Expressway to the Moon
Half the fuel, one quarter the time.
Why The Moon?
It has long been an inhibition of our species to explore and settle the other celestial bodies of our Solar System. This untamed curiosity was likely driven by the presence of our enormous moon, looming over us like a cosmic beacon, pleading for our attention. In 1969, the Moon finally won us over when we performed one giant leap to its surface, Armstrong depositing the first footprints in its dusty, gray regolith. In many ways, the Moon can be viewed as an exploratory checkpoint; one proverbial step closer to the countless other planets, moons, and other bodies orbiting the Sun. In the three ensuing years of Apollo 11, five more manned missions landed humans on the lunar surface, further fueling our drive to increase our knowledge base about our home star system. Then, in December of 1972, Eugene Cernan stepped from the lunar soil into the Apollo 17 landing craft with a return trajectory for Earth. We haven’t been to the Moon sicne.
America’s need to put footprints on the Moon in the 1960’s was primarily encouraged by the threat of Russia’s budding space program, and not directly in the name of science. In the wake of the Space Race era, two things occurred which dashed our hopes of further exploring our Moon; NASA’s budget was cut, and public interest in space exploration decreased. These two facts in tandem forced NASA to realize that sending humans to the Moon for the sole purpose of exploration was, in fact, much too expensive.
The Moon, however, may be more than simply a checkpoint in our exploration of the Solar System. Instead, it may be more akin to a stepping stone, with benefits far greater than using our home planet as a base of exploratory operations. Firstly, Luna has plenty of cryogenic water frozen into its poles to supply a permanent colony with both liquid water for plants and humans, as well as for the purpose of splitting into its atomic counterparts to use as rocket fuel. Secondly, the Moon’s lack of atmosphere provides an ideal environment for solar farms to produce power for a base’s energy needs. But most importantly, the Moon’s small gravity allows for a safer and less costly solution for missions, manned or otherwise, to the outer reaches of the Solar System. However, the problem remains that getting materials and equipment to the Moon with modern rocketry is still extremely expensive, dangerous, and time consuming. In order to allow for such a base to become a reality, humanity will eventually require a more efficient way to reach the lunar surface.
The Idea
In 1956, Dwight D. Eisenhower approved construction of America’s Interstate Highway System. This entailed the construction of 40-some thousand miles of freeway systems across the continental United States, fluidly connecting every end of the nation with a smooth, safe mode of transportation. Suddenly, the movement of goods and services from one side of the country to the other was faster, safer, and more affordable than it had ever been, effectively allowing the United States’ economy and GDP to skyrocket in the ensuing decades.
For a permanent lunar outpost to become economically feasible, such an expressway must be erected between the Earth and the Moon so that the expenses involved in transporting humans and materials to that outpost is outpaced by the profit (or lowered expenses) reaped by the colony itself. Launching spacecraft from Earth is expensive because of high fuel demands engendered from two main issues: (1) Earth’s large gravitational well, and (2) Earth’s drag-inducing atmosphere. If we could find a way to have a spacecraft start high above Earth’s atmosphere and gravity well, then launching regular spacecraft to the Moon would become much more practical. So, the question is then, how do we transport people and materials above Earth’s atmosphere and gravity well without using a rocket?
The Solution
Seeing as Iron Man suits have yet to be feasible, the best solution to transport payloads into space barring the use of rockets is a space elevator. A space elevator is essentially an immense tether extending from the Earth’s equator to beyond geostationary orbit (~42,000 km distant), with the center of mass at geostationary distance. That way, the tether “orbits” the Earth vertically while maintaining a fixed geographic location on the Earth’s surface. Due to the unique properties of a space elevator, the downward force from gravity on the structure would be identical to the outward centrifugal force from the Earth’s rotation, meaning that a space elevator would exert no force on the Earth’s surface at all. Instead, the trouble in designing a space elevator would come in manufacturing a material that can withstand the tension induced at the center of the elevator, where stress would be greatest.
Most designs for a space elevator utilize a counterweight (such as an asteroid or large station) beyond geostationary orbit, so that the tether doesn’t have to extend to the full length of 84,000 km. However, for an expressway to the Moon, extending this tether as far as possible may actually be beneficial. Because all parts of a space elevator would perform one rotation around the Earth every 24 hours regardless of where you are on it, your velocity actually increases the farther you are away from the Earth, like riding farther away from the center of a Merry-Go-Round. This means that the far end of a space elevator is constantly travelling faster than orbital velocity at its altitude, and is perpetually being pulled outward by strong centrifugal forces. The longer this tether is constructed, the more the far end is pulled outwards… right towards the Moon.
