What does it take to keep a healthy robot alive and thriving on Mars for more than ten years? For the NASA Curiosity rover, it is a mix of excellent engineering, unrelenting evolution, and a healthy dose of autonomy that any teenager who lives on the planet called Earth would be proud of. As Curiosity sets out on its thirteenth Martian year, the rover’s engineering team has revealed a catalog of new capabilities that have milked every watt it could out of its exhausted nuclear power source, so that science, and not survival, is the mission’s priority number one.
Curiosity drives a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), a mechanism that converts heat from radioactive decay of plutonium-238 into electricity. The technology, heir to the radioisotope thermoelectric generators that had propelled Apollo to Voyager, was meant to last and be reliable. The MMRTG supplies approximately 120 watts of electric power and 2,000 watts of heat power during launch and is sufficient to power Curiosity’s sophisticated array of instruments, radios, cameras, and heaters on the extreme Martian environment. As the plutonium ages, however, the energy return is less, requiring longer to replenish the battery and making the team have to make heartbreaking decisions about how to allocate power per sol.
The issue is compounded, though, by Mars itself. The planet’s notorious dust storms, described by planetary scientist Michael Smith as “continent-sized areas [that] last for weeks at a time,” are not only a threat to solar-powered missions but also coat every exposed surface with a fine, gritty layer. “If you’ve seen pictures of Curiosity after driving, it’s just filthy,” Smith noted. The dust finds its way into rolling components and adds to wear, most significantly for the wheels of the rover and the robot arm. The MMRTG is dust-immune as solar panels are, so energy arrives in a constant trickle even when the sky rains down in blackness, but the mechanical wear remains an issue.
To offset the gradual loss of usable energy, engineers at NASA’s Jet Propulsion Laboratory have implemented high-end autonomy and multitasking capabilities. “We were sort of conservative parents earlier in the mission,” stated Reidar Larsen, head of autonomy improvements. “It’s as if our teenage rover is maturing, and we’re trusting it to take on more responsibility. As a kid, you might do one thing at a time, but as you become an adult, you learn to multitask.” Curiosity’s daily cycle used to consist of activities performed sequentially with immense buffer times and a mandatory nap to nap. Now, the rover can overlap tasks like recording to an orbiting spacecraft while moving or taking photographs substantially lowering heaters and instruments’ power-up times. This combination of activities benefits energy conservation directly, increasing the scientific return as Curiosity explores the mysterious boxwork bedrock of Mount Sharp.
Another innovation enables Curiosity to choose automatically whether it needs a nap when it completes a task earlier than anticipated. By saving merely ten or twenty minutes on an operation, it limits the number of cycles through the battery recharges, which extends the useful lifespan of the MMRTG. These tiny efficiencies, sustained over thousands of sols, pay dividends in spades in terms of how much science the rover is able to generate.
Mechanical durability is also important. Decades of rolling over boulders have worn out Curiosity’s aluminum wheels, and NASA engineers are watching closely for cracks and punctures. A traction control algorithm now governs the speed of all wheels in real time, reducing pressure on weak areas and keeping them from being damaged further. “If it’s a pointed rock, it’s more likely to penetrate the skin between the wheel grousers,” said Art Rankin, team leader of the software testing. In ground tests, this algorithm shaved load on front wheels by 20 percent and mid-wheels by 11 percent remarkable improvement for a machine that does not even receive maintenance.”
When the unexpected hardware failures do arrive, creative thinking dominates. An errant signal in the color filter wheel of one of the Mastcam cameras prompted engineers to come up with a fix so that Curiosity can continue to capture the wide panoramas that are a signature of the mission. In the same manner that the drill’s sample collection mechanism has been re-designed to make up for wear, and the algorithms to drive the rover have been optimized to drive especially dangerous terrain.
The hard-learned lessons of Curiosity’s ongoing journey already are influencing the next generation of Mars rover explorers. Perseverance, Curiosity’s little brother, is even more autonomous, featuring an onboard scheduler that will conserve energy up to 20 percent and finish science campaigns 25 percent faster. These developments will enable future next-generation missions, which will need to be directed by hardly any Earth control, traveling even farther and enduring even harsher conditions.
Curiosity’s tale is one of survival engineering success and the incessant need for information. Every new ability, from multi-tasking to protecting itself, is less a technological breakthrough it is a survival strategy, allowing the rover to keep pushing its mission to uncover Mars’ secrets, one prudently-spent sol at a time.
