the problem with a space elevator is more likely to be economic: they just don’t pay for themselves.
It’s hard to calculate because the costs are extremely sensitive to the quality of your nanotubes, not to mention their material cost. But consider a space elevator that lifts 1000 kg at a time. It’s hard to believe that such a thing would cost less than $5B to build. It puts all other megastructures to shame.
The elevator brings payloads up to geostationary orbit–36,000 km up. Since the payloads have to physically grip the ribbon, speeds are necessarily limited–say, 200 km/h. So it takes 180 hours to make the journey.
Since the ribbon gets stronger as you go up, you can have multiple cars at once. But it’s still limited by the strength of the lower part of the cable. Let’s say you can have 4 at a time–that means 45 hours between payloads. That comes to 195 tonnes/year.
That $5B cost (which is already optimistic) has to be paid for. Let’s say that means 5% amortization, which means it needs to earn $250M/yr to be worthwhile. So the effective cost is $1280/kg.
That’s barely cheaper than a Falcon 9, which costs around $1600/kg (the price is higher, but we’re talking costs here). And the space elevator puts you in an orbit that you might not want–you need another rocket to get you to LEO or wherever else.
Falcon 9 costs will be drastically undercut by Starship (and other reusable rockets), putting launch costs at $100/kg or less. Even if I’ve been pessimistic about the space elevator, it still has to make up an order of magnitude vs. rocket tech that’s being built today. And the rocket doesn’t require dividing up your payload into 1000 kg chunks, or spending days on the cable (passing through radiation belts slowly), or any of that. And can go to any orbit you wish.
The problem with all of these types of projects is that totally independent of the tech issues, they all have huge capital costs. And they don’t launch at a high enough rate to pay for themselves vs. rockets.
Just because you can make carbon nanotube “in bulk” doesn’t mean that you can join them at a composite strength approaching even a moderate fraction of the strength of an individual fiber, especially at lengths of thousands of kilometers. In fact, the entire argument for viability of a beanstalk is based upon the theoretical intrinsic tensile strength of a defect-less carbon nanotube, even though we can’t make such perfect structures large enough to be seen by eye, nor have any practical means of joining them into a ‘rope’ tens of thousands of kilometers in length. Nor is there a guarantee that they can be made in situ without a large industrial infrastructure which would have to be hauled up from Earth and assembled on orbit. All of this actually assumes a substantial manufacturing infrastructure that does not exist even in the dreams off space enthusiasts and would take a vast amount of automation that would also function in freefall conditions.
The capsules or cars transferring people and goods up the elevator are going to have to be powered in some fashion (they aren’t going to be driven by fuel cells or heavy batteries) along the entire distance from Earth’s surface to geostationary orbit (over 35 thousand kilometers), and will not be able to mechanically tractor themselves up a carbon nanotube ‘cable’ or ‘ribbon’ because of the delicacy of such a structure to the highly localized contact forces that it would impart. The only practical way ride up such a structure would be by using some electromagnetic induction (basically turning the capsule into a solenoid) and magnetically driving itself up, which given the power requirements both for transmission over those distances and running such a powerful electromagnetic are going to require a room temperature superconductor, or else an enormous amount of refrigerant.
A space elevator that could only lift a metric ton at a time would be utterly useless; for it to be even remotely viable even assuming dirt cheap construction technology and maintenance it would have to carry tens of thousands of tons a day (comparable to transcontinental air transport today), and of course it can only deliver payloads to an equatorial orbit, so anything needing to go into another azimuth would have to carry sufficient propellant to make a plane change. The economics are certainly a limiting factor in the practical viability of such a system, as are the security concerns of protecting such a hideously vulnerable structure against sabotage or terrorism, as nearly any impact from errant or directed debris would almost certainly do significant if not catastrophic damage to the structure.
But frankly, if the purpose is to haul up basic materials and consumables as some marginal rate, it is missing the point that a practical large space industry or large scale habitation has to be self-sustaining using space-based materials and can’t be dependent upon dragging resources into orbit. The resources in space are vastly greater, and with pretty modest improvements in propulsion far more available, than hauling water and minerals up a gravity well. But it would also require a dramatic degree of automation to the extent that begs the question of why you would sent people up at all given the cost of habitation requirements and the paucity that any physical or even intellectual labor would offer in space.