Now, attach a spacecraft to the very top of the space elevator, load in your payload, and simply let it go. The craft will slingshot away from the elevator into a hyperbolic orbit, travelling faster than Earth’s escape velocity. We could wait for this craft to reach Moon before performing the maneuvers for it to land, unload, and return to the station, but instead why not construct a twin space elevator from the surface of the Moon? A lunar space elevator would extend outwards to 2x the Earth-Moon Lagrangian point (~58,000 km from the Moon; the place where the Earth’s and the Moon’s gravity’s cancel each other out). This would allow the spacecraft to use less fuel, and to not have to deal with landing in the hazardous lunar environment. Once at the lunar elevator station, the craft would deposit its payload, then mirror its maneuvers to arrive right back at the Earth elevator station ready for another trip.
Obviously launching a spacecraft between these two space elevator outposts uses less fuel than launching from the Earth’s and Moon’s respective surfaces; but would it save time? To mitigate stresses on the Earth space elevator and the passengers inside, Earth climber cars would be limited to accelerations of about 25% Earth gravity: 2.4 m/s². These climber cars would accelerate at this rate until about the halfway point, then spin upside down and decelerate the remainder of the distance to the orbital station. To traverse the full distance of 84,000 km, this trip would take a little over 3 hours. The transfer orbit between the Earth and Lunar orbital stations clocks in at about 12.5 hours. Though the Moon’s space elevator is longer in distance, the Moon’s small gravity and slow rotation induce less natural stress on the lunar space elevator. This allows the Moon space elevator climbers to be accelerated much faster than Earth ones. Even at a modest 0.5*g, the descent trip would take just shy of 3 hours. An executive taking a business trip to the Moon could schedule a trip and arrive there on the same day. The Apollo trips entailed a minimum approach of 3 days.
The transfer orbit between the Earth station and the Moon station would require a round trip delta-V (a way that astrodynamicists determine how much fuel a spacecraft needs) of less than 11 km/s. The Apollo missions, overcoming an enormous amount of gravity and atmospheric drag, required a round trip delta-V of 20–30 km/s; at least 2x as much as the Lunar Expressway. If an effective method could be implemented to power the two space elevators (such as solar power), such a system would save hundreds of millions of 2018 dollars every time a shipment was made to the Moon. Over the course of decades of operation, the Lunar Expressway would pay itself off.
Problems
The Lunar Expressway doesn't come without some potential issues. The maximum stresses induced at the center of the Earth space elevator would be a little over 63 GPa; 250 times the maximum tensile strength of steel. The only material that has shown promise in overcoming these enormous stresses is carbon nanotubes, which are still extremely difficult and expensive to produce. Probably the biggest issue facing the construction of the Lunar Expressway is the manufacturing of nearly 200,000 kilometers of these carbon nanotubes, or an equivalently strong material.
Advanced station-keeping techniques will also need to be devised in order to assure that the orbital stations maintain fixed orbits. The Earth-Moon L1 point, for example, is only quazi-stable, meaning that the center of mass of that space elevator will drift away over time, causing the entire structure to lose stability. Also, elevator cars climbing and descending the tether will induce both vertical and lateral forces on the elevator tethers which will require stabilization.
Though unlikely, impacts from asteroids and micro-meteoroids would pose a stark threat to the operation of a space elevator on either the Earth or the Moon. Precautionary avoidance systems (maneuvering thrusters) or active preventative systems (space guns!) would have to be installed to ensure that such events could be averted. Elevator cars would also have to be outfitted for radiation for passage through Earth’s Van Allen belts.
Conclusion
With a little human ingenuity, a Lunar Expressway comparable to the United State’s Interstate Highway System could be erected between the Earth and the Moon to reduce fuel expenditures and travel times enough to make future deep Solar System exploration more feasible. With such a system in place, deep space missions would no longer be limited to one probe per decade, perhaps allowing for the launch of tens of probes or manned missions per year to various destinations around the Sun. Having a permanent presence on the Moon would also make the colonization of other Solar System bodies, such as Mars or Venus, more realistic in the long run. The key to exploring and settling our Solar System is right in our cosmic backyard; we would be foolish to ignore the treasure trove of opportunity that is our very own Moon.
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