The Feasibility of Space Elevators
The Apollo program brought astronauts back as heroes and ignited the imaginations of young children, propelling a new generation to be excited about math and science. A space elevator could reinvigorate these benefits with a fraction of the cost of the Apollo program. However, despite the manner in which it would revolutionize the space program, a space elevator would be a monumental task. To overcome it, a 60,000km strong and flexible ribbon, radiation shield, counterweight, propulsion method, and debris avoidance systems would all need to be developed. While current research in this field is going into tackling these problems one at a time, much work remains to conquer these engineering challenges.
In order to ensure that a space elevator will remain intact, the ribbon (which provides support for the elevator climber) must be carefully scrutinized to ensure strength. This includes evaluating the likelihood of the ribbon being severed, torn, or being struck by lightning after deployment. The vast amount of space debris makes tears in the ribbon not only likely, but a near certainty; Pugno predicted to see 1012 – 1020 micro-cracks in a graphene cable. The strength was also measured both by internal fracture and sliding to be 35 Gigapascals (steel, in comparison, has a tensile strength of 450 Megapascals) (Pugno, et al. 2012). More structural problems are induced by librational forces, which involve oscillations between two orbiting bodies. They are introduced into the system by eccentricities of the moon’s orbit as well as the Coriolis effect (deflection of a moving object due to rotating reference frames) (Williams, et al. 2010). Most of the elevator has no surrounding air pressure, so there is nothing to stop librational forces; therefore, they must be stopped externally or by the climber, and this adds another layer of logistical problems.
Most of the logistical problems come into play with the deployment of the ribbon; the center of mass of the ribbon must end up slightly higher than geostationary orbit for the ribbon to remain stable. The counterweight must be carefully maneuvered for the ribbon to remain intact, but why is this? Think about how a figure skater tucks in their arms and body to go faster and extends them to go slower; this is because angular momentum is conserved. In a similar way, as the cable descends, it speeds up slightly, so a counterweight spacecraft is required to precisely adjust for this fact (Takeichi, et al. 2012). This trajectory must be painstakingly planned out and optimized, yet be adaptable enough to adjust for collisions with small space debris using a feedback system. Additionally, on the logistical front, space debris must be tracked and either disintegrated or be properly avoided, lightning storms must be tracked, evaluated for their danger, and the ribbon must be repaired if struck; perhaps sensors would be put along the ribbon to help detect micro-tears before they become a problem (Williams, et al. 2010). Then, there’s the issue of beaming up power, probably using a laser. This system would have to work reliably enough and have enough power to propel the climber all the way up the ribbon. Any deviations could cause a loss of power to the climber, and there would need to be a temporarily stored power supply to protect against such cases to ensure the safety of the passengers. Additionally, the entire trip of the climber must be mapped out to minimize librational movements and ensure ribbon and passenger safety. Using the final optimized simulation, Williams and Ockels found that in-plane librations could be completely eliminated by reversing the climber for a short time at about 80% of the way through the trip (Williams, et al. 2010). There are many other logistical problems, including coordinating all these systems as well as future systems together, getting international support, and ensuring the safety of the system and passengers using multiple levels of protection.
Even if a space elevator is built, there are many unsolved obstacles for humans in deep space, including radiation, muscle and bone atrophy, growing food, developing a system to allow humans to hibernate, and artificial gravity. Space elevators are designed to propel astronauts into space much more easily, but it cannot protect them once they get there. Radiation poses a major problem for deep space as well as during the trip to space. The most significant source of radiation is the Van Allen radiation belt, two bands of high energy particles around Earth. This belt was not such a big threat to astronauts because of their rapid speed throughout these regions (thus, Apollo astronauts received only 0.41 rad); space elevator passengers would be approximately 200x longer (10 days compared to less than an hour), so radiation doses would be much higher. In addition, the quick onset of solar flares poses a significant threat to astronauts. Finally, if all that radiation is avoided, there is still a background radiation of 15-20 rem / year, which would be difficult to shield and pose health problems for long-term space dwellers (rem is a calculation of equivalent dose based on rad and Relative Biological Effectiveness). 53 g / cm2 of aluminum shielding would be needed for only 1 rad exposure; however this would weigh 18 tons, making a space elevator sporting aluminum shielding impractical. Electromagnetic shielding is also possible, but larger climbers would need larger coils (greater power consumption), and would only be viable through superconducting magnets. A final option is to have two shields, one for weaker radiation fields, the other for stronger radiation fields; they would be exchanged at a certain point during the climb, but a stronger cable would be required to do so.
Space Elevator Alternatives
Space tethers require at least 35,000 km carried by at least 24,000 flights to geostationary orbit, further complicated by meteorite damage and lightning strikes. An alternative in the form of a free-standing pneumatically pressured tower could be assembled from the ground up. Quine, Seth, and Zhu propose a structure built of Kevlar pressurized with hydrogen or helium, control pods to adjust for instabilities, and walls with laminated polyethylene. The structure would be free-standing with pressures above 7 PSI and could be sustained despite leaks with a plumbing network and high pressure gas line, able to support payloads of more than 106 kg. Gyroscopes would also be added to greatly increase the angular momentum of the tower, making any small changes due to wind, libration, or other forces marginal. Sections of up to 150m could be constructed with gyroscopic stabilization with a roller system and pneumatic pressure, so to construct 20,000km of tower (the proposed height) only 134 150m sections would have to be installed (Quine, et al. 2009). Stabilizing this project would be less difficult, and certainly could serve as a stepping stone for a potential space elevator; however, no costs are cited for this alternative, as it seems to be more expensive than a space elevator, even if it would be more feasible, and there is only a 26% gain in efficiency over traditional chemical rockets. This article also cites a dynamic (moving) ribbon as a potential space elevator substitute, but the risks in terms of damaging the ribbon would be much too large (and certainly much higher than a static ribbon) to warrant such an option.
Like the issue of going to Mars, the space elevator can conceivably be realized if given a good enough reason; humans tend to make things recently considered impossible a reality when we really put our mind to it. The major issues at this point are constructing a ribbon long enough, dealing with deep space issues to be able to use the space elevator effectively, and convincing, policy makers, funders, and most importantly, scientists that it’s worth the effort. More focus needs to be put into finding new ways of making longer carbon nanotube and graphene strands (or maybe even some composite of the two of these materials) in order to make this a reality. Competitions that reward success among competing elevator climbers (such as google x) are a step in the right direction, but more need to take place in order to gain the full breadth of creativity possible. Additionally, a much better radiation shield material needs to be found to protect future elevator passengers. Many of the research articles have good ideas, but they need to be reviewed and put into a comprehensive plan.
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