Space elevators aren’t as obviously physically impossible as orbital ring megastructures or as dumb as Dyson spheres but they are just about as useless without some pre-existing vast in situ resource extraction and manufacturing infrastructure, which begs the question of why you need to haul stuff up from the planet at all. Fundamentally, they are systems for facilitating space tourism but are unlikely to ever satisfy a fiscally valid function.
Stranger
A general observation / question about the tradeoff between using earth-based resources in space and using space-based resources in space …
It’s often said that hauling Earth-based resources up the gravity well to Earth orbit for use there is dumb, expensive, etc. As @Stranger_On_A_Train just said so well in the post above mine.
As a separate matter it’s often said that dumping things into the Sun (IOW down into a big gravity well) is real difficult too, albeit for the different reason of needing to reduce the Sun-relative orbital velocity of whatever you’re dumping to get it to spiral in. Simple in principle, but a LOT of energy is needed. And energy is costly / heavy / etc.
Which dichotomy leads to my question / dilemma.
A solution often offered to the “up the gravity well” problem is using space-based resources. To me it makes complete sense to build, e.g. a Moon surface base from materials found on the Moon’s surface. Or even an orbiting Moon station using Moon-based materials; much smaller gravity well there.
But for any economic effort anywhere, it doesn’t deliver value until the value is on or near Earth. Because that’s where the people are. Scientific research (or even the applied research of prospecting for resources) just results in data that can be easily and efficiently radioed to Earth. But any economic use of space requires moving stuff. And stuff is hard to move in space; Delta-V is a real killer.
e.g. mining nickel in the asteroid belt is nice and all, but until you put the shiny new nickel where humans can use it, it doesn’t do anybody any good. So they won’t pay for it. Getting the nickel “down” from e.g. Ceres to Earth orbit is as hard or harder than dumping Earth garbage into the Sun. Which is widely acknowledged to be very difficult in terms of energy investment needed.
Yet somehow that part of the “just use space-based resources” claim is usually hand-waved away.
How do we square this circle? Am I missing something, or are the space resource cheerleaders just magicking up a thruster system much as the elevator cheerleaders are magicking up their tether material?
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I’m sure there are posters here who can address this much more knowledgeably than I, but I’d venture two observations. The first is that the sun’s gravity differential between Earth and the nearby planets isn’t really all that great, and this is reflected in the fact that the orbital speed differentials aren’t all that great. For instance, the Earth’s mean orbital speed is 29.78 km/s, while that of Mars is 24.08 km/s. Farther out in the asteroid belt, Ceres has a mean orbital speed of about 17.9 km/s.
So getting over to Mars or the asteroid belt is, relatively speaking, not all that demanding compared to getting to the sun, and conversely, so is getting back.
The other point is that some of the proposals I’ve seen involve capturing an entire asteroid rich in some desirable metal(s) and basically “towing” it into earth orbit. While this would involve a lot of energy it’s not in the same ballpark as sending something into the sun, and the economy of scale is probably far more efficient than many back-and-forth missions.
To land on the moon, you have to slow from escape velocity to 0 m/s.
To land on Earth, you can slam things into the atmosphere at many hundreds of m/s, then use the air to stop and land.
Speaking of asteroids rich in metals, it’s possible that such a metallic asteroid once slammed into the moon, creating one of the largest craters in the solar system and depositing its mass below the surface of the moon in a structure 300 km deep and nearly 1000 km across, a total mass of about 2.18 billion billion kg. It might not be metal at all but if this is a useful metal like nickel, or effectively an ore that can be turned into metals, the moon itself may prove to be an invaluable source of raw materials.
You make salient points about the energy required to get from one (solar) orbital state to another, the need to have materials and resources physically where they are processed or consumed, and in general a lot of the vagueness about space industries and habitation. I’ve often made the point that the reason to go mining asteroids (if it makes sense at all) is to use those resources in space rather than deliver them back down to Earth because for the most part we have plenty of resources and if anything we tend to waste more than we use and recycle. But the following are a few points for consideration of the logistics of moving mass around, and into or out of a gravity well.
