# Engineers monitor the fuel supply for NASA’s oldest spacecraft around Mars

Measuring the fuel supply of the Odyssey, a decades-old spacecraft without a fuel gauge, is no easy task.

Since NASA launched the Mars Odyssey Orbiter in 2001 to the Red Planet nearly 22 years ago, the spacecraft has orbited Mars more than 94,000 times. That’s the equivalent of 1.37 billion miles (2.21 billion km), a distance that required very careful management of the spacecraft. fuel supply. This feat is even more impressive considering the Odyssey has no fuel gauge. Engineers had to rely on mathematics instead.

Their work has helped Odyssey build a scientific legacy: spacecraft have mapped minerals across the surface of Mars, allowing scientists to better understand the planet’s history. Odyssey has found ice deposits that could be used by astronauts in the future. Radiation, which can harm the breath of astronauts, has been studied. And scout potential landing sites for upcoming missions. Odyssey is also among a small group of orbiters that relay data to Earth from NASA’s rovers and landers (approximately 150 GB so far, and counting).

But last year, Odyssey seemed to be running out of gas: Calculations indicated that there was much less hydrazine fuel than expected.

The Odyssey was launched in 2001 with approximately 500 pounds (225.3 kg) of hydrazine fuel. Since there was no fuel gauge, engineers used a variety of methods to deduce how much hydrazine the spacecraft had consumed over time. One way to measure Odyssey fuel is the application heat to the spacecraft’s fuel tanks and monitor how long it takes them to reach a certain temperature. As with a teapot, an almost empty fuel tank heats up faster than a full tank.

And that, in fact, is what appeared to have happened with a fuel estimate made for the Odyssey in the summer of 2021. The math seemed to show that about 11 pounds (5 kilograms) of propellant was still available—less than the modeling predicted. Another estimate in January 2022 indicated that 6 pounds (2.8 kilograms) of hydrazine remained.

If the numbers are accurate, Odyssey will run blank in less than a year. Either the spacecraft had some kind of failure, like a leak, or something was off in the team’s measurements.

Months of extensive testing and investigation followed. After studying the mystery of the “missing” fuel, mission engineers have learned new things about how the spacecraft’s complex fuel system behaves in flight. Their conclusion: The spacecraft should have enough to survive at least until the end of 2025.

## How to use hydrazine odyssey

Odyssey doesn’t need much hydrazine any day. Solar panels power its systems, while three strategically placed reaction wheels help the orbiter steer its science instruments on the surface of Mars. When the reaction wheels inside a bus or spacecraft body rotate, they create a torque that causes Odyssey to move in the opposite direction.

“These reaction wheels have to work together to keep the spacecraft oriented,” said Odyssey mission manager Jared Cole of NASA’s Jet Propulsion Laboratory in Southern California. “But with Odyssey completing a full lap in each orbit, you need a way to offload the increased momentum.”

This is where hydrazine comes into play in Odyssey. The spacecraft’s thrusters release this fuel in small bursts calculated to counteract the momentum of the reaction wheels building up.

## work as one team

So when the team’s calculations showed fuel supplies were lower than expected, engineers at JPL worked with those at Lockheed Martin Space, which built the Odyssey, maintains mission operations, and provides engineering support for the spacecraft.

“First, we had to verify that the spacecraft was okay,” said Joseph Hunt, Odyssey project manager at JPL. “After ruling out the possibility of a leak or that we were burning more fuel than expected, we began looking at our metering process.”

The team agreed that they needed some fresh eyes to assess the situation. They brought in Boris Endler, an outside consultant who also specializes in spacecraft fuel estimation.

Like all spacecraft, Odyssey relies on heaters to keep various parts, including fuel tanks, running in the coolness of space. Endler wondered if heat was being added to the propellant from another source on the spacecraft, complicating the measurement of the fuel. After much experimentation, the team confirmed that this was the case: Geysers along the fuel line connecting the tanks were heating them up faster than expected, making the tanks appear to be nearly empty.

“Our method of measurement was good. The problem was that fluid dynamics Speaking aboard Odyssey is more complicated than we thought,” Cole said.

After figuring out how much heat didn’t account for their calculations, the team concluded that Odyssey had about 9 pounds (4 kilograms) of heat. hydrazine; Leave. It is enough for the mission to last a few more years. Although the number could change as the team works on refining the measurements and improving their accuracy, the team rests easy now that they have a better understanding of their spacecraft.

“It’s a bit like our process of scientific discovery,” Cole said. “You’re exploring a system of engineering and you don’t know what you’ll find. And the longer you look, the more you find than you expected.”