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“Earth is the cradle of humanity, but mankind cannot stay in the cradle forever.”
These words, attributed to Tsiolkovsky, remain true today. Humanity will eventually become extinct if it is forever confined to Earth. Yet it’s Tsiolkovsky's own rocket equation that keeps us confined to the cradle; as the payload mass to space increases, fuel requirements increase exponentially. Even with our best materials and most efficient engines; we can barely make it to Earth orbit, let alone other planets. Chemical rockets expend about 95 percent of their mass getting 5 percent of useful payload into orbit. To become multi-planetary we will need to “cheat” the rocket equation. That is where non-rocket travel may plug the gap. Space elevators and space tethers could greatly reduce the energy requirements and increase the safety profile of space access.
Space Elevators
A “space elevator” is a system that lifts payloads directly into space along an extremely strong cable, often called a “tether” or “ribbon.” This cable is affixed or anchored to the Earth’s surface on one end and connected on the other to a counterweight beyond Geostationary Orbit (GEO). One might think of a space elevator as a subway to the heavens. “Climbers” traverse up and down the cable using energy produced on Earth, possibly beamed from the ground via powerful lasers. The climbers don’t need to travel quickly or carry fuel, thus greatly reducing the energy requirements for getting into orbit and ostensibly making space travel much cheaper.
The challenge, of course, is building a space elevator in the first place. The initial infrastructure requirements are almost unfathomable. It would take dozens, if not hundreds, of traditional rocket launches, using the most capable rockets today to build one. The length and strength of the cable are the primary challenge; most designs call for a cable length of about 100,000 km. The cable will need to be extremely strong, able to survive micrometeorite and space debris impacts, radiation, gravitational tidal forces, and terrestrial weather, while also being extremely lightweight and flexible. Carbon nanotubes are one possible build material, but producing them at scale and free of defects, so far, eludes materials science. For now, at least, space elevators are relegated to science fiction.
Space Tethers: Orbital Slinging
But what if we didn’t extend a space elevator all the way to the ground? Instead, what if we extended it only far enough such that a launching rocket could catch the tip of the tether just outside of Earth’s atmosphere where the spacecraft could then be lifted along the cable to a higher orbit? This concept, often called a “space tether,” could be shorter, lighter, and would engender less strain on materials because it wouldn’t have to deal with terrestrial weather. Building such a tether would still, however, be beyond the realm of feasibility today.
We could, however, make the tether much shorter and lighter if we rotated it; using centrifugal force to transfer momentum to incoming spacecraft. A “skyhook,” for example, utilizes a relatively short cable, perhaps just a few hundred kilometers in length, with a heavy counterweight on the “short” end, and the long end extending almost into Earth’s atmosphere. A rotating skyhook would “catch” spacecraft as they depart Earth, flinging them into higher orbits by transferring momentum. This would enable orbital and velocity changes without chemical propulsion, circumventing the rocket equation.
Compared with a space elevator, a skyhook promises to be much more realistic in the near term. At less than a fraction of the size, a skyhook requires far less upfront infrastructure investment, especially if we taper the tether from base to tip, further reducing its total mass. Additionally, it wouldn’t require breakthroughs in carbon nanotubes or other supermaterials; we could probably build one from existing materials.
suggested that we might even build one out of glass fibers today if we wanted. Glass fibers can handle temperature extremes, are corrosion and UV-resistant, and can be made from space-based resources. Tethers of this kind can greatly reduce the fuel required to reach space. A skyhook to LEO, for example, could cut fuel requirements by over 80 percent.Beyond Earth
More broadly, space-based tethers could unlock a host of destinations for human travel. We can map out the “Delta-v” and rough fuel requirements of any solar system destination fairly easily.
calculates, assuming a rocket is already in Earth orbit, a tether can reduce the amount of fuel required to reach the Moon or Mars by about 70 percent. This figure includes the upfront fuel cost of building the tether. Eventually, long-lasting tethers that are reusable 1000 times or more, could lower the fuel requirements of reaching the Moon or Mars by an order of magnitude. Further, a pair of tethers placed in Earth and Mars orbit could be used to sling spacecraft back and forth, shortening the travel time between the planets from 9 months to five or fewer, while reducing the spacecraft’s total mass by 90 percent or more.Part of this incredible mass reduction comes from reducing the stress on the heat shield. Just as a space tether can accelerate a departing craft, it can also decelerate arriving spacecraft. Capturing this energy and slowing the incoming spacecraft reduces the heat load during reentry, allowing for a thinner and lighter heat shield. This property, the ability to both accelerate and decelerate spacecraft, will be crucial to landing cargo ships on asteroids that have no atmosphere for braking at all. A tether positioned around a large asteroid would make asteroid mining possible, unlocking trillions of dollars of valuable resources for further human expansion into the cosmos.