First, it takes a net change in velocity (∆v) of about 7.8 km/s for the payload of a launch vehicle to achieve Low Earth Orbit (LEO). However, the actual impulse required is equivalent to around 9.5 km/s because of all of the energy required to continually lift the vehicle against gravity and above the atmosphere until it achieves LEO; in essence, until it is going so fast that it falls above the horizon rather than back to the surface. So the thrust developed has to exceed the weight of the vehicle, which is done by expelling the carried propellants that are combusted and then ejected to produce sufficient momentum to counter gravity as well as impart orbital speed, and by necessity this has to be done as fast as possible even if it isn’t very efficient in terms of mass utilization as quantified by specific impulse, or the unit impulse per mass expended. (Although rockets are quite efficient in a thermodynamic sense, the mass efficiency is poor because the exhaust velocity of the combustion products is limited by the enthalpy available from the combustion and the temperature that the rocket engine can endure.) So space launch vehicles have to impart all of their impulse over a short duration of time to minimize gravity losses even if it isn’t a very effective use of mass, and they are mostly propellants with only a few percent of the total mass available for payload and inert mass.
A spacecraft out in solar orbit, on the other hand, isn’t going to fall back onto anything, so the propulsion system can deliver impulse at a much more leisurely rate and at higher mass efficiency even if it can’t developed high thrust. This is why proposed spacecraft using ion or low thrust plasma drives can go from Earth orbit to Mars much quicker than chemical rockets; even though they only provide a tiny fraction of the thrust, they can operate continuously for much longer durations and higher effective exhaust velocities, requiring less propellants for the same amount of ∆v. (Although to be fair, they operate at far lower thermodynamic efficiency than chemical rockets and produce large amounts of waste heat, which carries its own set of problems.) So, provided you have a source of propellant mass than can be extracted in situ and a ready supply of energy (i.e. solar or fission) it can be far more efficiency to move mass from one orbit to another versus lifting it up from a gravity well even at equivalent change of specific orbital energy, especially if scaling up to very large masses
It is also possible to use the gravity of other planets to ‘steal’ some impulse; the most obvious of these are gravitational swing-by maneuvers such that many spacecraft perform but it is also possible to trace low impulse routes through the solar system that require a minimum of energy, provided you have plenty of time to spare waiting for your spacecraft to get where it is going. That isn’t a very good option for moving people around or space missions where scientists are eagerly awaiting data before the end of their careers, but for moving bulk materials around it may be fine. As long as there is a steady supply of consumables like water or carbonaceous minerals, it doesn’t really matter how slow they travel between orbits, and avoids the cost, difficulty, hazards, and pollution of encapsulating and moving ‘small’ payloads of material up fro Earth (or for that matter, the Moon or any other planetary-like body).
However, it is important to not gloss over the complexities of developing any functional and self-sustaining space industry. On Earth we have an extensive infrastructure and decades of experience in turning raw materials into finished goods or useful materials, as well as gravity which makes things all fall in one direction and stick to the ground. Developing a mining, processing, and manufacturing industry in freefall would require throwing away most of what we do on Earth and coming up with entirely new methods for processing and handling materials. All of this would require enormous investment to get such an infrastructure up to the point of being self-sustaining or even marginally bootstrapping, and would take many decades.
Furthermore, it would require extensive and almost completely autonomous capabilities; the idea of space miners in vacuum suits laser drilling valuable minerals out of an asteroid is a quaintly antiquated notion; aside from the enormous costs of keeping people alive and the exertion of doing practically anything in freefall, what we now know about human physiology in the space environment makes it clear that people just cannot remain healthy and fit for long intervals in space. For people to live off-Earth will require the recreation of something close to a territorial environment including simulated gravity as well as protection from both solar and galactic cosmic radiation, which is inconsistent with relatively mobile spacecraft. In fact, I’ve become pretty dubious about the entire concept of people living in space or on another world (the Moon, Mars, or the moons of Saturn) in general because it has become apparent that even moderate fractions of gravity are likely to be inadequate to prevent various physiological and developmental problems; not just musculoskeletal issues but basic problems with cellular metabolism and the immune system. Barring magical propulsion and speculative physics, space is going to be the domain of autonomous systems and robots for the foreseeable future.
