Theia is a modified version of a standard radioisotope thermoelectric generator (RTG) that overcomes the high pressures and temperatures of Venus’ surface through redesigned component structures and materials. This includes an internal complex of 15 stacked modules containing Pu-238 with aerogel-insulated SiGe thermoelectric converters used to generate power from the temperature differential. This complex is contained within a gold-plated structure made of chromium-molybdenum alloy steel with 8 fins to help remove excess heat. This redesign creates an energy source/storage system that could effectively function for over 10 years on Venus.
Various methods of creating effective energy sources for long-duration Venus missions have been visited in the past. The majority of these methods had evident limitations prevented them from being viable systems to use on long-term missions on Venus’ surface.
Previous Venus missions consisted of spacecraft that relied on primary batteries as the main power source for the vehicle. Primary batteries revolve around the concept of 1-time use and discarding them afterward. Previous Venus surface lander concepts consisted of LiSO2 batteries, which would corrode in the extreme temperatures of Venus. The power output of such primary batteries is solely dependent on the capacity of the battery itself. Primary batteries are unstable at high temperatures and restrain the maximum duration of the mission. The extensive thermal maintenance of these systems also increases the overall power requirements. Rechargeable batteries have the potential for longer-duration missions although a concrete energy source for these batteries is currently lacking.
Figure 1
There is massive disproportionality between the solar concentration/intensity of solar rays as altitude changes on Venus. The thick Venus atmosphere absorbs most of the sunlight or scatters it heavily. The solar intensity of these rays ranges from 2622 W/m2 above the atmosphere to a staggering <100 W/m2 at the surface of Venus. Figure 1 depicts this disproportionality as a calculation of the downward solar flux in Venus. Creating aerial vehicles to supply power was visited by NASA as an option and still remains viable although the logistics of this approach become significantly more complicated, and in some ways, impractical. The operation team would need to consider the power sources for both the aerial vehicle and the lander alongside the high-velocity winds in the upper atmosphere of Venus (360km/h).
Figure 2
Venus has an abundance of ions in its ionosphere located between the altitudes of 120 - 300 km (see Figure 2). Using an ion collector and a conductive tether, it’s possible to reach higher altitudes and send ions through the conductive tether to the surface where another conductive wire takes them to a processing unit to be converted into energy. There have been theoretical research papers on ion harvesting applications for the Martian surface. The same architecture is mimicked on Mars although the optimal period of time to harvest the ions is during only global electric circuits (caused primarily by dust storms). Triboelectrically active dust storms and incident solar radiation excite neutral atmospheric conditions. This increases the overall electron density as displayed by the figure above.
While this same system could be implemented on Venus, the implementation and logistics would be very difficult to overcome. The processing unit must be able to withstand the heat and corrosive sulfuric acid in the atmosphere of Venus. Implementing a huge tether of 120 km in distance is also impractical. These limitations pose a barrier to implementing ion harvesting on the Venus surface for landers.
After taking into account the various limitations of the power sources/storage systems above, our team took the executive decision to focus on a method that is technically viable and logistically more practical. The logistical complexity of the other methods of energy generation and storage create a sense of ambiguity in which assumptions have a high probabilistic outcome to be inaccurate for precise energy requirements. When analyzing previous research for other viable options, we found that the use of radioisotopes in thermoelectric generators (RTGs) to generate power seemed fairly reliable. There have been many a multitude of previous space exploration missions that used RTGs as the primary source of power, making them an attractive option to be reused.
Figure 3
RTGs work by converting a temperature differential into power without using moving parts. A heat source, which contains a decaying radioisotope (often Pu-238) is used to warm a conductive heat shoe. A cold shoe is placed opposite of this heat source to create a temperature difference. Then a set of thermocouples are used, generating a magnetic field for the electrons to flow in. This magnetic field creates a constant flow of electrons through the 2 thermoelectric legs of a thermoelectric converter (p and n legs). Figure 3 depicts the general schematic of an RTG. The governing principle associated with RTGs is the Seebeck effect. It states that 2 different metals with a great enough temperature differential within a closed circuit can generate voltage; this thermoelectricity is created through thermocouples.