Approaches to Technological Challenges of the Space Elevator

The space elevator is a future space transportation system that connects Earth and space by a cable and climber (vehicle) back and forth. When completed, the system will have the following advantages: “the cost of going to space will be greatly reduced," “space travel will be less physically demanding," “it will serve as a starting point for other celestial bodies,” and “solar power generation will be possible without loss.” However, the elevator’s mass was too large, and the materials were not strong enough to make it feasible as a structure.<br/>Carbon nanotubes (CNTs), discovered in 1991, are composed of carbon atoms and are lightweight (specific gravity is about half that of aluminum), and have high mechanical strength (tensile strength is about 20 times that of steel). CNTs as a material for cables have increased the feasibility of space elevators. However, to make it viable as a transportation system, it is necessary to manage the same processes as on the ground, such as design, construction, and maintenance, and to deal with the special movement mechanism that moves vertically through the Earth’s gravity zone and exposure to the unique space environment.<br/>This paper reports on the current status of efforts to test space elevator cable materials for exposure to the space environment. The space environment resistance of twisted CNT yarns, which are assumed to be the cable material for space elevators, was investigated using the Exposed Facility of the Japanese Experiment Module “Kibo” on the International Space Station (ISS) at an altitude of about 400 km. At altitudes between 200 km and 600 km, atomic oxygen (AO) is a major component of the atmosphere, accounting for more than 80% of the atmospheric composition at an altitude of 400 km, where the ISS orbits.<br/>The 1-year exposure test specimens were placed at the rear and front of the ISS, and the 2-year exposure test specimens were placed at the rear of the ISS. For the ground control tests, the first test was conducted from November 12, 2014, to March 2, 2015, and the second from June 8 to 30, 2016, at the JAXA-owned vacuum complex environment test facility. The test conditions were full-period irradiation, with the irradiation dose corresponding to approximately six months of space environment exposure. Strength tests were not conducted on thin-stranded CNTs due to their serious damage. For thick-stranded CNTs, the tensile strength was measured to be lower on the front than on the back. The results of the ground control test were reproducible. The tensile strength of the flown product was lower than the pre-flight product. For the specimen of the flown product, the portion covered by the cover for fixing the specimen and the portion exposed to the space environment were compared. The diameter of the exposed part was smaller than that of the part covered by the cover. It was also found that CNTs were gouged from the surface to a depth of several CNTs in the exposed area.