NASA investigates veteran Mars orbiter to solve a missing fuel mystery

Engineers at NASA and Lockheed Martin Space have performed the difficult task of measuring the fuel supply of the oldest Mars orbiting spacecraft without the use of a fuel gauge, after calculations indicated the probe may be close to running on empty.

Estimates from 2021 and 2022 indicated the Mars Odyssey orbiter has exhausted its propellant much faster than expected, prompting the investigation of the craft's "missing fuel." Eventually, the scientists concluded that the missing fuel was never missing at all! That means Odyssey should have enough propellant to keep going until the end of 2025.

The Mars Odyssey orbiter has been in space for 22 years. During this time, the mission has completed over 94,000 orbits of Mars and delivered a wealth of impressive discoveries such as the detection of water ice under the planet's surface that could be used by future astronauts.

Related: The big reveal: What's ahead in returning samples from Mars?

During its mission, Odyssey has traveled the equivalent of 1.37 billion miles (2.21 billion kilometers) around Mars, which has required careful fuel management. The problem is that the spacecraft, launched in 2001, isn't fitted with a fuel gauge.

That means NASA operators have to rely on mathematics and ingenious tests to calculate how much of the 500 pounds (225.3 kilograms) of hydrazine propellant the craft lifted off from Earth with is left.

The remaining fuel supplies are measured by applying heat to Odyssey's two propellant tanks to see how quickly they reach a set temperature, which indicates how full of fuel they are. Just like an empty teapot which heats up faster than a full one, an empty fuel tank should hit a target temperature more rapidly than a full one, NASA said in a statement.

In 2021, the test indicated that Odyssey had just 11 pounds (5 kg) of propellant left, which is less than mathematical modeling of the probe's fuel consumption predicted. In January 2022, the NASA team used the method again, and again came up with a lower than expected amount of remaining fuel: just 6 pounds (2.8 kg) of hydrazine. If the results are correct, Odyssey would be running on empty in less than a year.

The engineers didn't know how to explain the discrepancy between the test results and the expectations and set out to conduct a deeper investigation, which revealed hitherto unknown details about how the craft's complex fuel system has aged while in flight.

So where did Odyssey's "missing fuel" go?

Following the calculations that indicated the Mars Odyssey Orbiter may be running low on fuel, NASA Jet Propulsion Laboratory (JPL) scientists drafted in Lockheed Martin Space engineers, who not only built Odyssey, but also maintain its mission operations and provide engineering support for the craft.