Long an in-joke used among senior staff in factories and shipyards, to send new recruits on a fruitless search, skyhooks might actually become a real thing one day. In the latter sense, a skyhook, as the name suggests, is a theoretical momentum transfer device that could be used as an alternative way to launch space vehicles.
Based on a very old principle, similar to that behind some of the pre-gunpowder war machines of old, the skyhook is a perfect blend of our past with our near-to-medium future.
But more on the technical details later.
These structures are incredibly interesting, and might just be our ticket to sending astronauts (and all their stuff) to distant worlds. Who knows, they may even become the powerhouses for building a galaxy-spanning empire?
But, let's not get too ahead of ourselves. What exactly are these things, and why are they potentially so revolutionary for space exploration?
Hold on tight, and enjoy the ride.
What are skyhooks?
Skyhooks might just be the best thing to happen to space exploration since the invention of the space telescope. Relatively simple to build and operate, they could make space exploration and travel a more routine affair for our species.
According to various studies, such a structure might be possible to build using our current technological knowledge and materials. Such a structure would be completely reusable with very high propellant efficiency and could provide the launch capability needed for future planetary science missions.
Interestingly, similar momentum transfer-based spacecraft launch systems are not a new idea. Konstantin Tsiolkovsky, for example, is widely credited with the concept of multi-stage rocket vehicles, but he also proposed an innovative orbital tower as far back as 1895.
While slightly different from the modern concept of a skyhook, the idea would inspire later scientists to build on the concept. For example, Yuri Artsutanov made the first realistic proposal for this kind of structure in 1960, called a "space elevator." A similar concept was later developed, independently, by the U.S., when the term "skyhook" was first coined.
However, it is important to note that "space elevators" are a distinctly different solution. Skyhooks would be much shorter, and, unlike space elevators, would not ever touch the Earth's surface. "Space elevators" would also be geostationary structures, whereas there is no real need for skyhooks to maintain their relative position to the Earth's surface.
Most proposals for a skyhook consist of a rotating satellite of some kind with a very long tether on one side and a small tether with a counterweight on the other side. During a launch, a spacecraft attaches to the long tether side and is flung out into space by the rotation.
The idea is that a craft would dock with the longer tether in low-Earth orbit, and would build up angular momentum as the skyhook rotated, while also orbiting the Earth. Once the longer tether reached its maximum distance from the Earth, the craft would be released and propelled into space.
However, it would not be possible to keep using such a device for one-way trips into space. It would soon run out of rotational momentum and the satellite would stop spinning.
To counteract this, it would be necessary to add some mechanism of "recharging" the skyhook. This could be done using thrusters of some kind, or some other means of replenishing the lost energy.
One solution could be to use the skyhook in reverse so that it "bled" energy from incoming spacecraft in preparation for re-entry into Earth's atmosphere.
This would involve "catching" a fast-moving spacecraft, or another payload, in high-orbit, and then slowing them down ready for release in low orbit. In addition, extra kinetic energy could be fed to the skyhook using electromagnetic or rocket propulsion devices, too, if needed.
Theoretically, this will enable the skyhook to be used ad infinitum. Of course, so long as the structural integrity of the skyhook is maintained.
Try to get that kind of return on investment from other reuseable alternatives currently in development.
What would skyhooks be used for?
At present, the only practical way to get stuff from the Earth's surface into space is to strap it to giant controlled explosions — rockets. This is very expensive, technologically challenging, and until recently with the likes of SpaceX, etc., very wasteful.
Most existing space launch vehicles, like rockets, consume a vast amount of fuel to overcome Earth's immense gravitational force and make it into space. In fact, so costly is this enterprise, that most of a rocket's mass (and design) is given over to the storage of this fuel. This tends to make the actual reason for going into space (ferrying astronauts or carry payloads like satellites, or the odd Tesla car) something of an afterthought.
Most rockets need to reach a velocity of around 24,855 mph (40,000 kph) to escape Earth. This means that most of a rockets' mass is made up of fuel and containers for the fuel. As a consequence, only a small portion of the rocket consists of the payload.
This is somewhat tolerable for just getting stuff into orbit, but a serious drawback if we want to go further afield in the solar system, like to another planet. Such a venture, especially for long-term colonization, would require a fair amount of material.
Such a venture would, with our current technology, require stockpiling stuff in space through a series of rocket launches, before making the trip to, say, Mars. Unless, of course, we found a way to rapidly synthesize supplies en route, or in situ.
