by Justin Kerr
With the recent landing of the Perseverance rover in Jezero crater, exploration of the Martian surface is now all over both the news and general scientific conversation. Perseverance is the latest in the ever-expanding list of vehicles successfully landed on Mars, and a large one at that. It weighs in at 1025kg and has dimensions of 3x2.7x2.2 meters, making it only a little smaller than the average car. But unlike a car, Perseverance obviously cannot just drive up to the local gas station for a refill. The amount of gasoline needed for a decently long mission would also be far too heavy and volatile to bring along for the launch from Earth and subsequent landing, so just how do we go about powering Perseverance and the vehicles before it while they explore the red planet?
Most past rovers and landers have utilized solar power to generate the electricity they needed to operate. Some recent examples of vehicles using solar power include the InSight lander alongside the Spirit and Opportunity rovers. While Mars is further from the Sun than Earth and thus receives less sunlight, solar panels on the surface can still produce enough energy to power a rover. They also have the huge benefit of never running out of fuel so long as the panels are still functioning, which allowed Opportunity to last for 14 Earth years, far longer than its expected lifespan. That being said, power outputs from the panels may themselves seem quite low compared to the usage of everyday devices on Earth. The panels on InSight are capable of an output of 600 Watts, and the Spirit and Opportunity rovers only 140W – a pittance compared to the 850W power supply in use on the computer on which I am currently typing this! The Mars vehicles make up for this relatively low power generation rate by storing power in lithium-ion batteries much like those used in modern smartphones to use for larger expenditures or during the night – which brings us to some of the problems associated with solar panels.
The biggest problem with solar energy production is the inconsistent availability of sunlight with which to generate power. The most obvious source of this problem is nighttime, but there are others. Seasonal variations cause decreases in solar power output, with power generation being more difficult during winter. Solar power is most effective at the equator where the most sunlight is received, making it much more difficult to power vehicles closer to the Martian poles (you can actually expect a short paper related to this topic from me in the future!). Dust is a major problem on Mars, able to settle in a fine layer on top of vehicles we land there even during normal weather. Spirit had its solar panel efficiency drop to roughly 60% due to dust coverage in its first year – although the rover actually got lucky by having its panels cleaned off by a dust devil in early 2005. Of even larger concern are the global dust storms that can occur on Mars which put so much dust into the atmosphere that solar power generation becomes essentially impossible. You can see the extensive dust buildup on Spirit from one of these storms in 2007 at the top of this article. One such storm was famously responsible for the loss of Opportunity in 2018.
While solar power may seem like the obvious solution for power on another planet and is indeed effective in many situations, it clearly isn’t perfect – so what else could we use? Perseverance, Curiosity, and the Viking landers of ages past instead utilized the radioactive decay of plutonium-238 for power generation. Specifically, the power is generated by a device known as a radioisotope thermoelectric generator (RTG). When the plutonium in the MMRTG (Multi-Mission RTG) decays into Uranium, it produces significant amounts of heat as the released radiation is absorbed by materials which can then be converted into electricity. In the Perseverance and Curiosity rovers, the excess heat lost in the conversion process can even be put to use keeping the delicate electrical components of the rover warm. This electricity production method gets around the issues of solar power not working at night, during winter, or when covered by dust – radioactive decay will occur regardless of the environmental conditions.
Nuclear power generation with MMRTGs still has some downsides compared to solar, such as the amount of power that can be generated by an appropriately sized RTG. Perseverance can currently only generate 110W of power, which is used to charge batteries in the same manner as the solar powered vehicles. This amount will also reduce over time as the amount of plutonium decreases as it decays. Plutonium-238 has a relatively short half-life of 87.7 years, meaning there will be a noticeable drop in the amount of available power by the end of the rover’s 14-year lifespan. There is also a concern with the amount of available plutonium-238 for future missions, as the United States only has enough left in their cold war era stockpiles for a few more missions. Thankfully there are plans to begin production of the isotope right here in Ontario at the Darlington nuclear power plant in the near future. In the end, neither RTGs nor solar power provide a perfect solution to the power requirements of the vehicles we send to Mars. We can expect to see a mix of these two methods in upcoming Mars missions, with the next two vehicles set to land (China’s Tianwen-1 and the ESA’s ExoMars) both utilizing solar power. Just like here on Earth, there seems to be no single best answer for power generation on Mars.
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