There have been such proposals but I’m not sure they really make much sense. If you are going to do processing in space, it likely makes far more sense to move your processing facilities to where the resources are, and extract and move just what is actually needed. There is nothing special about Earth orbit in terms of needing to process there (again, assuming extensive automation), and then what ever slag and residues are produced have to be moved out of orbit so as not to pose a debris risk to satellites or navigation hazard for spacecraft. And while given sufficient power, propellant, and time you can move any solid object, anything larger than a very small asteroid is going to act like a very flexible (and likely friable) body under point impulse, and most asteroids appear to be loose aggregations of material collected under their very small self-gravitation rather than solid objects that can be tractors around. If you want water, or carbonaceous material, or fissile ore, or whatever from an asteroid, it likely makes sense to break it down and extract the material you need where it orbits, and then send the processed material to where it needs to be used than to try to. drag a large mass around the solar system like a recalcitrant mastiff.
Stranger
Excellent post as always, @Stranger_On_A_Train , though focusing largely on the logistics of space mining. But I think one of the points that @LSLGuy raised that “Ceres to Earth orbit is as hard or harder than dumping Earth garbage into the Sun” is not correct; it seems to me that the most important criterion for the energy required to travel between solar orbits is the sun’s gravitational differential at the two orbits and consequently how different the orbital speeds are. IOW, sending a given mass to the asteroid belt (or back) is, in terms of energy requirements, a much lesser deal than trying to send something into the sun.
One can see this by comparing the Parker Solar Probe with missions heading toward the outer solar system. The Parker Solar Probe launch mass was 685 kg. Despite its relatively small mass, it required a Delta IV Heavy launch vehicle to help send it close to the sun, along with multiple flybys of Venus to further slow it down. The Delta IV Heavy, with a mass of 733,000 kg, was the world’s third-highest capacity launch vehicle at the time. Yet despite all this energy to slow it down plus the Venus flybys, the enormous potential energy from the earth’s high orbit will, by around the end of the year, give it an orbital speed of 692,000 km/h as it spirals closer to the sun, making it the fastest manmade object ever made.
The overall Curiosity spacecraft, OTOH, had a total mass of 3,893 kg. Despite being nearly six times more massive than the Parker probe, it was launched to Mars on a much smaller Atlas V-541 weighing just 531,000 kg.
For anyone interested, extensive crunching of the delta-v numbers is available at the excellent Atomic Rockets website here:
https://www.projectrho.com/public_html/rocket/appmissiontable.php
This may have been a joke, but no, the law of gravity (or more formally, Newton’s law of universal gravitation) is a scientific law.
A scientific law is a summary of observed behavior of the universe with no explanation as to how or why the behavior works as it does. So for example, Newton’s law of universal gravitation is a mathematical equation that can be used to predict the attraction between bodies, but it is not a theory to explain how gravity works.
A scientific theory, on the other hand, provides an explanation for observed behaviors. The best theory we have to explain the law of gravity is Einstein’s theory of general relativity (aka Einstein’s theory of gravity).
So depending on what you are talking about the observed behavior or the explanation for the behavior, “gravity” can actually be a law or a theory. The genius of Einstein’s theory was that it actually predicted behaviors that had not yet been observed, such as gravitational lensing.
And [personal nitpick], there is no “just” when referring to scientific theories. Scientific theories are robust explanations of the observed behavior of the universe, and are the best explanations that humanity has come up with to date.
Do you have some material to read about this?
As far as I know, the only mesogravity experiments to date have been through JAXA’s Multiple Artificial-gravity Research System. And only very recently have they done a test in lunar gravity:
https://www.nature.com/articles/s42003-023-04769-3
While their system is capable of it, I don’t think they’ve done a Mars (1/3 g) gravity test yet.
Lunar gravity does prevent some but not all muscle atrophy. But this is a very limited experiment on mice, not humans. And it’s so small that even the mice aren’t capable of getting a normal level of exercise–it seems entirely possible that the effects would be erased simply by being able to move around normally.
I guess I am missing something, but there is one thing that buggers me when I think of this space elevator idea:
Suppose one is built, and forget about the technical difficulties and magical materials for a while. There it is. A cable from the Earth’s surface to geostationary orbit, and the mechanism to lift cargo along the cable. Great! Now when we lift a weight up the cable are we not pulling the whole thing down at the same time? Why does actio = reactio and angular momentum conservation not hold there? Either we are pulling the whole structure down, or we are slowing the rotation of the structure (thus it is not geostationary anymore but it bends westwards with catastrophic consequences) as we go up. Or both.
How can we get more out of this structure than we put in it to start with? What am I missing?