Proposed Tether Mission

Born as a side product of the semi-conductor industry, MEMS devices integrate both electronic and mechanical components onto a single substrate. Sensors, such as accelerometers, comprise the majority of existing commercial microdevices. The reason for their early success arises from the mechanical and electrical integration, as a microsensor can translate physical phenomena into electrical signals all within a single electronic chip. However, the numerous advantages imparted by the size scale offers great potential outside the realm of sensors. For example, in the realm of fluids microdevices can control the flow at a very local scale resulting in more efficient devices, a fact Massachusetts Institute of Technology is trying to exploit by developing a microengine. With the electronics industry possessing half a century of experience using silicon, most mechanical MEMS components are formed from polysilicon using the same technology developed to make transistors. However, a number of other materials are also employed including Silicon Nitride, Silicon Germanium, electroplated Nickel, Amorphous Diamond, and Silicon Carbide as used for the turbine blades in the engine mentioned earlier. The Microsample Laboratory at Johns Hopkins University conducts testing on these materials to determine the basic material properties, including the Young's Modulus, the failure strength, and the coefficient of thermal expansion. Due to the higher surface area to volume ratio, unique manufacturing process, and greater influence from grain size and orientation, micron sized materials behave completely different when compared to their bulk relatives. For example, the strength of polysilicon, roughly 1 GPa at room temperature, far exceeds the ultimate strength of bulk steel (~ 0.5 GPa), with silicon nitride approaching 7 GPa. The space industry provides ample opportunities for microstructures. For instance, a next generation telescope could employ an array of thousands of individually controlled micron-sized mirrors to allow for precse light gathering and focusing. The figure, from Sandia National Laboratory, shows a hinged mirror oriented by a series of comb drives.

Figure 5: Hinged Silicon Mirror and Drive Motors [I-1]

However, little research exists on how MEMS materials will react to the space environment, under extreme thermal fluctuations, intense UV and high energy radiation, and, near the Earth, atomic oxygen. Part of this research can be conducted by simulating the conditions on the ground. Research conducted by Sharpe, Eby, and Coles, at Johns Hopkins has established the mechanical property variation for polysilicon between 0° and 250° C. [7]

Figure 6: Young's Modulus of Polysilicon

Figure 7: Failure Strength of Polysilicon

A space-based experiment would complement the ongoing tests providing more reliable material properties for designers to work with. An optimal mission would expose separate material samples to different space environments and for a different duration depending on the particular application. This proposed mission would examine three material scenarios. First, a low altitude, short duration exposure, designed to simulate a pass though the upper atmosphere by a MEMS device. For example, a micro-rocket motor will operate for a short period as it lifts its payload through the upper atmosphere. Atomic oxygen degradation serves as the primary hazard in this stage. Second, a middle altitude, clear of the atmosphere, for a moderate duration, designed to simulate the exposure received by MEMS device on a short space mission. The space shuttle, for instance, stays aloft for only a few days and operates in a low altitude orbit. Thermal cycling, and high-energy radiation play prominent roles for the second stage. Finally, a longer exposure at a higher altitude designed to test the response of the materials to the environment typical to low earth orbiting satellites. For a longer duration higher altitude orbit, other environmental factors arise beyond those mentioned for stage two, such as micrometeorites.

Several methods exist to conduct the experiment. The space shuttle or International Space Station could carry out the tests, but forcing either to change altitude twice for this small experiment would prove a very costly option. Alternatively, a small satellite, deployed at the upper test altitude, could maneuver to the other test altitudes while also controlling the sample exposure. Released from the shuttle or riding piggyback on another satellite deployment mission operating near the top altitude appears possible. Since the samples require testing back on Earth after the exposure, this option also requires a method to recover the satellite. A full-scale satellite with a propulsion system capable of executing multiple orbital changes, and returning to the shuttle or Earth would be prohibitively expensive for a university research project. However, employing a space tether alters this cost equation dramatically. A short space tether, taking advantage of the induced electrodynamic drag, could rapidly move the satellite to the lower two test altitudes. Removing the thrusters, fuel, and related components results in an extremely simple satellite, composed of the following components.

Figure 8: Satellite Diagram

• Sample Material: Each stage will expose a sample chip, as pictured, to the environment for a total of three chips for the mission.
  
  Figure 9: Polysilicon Test Chip
  The 1 cm square die contains fourteen tensile test specimens, each with a 3.5 by 50 micron cross section. For reference the two gold bands, used for interferometric strain measurement, at the center of each specimen have a separation of 200 microns.
  
• Tether System: The tether, a single aluminum rod, connects the two end masses. The selection of aluminum yields an inexpensive tether with a high strength to weight ratio, and good electrical conductivity. Insulating the length of the tether, with Teflon, will limit the electrical contact with the surrounding plasma to the two ends, with an anode electron-collecting device on the main sphere, and the metal ballast mass serving as a cathode electron-disseminating source. This method allows for the control of the current and thus the drag acting on the tether. A simple switching device can break the circuit preventing the flow of current and eliminating the electrodynamic drag.
  
• Main Satellite: The main body contains the material samples, recovery hardware, and electronics. Conceptually, the satellite controls the material exposure with a small tray that extends outside of the capsule. The tray holds a dish with three partitions, one for each sample. A rotating cover with an opening allows one sample to be exposed to the space environment at a time while protecting the other two. The recovery hardware consists of three components. First, an ablative material covers the outside surface area of the aluminum shell. This silicon foam vaporizes during atmospheric reentry thereby absorbing the heat and protecting the main sphere. [5] The second component, a parachute, slows the descent of the capsule through the lower atmosphere. Finally, a radio beacon guides a recovery crew to the spacecraft. The electronics portion controls the operations and also contains a battery to power the mission.
  
• Small Ballast Mass: The ballast mass serves a dual role. In the launch configuration, the ballast mass stores the tether around a central reel. In orbit, the mass serves to provide tension in the tether and maintain a gravity gradient stabilized configuration.