# Toward Sustainable Mars Exploration: A Perspective on Collaborative Intelligent Systems
## 3 Energy Management
### Conventional Energy Sources  
Solar radiation and heat energy generated from the natural decay of radioactive materials have been the predominant energy sources for the majority of Mars rover missions. During the early days of Mars explorations, solar energy was the standard choice due to its well-established technology and ease of implementation. Converting solar energy to electrical energy can be easily accomplished with the help of solar cells. Typically, a combination of solar cells with rechargeable batteries is used. The rechargeable batteries act as a backup source and supply energy to the system when the Sun stops illuminating the panel. For instance, NASA's _Mars Pathfinder_ used gallium-arsenide solar cells with a maximum production of 180 W in combination with 27 V, 40 A h silver-zinc batteries [14]. Similarly, many other solar energy-powered rovers were deployed. The selection of **solar energy** for powering Mars rovers was primarily motivated by their widespread use in many space applications and satellites, as well as the technological maturity it has gained over time. However, the Martian environment presented several challenges that restricted the performance of solar panels:  
1. The intensity of solar radiation decreases with the distance from the Sun to a planet. Consequently, Mars receives weaker illumination from the Sun due to its relatively distant placement in the solar system. 2. Relying on solar energy limits choices for the landing location of the rover to those close to the equator, where the radiation is abundantly available [15]. 3. A major factor that negatively impacts the performance of solar cells is the occurrence of frequent dust storms and dust devils on Mars. These dust storms lead to the accumulation of dust particles over the cells, resulting in the degradation of their performance. The fact that dust particles in the Martian atmosphere are electrostatic further elevates the concern [16]. Over time, the accumulation of dust particles on solar cells has led to the temporary or permanent failure of several missions. Cleaning of dust particles caused by local turbulence has been highly beneficial in mitigating the impact of gradually accumulating dust during certain surface missions reliant on solar power. However, predicting these events has proven to be challenging. In April 2022, NASA's Ingenuity Helicopter became inactive for around 6 days as the solar arrays stopped functioning due to dust cover [2]. In another instance, China's first Mars rover _Zhurong_, surrendered to dust storms [17]. The rover was commanded down to sleep mode in response to an expected dust storm, but it never woke up from its sleep. In an attempt to overcome the limitations of solar energy sources, **Radioisotope Thermal Generators (RTG)** were introduced [18]. RTGs are nuclear generators that rely on heat energy released during the radioactive decay of materials to generate electricity. Though RTGs are nuclear power generators, they are direct converters and do not rely on moving or mechanical converters to generate electricity. Specifically, an RTG uses solid-state heat engines, called thermoelectric generators (TEG), to convert heat produced from the natural decay of radioactive materials into electricity. When heat passes through two dissimilar metals that are joined together, a voltage is generated at the junction of these two materials. This phenomenon is known as the Seebeck effect and lays the foundation for TEGs [19]. The use of RTGs has significant benefits. The lack of moving parts to convert heat energy into electrical energy enables a compact, maintenance-free, and noiseless design for RTGs. Moreover, unlike solar cells, they are not impacted by the atmospheric conditions of Mars, such as dust storms. Considering these advantages, many missions to Mars have started using RTGs as energy generators. For example, the Curiosity rover, which has been operational on Mars since August 2012, is sustained by an RTG that guarantees a lifetime of a Martian year (i.e., 687 Earth days) or more [20].