Could there be a way of getting stuff to space with less fuel and more actual useful stuff? There might just be.
Skyhooks could eliminate many of the drawbacks of rockets. If one could be built, it could allow space travel to become very cheap and would, in theory, solve many of the major problems with, particularly large payloads.
However, despite its clear benefits, it would still require us to get into low-Earth orbit in the first place. This is because of the inherent problems of friction with the Earth's atmosphere.
Realistically, a skyhook would have the maximum extent of its launch tether at an altitude of between 62 miles (100km) and 93 miles (150 km). At these heights, the Earth's atmosphere is relatively thin to non-existent.
Any lower, and air friction would significantly impact the efficiency of the skyhook, potentially resulting in the entire structure slowing down considerably, ultimately leading to a catastrophic impact with Terra Firma.
However, this is a much less taxing prospect compared to current solutions for getting things into high-Earth orbit and beyond. In fact, any rockets used could be reduced in size by more than 80 percent - perhaps more.
Alternatively, specially designed shuttles could be used in place of mini-rockets, to act as a kind of taxi service of a kind. However, these would need to be very sophisticated pieces of kit in and of themselves.
Such a craft would need to fly just outside the atmosphere, and reach speeds in excess of around 12,000 kph, or so, in order to actually catch up with the skyhook's tether. Currently, the fastest thing in the sky is the American X-15 craft.
This can travel at an impressive Mach 6.7 (8,273 kph), but even that is not fast enough. But, it also lacks the ceiling needed to get to the tether. The SR-71, which still holds the record for the highest-flying aircraft, can only reach 90,000 feet (about 27.4 km) above sea level, which is impressive, but still not enough.
For this reason, if some future combination of the X-15 and SR-71 proves too technically challenging, small reusable rockets will probably need to be developed instead.
This is where developments from companies like Blue Origin and SpaceX could prove pivotal.
You can conceptualize a skyhook as something like a trebuchet in space, except the counterweight is fixed, it's not made of wood, and it freely rotates around its frame.
The business end of this space-trebuchet would, consequently, be able to launch a payload (stuff or a spacecraft) at potentially very high speeds. Depending on the design (most proposals tend to require the payload to make it to low-Earth orbit), such a relatively simple structure would make space travel considerably cheaper and more efficient.
A skyhook orbiting at 621 miles (1,000 km) above Earth, with a tether 559 miles (900 km) long, would need to travel at 12,035 mph (19,368 kph) and rotate around its axis every 53 minutes. With these dimensions, the craft will be traveling in excess of 26,770 mph (26,770 kph) at an altitude of 1,181 miles (1,900 km) at the time of release.
This is not quite the full speed needed to escape Earth's gravitational pull, but a quick burst of the craft's rockets (and no air resistance) will make this relatively easy to achieve.
All-in-all, depending on the dimensions of the skyhook, this should mean that craft can connect to the tether in low orbit, be spun out to the full extent of the tether above Earth, and be released in around 25 minutes. Not too shabby.
If we were to build several of these structures around Earth, and other celestial bodies, like the Moon or Mars, they could be used in tandem to deliver, receive, and return spacecraft almost as routine as catching the train. Much like a pair of baseball players throwing and catching balls in the backyard.
The principle is also not limited to building artificial satellites as the anchor points for these gigantic tethers. Theoretically, existing natural satellites, like the moons of Mars, could also be modified to act as even more enormous skyhooks. If we did this, it would form an important part of trans-solar system infrastructure.
A mixture of artificial and natural satellites in this way could mean our species could rapidly, efficiently, and cheaply travel to most planets in our solar system and send stuff back and forth. It may even be the secret to taking full advantage of the enormous amount of resources that can be found in the asteroid belt and beyond.
The potential for this kind of setup is so huge, that it could drastically reduce interplanetary travel by months (inside our solar system) to years/centuries beyond it.
It really is a very promising piece of technology - with regards to exploring the universe around us.
Would skyhooks make space exploration easier and cheaper?
Apart from the initial capital expense, and cost of material, to build the skyhook in the first place, how much cheaper would a skyhook make space travel?
The answer is a lot.
To make a back-of-a-cigarette-packet kind of calculation, let's use SpaceX's Heavy Falcon as a point of comparison. Currently, each launch of this rocket costs around $115,000,000.
This cost allows about 64 tonnes of stuff to be delivered to low-Earth orbit (which is about the altitude of the ISS, 250 miles, or 400 km). To get there, the rocket needs to reach speeds in excess of 24,606 mph (39,600 kph) - which is also about enough to theoretically get us to a skyhook tether.
