NASA reveals how it measures fuel supply on decades-old Odyssey spacecraft without a gauge

NASA's Mars Odyssey orbiter has been exploring the Red Planet for more than two decades since its launch in 2001. According to the agency, the spacecraft has looped around Mars more than 94,000 times, about the equivalent of 1.37 billion miles, a distance that has required extremely careful management of its fuel supply.

Because there's no fuel gauge, it's not an easy task for the engineers to measure the fuel supply on the decades-old spacecraft. Then, how do they keep an eye on Odyssey's hydrazine fuel supply?

According to NASA, one way to do so is to apply heat to the spacecraft's two propellant tanks and observe the time they take to reach a certain temperature. A fuel estimate performed by the mission team in 2021 indicated that about 11 pounds (5 kilograms) of propellant remained available - that's less than the mission's modeling had predicted. Another calculation in January 2022 indicated only 6 pounds (2.8 kilograms) of hydrazine remained.

If the calculations were correct, Odyssey's fuel would have been depleted in under a year. The team also ruled out the possibility of a leak or that they were burning more fuel than estimated.

After months of testing and rigorous investigation, mission engineers delved into the mystery of the "missing" fuel, uncovering new insights into the behaviour of the aging spacecraft's complex fuel system during flight. Their findings indicated that the orbiter's fuel supply, which was previously thought to be insufficient, is actually enough to keep the spacecraft active for a few more years.

The team concluded that Odyssey has about 9 pounds (4 kilograms) of hydrazine left, which is enough to last the mission until the end of 2025.

"Our method of measurement was fine. The problem was that the fluid dynamics occurring on board Odyssey are more complicated than we thought. You explore an engineering system not knowing what you'll find. And the longer you look, the more you find that you didn't expect," Odyssey's mission manager, Jared Call of NASA's Jet Propulsion Laboratory, said in a statement.