A space elevator is a long time dream of scientists and engineers. It consists of a carrier cable and a climber transporting loads between Earth and Space. The most challenging problem with a space elevator was finding a suitable cable material until carbon nanotubes have come to the rescue.
Today, the only system for space exploration and transmission is rockets. No successful alternative that would offer lower costs, ease of operation, or better results is currently realized. However, at the beginning of the space age, 60s, and 70s, an innovative alternative was proposed: the space elevator. What is a space elevator? It is a transportation system that is planned to be built on the Earth’s equator to a height exceeding the geostationary orbit (35,800 km altitude). Theoretically, a cable at the length of 144,000 km would be balanced in equilibrium. However, this length could be shortened when balanced with a counterweight. Its center of mass will have an orbit of 24 hours and it will stay on a geostationary point above the equator while rotating around the Earth’s axis. A space elevator basically consists of a cable, also known as the tether, attached to the Earth’s surface and the counterweight and an electrically powered climber to carry loads along this cable. This innovative transportation idea would revolutionize the transport and connection between Earth and space by providing easier, safer, faster, and more economic access to space. However, this idea was left behind because of the lack of suitable cable material. The long carrier cable must be built from strong yet light material for the construction of the space elevator. This is why until the discovery of carbon nanotubes (CNTs), the idea of a space elevator was merely science fiction.
Carbon nanotubes were identified in 1991 by Iijima. These nanoscale tubes are entirely made up of carbon, have diameters of several nanometers and theoretically km long lengths. Two types of CNTs are currently available and used for different applications, single walled carbon nanotubes (SWCNTs) and multi walled carbon nanotubes (MWCNTs). CNTs have many desirable features: ballistic transport characteristics, light weight (with approximately half the density of aluminum), high mechanical strength (approximately 20 times the tensile strength of high-strength steel), high elasticity, high electric current density (approximately 1000 times that of copper), and high thermal conductivity (approximately five times that of copper). For the space elevator applications, the tensile strength and density of CNTs are the most important characteristics. The tensile strength of CNTs has been theorized as ≈150 GPa which is superior to steel at 5 GPa. Furthermore, the density of CNTs is 1300 kg/m3 almost six times lower than that of steel. Furthermore, the unique electrical properties of CNTs allow the detection of any atomic deformation within the cable structure rapidly. Therefore, the breakage of the CNT tether can be detected almost immediately and can be repaired before any serious accident. The studies on CNTs show that a CNT based cable can provide the required strength-to-weight ratio. Theoretically, the strength of CNTs is about three times the strength needed for the construction of the space elevator cable. But practically, CNTs can provide only two-third of needed strength. Due to several different factors such as structural defects and external effects, the practical strength of CNTs cannot reach the theorized values. The strength of a real, thus defective, carbon nanotube based space elevator mega cable is expected to be reduced by a factor of at least ≈70% with respect to the theoretical strength of a carbon nanotube and reach an approximate strength of 45 GPa. Thus, the studies on optimizing the CNT mega cable structure are still in progress.
What are the Challenges for Building a Space Elevator?
The studies on space elevator focus on several different issues apart from the practical strength of CNTs mega cable. The general concerns are about the micrometeorite impacts, spacecraft impacts, radiation damage, and effect of natural frequency and oscillations on the cable, deployment locations, risks of damaged cables, and malfunctioning climbers.
One of the first concerns that come to mind is micrometeorites with dimensions of 1-5 cm. Due to meteoric effects; researchers have concluded that the optimum design of the cable would be ribbon-type which means one cross-sectional dimension of the cable is smaller than the other. Assuming that these meteorites are not scattered by the external parts of the space elevator, they can cause serious damage to cables with maximum dimensions of less than several centimeters. Hence, a bundled structure is suggested for the cable design. A bundled design gives the opportunity to repair the structure before irretrievable damage.
Even though space exploration has just started, we have already collected a considerable amount of junk at the low-Earth-orbit. This cloud of junk orbiting the Earth is called debris which includes meteors and leftover spacecraft parts from previous missions. The dimensions of objects in the debris can vary from 10 cm to several meters. One way to avoid damages by these objects is to incorporate tracking systems and movable anchors on Earth to try and circumvent collisions.
The Earth’s radiation belt containing energetic electron and protons would create radiation damage on the CNT cable however; the studies show that radiation damage is not as destructive when compared to other environmental problems. CNT cable can survive Earth’s radiation for over 1000 years. Atomic oxygen content in the atmosphere causes more severe damage to the cable and requires durable coating applications for protection.
Natural frequency and oscillations caused by the gravitational force of the Sun and the Moon create stress on the cable. However; these problems can be avoided by the variations in counterweight location and the damping at the anchor.
An important problem which requires a detailed analysis and optimization is the choice of anchoring station on Earth. The location would affect the power transmission to climbers and the effect of extreme weather conditions on the cable. Anchor location should be on the equator otherwise a constant out-of-plane force on the cable and counterweight and an additional time-variant force when climbers are on the cable would be observed. The altitude of the station must be decided considering the absorption of microwaves for powering the climbers and the human operation conditions. Studies suggest a 5 km altitude can be suitable considering both factors. Avoiding the effects of extreme weather conditions such as lightning strikes, storms, and jet streams is tricky. These factors can severely damage the cable. Selecting an equatorial site avoids both the jet streams and cyclonic storms, however; the thread of lightning strikes remains. Heavy lightning frequencies can be avoided at higher altitudes or on ocean sites.
In order to obtain a feasible operation, the design and deployment of the space elevator should be optimized considering all of these concerns.
A space elevator has been discussed since the beginning of the space age. This system simply consists of a carrier cable and a climber cabin to transport loads between Earth and space. The idea of this innovative system has remained a fantasy until the discovery of carbon nanotubes. Before carbon nanotubes, no other material was suitable to build such a long and durable structure. However, with its excellent tensile strength and low density, CNTs rekindled the hope of space elevator. Using CNTs as cable material answers the biggest problem in the construction of a space elevator. However, there are still some problems standing in the way of realizing this dream. For starters, carbon nanotubes produced currently cannot reach the theorized high tensile strength due to structural defects. Thus, obtaining high quality CNTs is the key to progress. Other problems in the way of constructing a space elevator are micrometeorite impacts, spacecraft impacts, radiation damage, and effect of natural frequency and oscillations on the cable, deployment locations, risks of damaged cables, and malfunctioning climbers. Scientists and engineers are currently working towards eliminating these problems. Even though constructing a space elevator seems like a challenging task, we shouldn’t forget that all engineering projects are challenging at the beginning but engineering is all about creating solutions to tough problems.