If we had a working skyhook, we would not need to reach this kind of height above Earth. In fact, only about a tenth of the way. We also wouldn't need to go quite so fast.
Now, remember that a rocket needs fuel to take off, more fuel to carry itself (and fuel) into orbit and beyond. Since we don't need to go as high, or as fast, we can drastically reduce the size, weight, and critically, fuel reserves used with other rockets like SpaceX's Heavy Falcon.
For this reason, it should be possible to reduce the size of skyhook taxi rockets by as much as 86 percent, give or take. We should still be able to lift the same payload as that of the Heavy Falcon but at a fraction of the cost.
Assuming such a rocket could be built, we should be able to reduce each launch to as "little" as $12,600,000 - a massive $77,400,000 savings!
So definitely cheaper. But, what about easier?
The initial construction would clearly be very technically challenging, but that is only just the start of it.
To actually use the skyhook effectively would require exquisite timing. This is because the tether of the skyhook would only be at its closest point for a few minutes maximum. For this reason, any docking craft will need to be in the right place at the right time.
Thankfully, we can probably use computers to do all the heavy lifting with regards to making the necessary calculations. But, even so, this will not be a simple task.
The actual superstructure of the skyhook would also be a very tricky thing to assemble. It would need to be the right size, and made of the strongest and lightest material we have available.
The main anchoring satellite will need to maintain a precise orbit at all times, while also spinning about its axis. For this reason, a counterweight is a must to ensure that the satellite anchor spins and orbits in a predictable fashion.
The main tether itself will also need to be made of something that is very tough. For this reason, a substance called Zylon, or carbon nanotubes, might just do the job. Zylon is a synthetic fiber that is currently used to manufacture strong, lightweight components like the safety cabin in single-seater racing cars, like those used in Formula 1.
This material is over one and half times stronger than kevlar, and barely changes length when put under intense tensions. All very useful properties.
Could we ever build a skyhook for real?
In short yes. In fact, it has already, sort of, been done - just on a much more modest scale.
For a "true" skyhook to be built, it would require a significant investment in time and resources. But, once built, a skyhook could be used for many years to come.
Depending on the design, but in most cases a tether solution would be best, the biggest problem is finding the right material to make the tether cable from.
If a very long tether, the chosen material would need to be able to withstand the massive tensional forces involved in the process while also being resistant to the rigors of unfettered solar radiation and the odd impact from space debris and micro-asteroids.
The latter is a relatively simpler problem to overcome as we could simply weave said material into a thread of redundant fibers. As one particular fiber takes a hit or degrades, it will fail to leave the rest unscathed.
We already have the means to do it, so, you might ask, "why aren't we funding this?"
The first reason, while not necessarily an actual "problem" per se, is that such a structure could render the rocket industry obsolete overnight. With so many private enterprises currently staking their future on reuseable rockets, there is little incentive for them to make a skyhook a reality.
However, and probably most importantly, is the problem of space debris. According to current estimates, there is in excess of 9,500 tonnes of junk orbiting Earth (according to the ESA at the time of writing).
The remnants of decades of rocket launches, these pieces of debris range in scale from fairly large chunks, to nearly undetectable microscopic pieces of stuff. This is all a real problem for something like a skyhook as it needs completely empty space to rotate safely and efficiently. On top of this is the safety aspect of a rotating cable and its platform located so close to Earth, which has not yet been fully explored.
Before we even begin to plan to build something like a skyhook, we will need to clean up the debris-laden space around the Earth. This will take a significant investment in research and development, and will take some time to achieve.
But, this might be happening sooner than most expect. Announcements like Steve Wozniak's new space-debris clearing startup might just be literally preparing the ground (well, space) for a genuine skyhook to be built in the not too distant future.
It is unlikely we'll see a real skyhook anytime soon. However, as the benefits of such a system are potentially so great, it is not inconceivable that companies like SpaceX, Blue Origin, etc, might decide to invest in such a system in the future - if they still exist, of course.
But, who knows, if enterprises for cleaning up space debris in orbit are successful, we might very well see an effort to build a skyhook within our lifetime. All it takes is a little long-term vision and courage to be the first to attempt it.
After all, "the sky[hook] is the limit," for whoever manages to be the first to build one. Not only that, but the very fate of our species may depend on it!
If that isn't enough of an incentive to build one (or a few) we don't know what is.