Staying in Orbit
Should we one day build a space tether, the challenge will be keeping it in orbit. Every time the sling is used, momentum is lost; they aren’t perpetual motion devices after all. Atmospheric drag, gravity gradients…etc will slow and eventually stop the tether’s momentum. This challenge, however, is manageable. We could, for example, do as we do with space stations and install a small thruster onto the tether’s counterweight that is powered by fuels that do not boil off in the vacuum of space. When the tether loses too much momentum, it reboosts itself using onboard fuel. Incoming spacecraft could provide a fuel top-up when needed.
Alternatively, instead of a traditional chemical thruster, we could use a Hall thruster. Although they provide little thrust, they do so with an extremely high specific impulse (~1600 seconds) compared to a chemical thruster (~300 seconds) greatly reducing the fuel required. Another promising approach is an Electrodynamic Reboost tether. These require a conductive cable material; as the tether rotates and moves through the Earth’s magnetic field, a voltage is induced inside the tether, much like a generator on Earth. The interaction between the magnetic field and the electrical current produces a Lorenz force. By controlling this current, we can use this small force to keep the momentum of the tether going almost indefinitely.
My favorite approach to this problem, however, is simpler; just balance incoming and outgoing spacecraft. We might think of a space tether as a kind of rechargeable momentum “battery.” Every time it slings a spacecraft outward, it loses some momentum, but every time it catches and slows an arriving spacecraft, it gains momentum. We can greatly reduce the required reboosts by balancing incoming and outgoing transfers so that momentum energy is conserved. This will work especially well when there is regular travel of ships between celestial bodies, such as cargo resupply flights to and from the Moon.
Exploring the Cosmos
We humans naturally seek to expand and explore. Tens of thousands of years of human history, much lost to time itself, have been spent in the cradle of Earth. In the 20th Century, we reached beyond that cradle for the first time, but only under the extreme force of government funding and political will. If we are to truly expand, to take our first steps into the cosmos in the 21st century, we will need to make space economical. Reusable rocketry is only the first step. Once perfected, we will be able to affordably build space tethers that circumvent the rocket equation. Once this infrastructure is in place, will we be able to build a self-sustaining colony on the Moon or Mars and mine asteroids for space development. These first small steps will enable us to conquer our solar system and hopefully, ensure the survival of our species.
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* while not a religious believer I am enthralled by the concept of the "design solution" offered by evolution providing for a variety of random (or maybe "random") changes (aka adjustments) among which to select for continued survival as changing conditions warrant. If the "design" goal is continued existence of "life" in toto rather than worrying about continued existence of a particular species .... or maybe there is no "goal" at all????
Plus Matt Ridley in The Evolution of Everything expands this idea to a wider set of domains.
** on green skinned solar energy absorbing animals, I am surprised we have not seen more of this in nature so far. Why have only the typically stationary plants evolved this capability? Of course cold blooded animals sun themselves to warm up enough to get moving, but that is only a suboptimal answer for maximizing energy gain from the sun. What about evolving the equivlaent of photovoltaic doped silicon scales or plates? Or exoskeleton? Or using other materials more suitable for other ranges of the electromagnetic spectrum? Electromagnetic chameleons? Part of what might be needed by space living entities of some sort?
Thanks for the opportunity to wander and wonder in these comments. :-)
Something of a philosophical question as to whether we can or should think we can forestall or even side step our species extinction; or even that we should truly want to do so? We see death as part of the "natural" cycle to renewed and potentially "improved" life via evolution*. If our environmental conditions change radically enough, our nominal human form (and mind?) may be inadequate to survive without some significant changes in phenotype?
Thinking about this just a little, I asked how might we evolve into something that can survive either "in space" or have the ability to "cross space"? Perhaps we have time over the next 10 billion years before Old Sol goes Nova to become some sort of "energy beings" or whatever?
This led me to fantasize about evolving into lizard-like animals with green skin that contained chlorophyyl** to absorb solar energy. Not sure what the next step could or should be towards a free floating space occupying entity.
Also, the distances and time required for interstellar travel are so great (even at 0.95 speed of light c) that unless we find a way to create or find "worm holes" or "fold space" or "cross dimensional barriers", or ??? I don't really see us leaving our particular star system. This of course might also explain why we don't see other signs of life, as whatever civilizations might arise elsewhere can't or don't survive long enough to make their presence known outside of their star system??
Footnotes in next comment :-)