Engineers at NASA and Lockheed Martin Space have taken on the difficult task of measuring the fuel supply for the oldest spacecraft orbiting Mars without the use of a fuel gauge, after calculations indicated the lander might be close to operating empty.
Estimates from 2021 and 2022 indicate that Mars Odyssey orbiter Its propellant was depleted much faster than expected, prompting an investigation into the craft’s “missing fuel”. In the end, the scientists concluded that the missing fuel wasn’t missing at all! This means that Odyssey should have enough propellant to last until the end of 2025.
The Mars Odyssey orbiter has been in space for 22 years. During this time, the mission has completed more than 94,000 orbits Mars And it made a wealth of impressive discoveries such as the discovery of subsurface water ice that could be used by future astronauts.
Related: The big reveal: What awaits us in returning samples from Mars?
During its mission, Odyssey traveled the equivalent of 1.37 billion miles (2.21 billion kilometers) around Mars, which required careful fuel management. The problem is that the spacecraft, which was launched in 2001, is not equipped with a fuel gauge.
That means NASA operators have to rely on ingenious math and testing to calculate how much of the remaining 500 pounds (225.3 kilograms) of hydrazine propellant the rover has left from Earth.
The remaining fuel supply is measured by applying heat to the Odyssey’s two fuel tanks to see how quickly they reach the set temperature, which indicates how full they are. Just like an empty teapot heats faster than a full tank, an empty tank must reach its target temperature more quickly than a full one, NASA said in a statement (Opens in a new tab).
In 2021, testing indicated that Odyssey had only 11 lb (5 kg) of propellant left, which is less than mathematical modeling of the probe’s projected fuel consumption. In January 2022, the NASA team used this method again, and again came up with a lower-than-expected amount of fuel left: just 6 pounds (2.8 kg) of hydrazine. If the results are correct, Odyssey will be running blank in less than a year.
Not knowing how to explain the discrepancy between the test results and expectations, the engineers embarked on a deeper investigation, which revealed hitherto unknown details of how the vehicle’s complex fuel system progressed in flight.
So where did Odyssey’s “missing fuel” go?
After calculations that indicated the Mars orbital Odyssey might run out of fuel, scientists from NASA’s Jet Propulsion Laboratory (JPL) drafted the aerospace engineers at Lockheed Martin, who not only built Odyssey but also maintained its mission operations and provided engineering support for the rover.
“First, we had to verify that the spacecraft was OK,” Joseph Hunt, JPL’s Odyssey project manager, said in the release. “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 joint JPL/Lockheed Martin space team decided they needed “fresh eyes” to assess the Odyssey case. These new eyes came in the form of the spacecraft’s propellant estimation consultant, Boris Endler.
Yendler related the lost fuel to the fact that, like all spacecraft, Odyssey uses some fuel to keep its systems at optimal operating temperatures and protect them from the cold of space. The consultant wondered if some source in the Odyssey was adding heat to its fuel. This may have the effect of causing temperatures to rise faster during the fuel tank heating test, thus giving inaccurate fuel measurements.
Painstaking investigation led to the discovery that heaters along the Odyssey’s fuel line heated the connected fuel tanks, making them appear more empty than they actually are. By factoring this into the measurement method and considering the more complex fluid dynamics, the team was able to determine that Odyssey had about 9 pounds (4 kg) of hydrazine remaining.
Mars Odyssey doesn’t burn a lot of fuel each day, as solar panels on the rover provide the power needed to power its systems. In addition, the probe uses reaction wheels to stay pointed at Mars during science investigations. These wheels rotate inside the body of the spacecraft, generating a torque that allows the Odyssey to maintain its position without being pushed.
Where hydrazine really comes in is when Odyssey completes a full orbit and needs to dump the increased momentum into her reaction wheels. The spacecraft does this by firing small, precisely calculated bursts through its thrusters.
With this limited use, the Odyssey has enough fuel to last at least a few more years. However, the team acknowledges that the amount of remaining fuel they calculated may change as they refine their calculations and improve measurement accuracy. However, the team is sure that they now understand the craft better than they did previously.
“It’s a bit like our process for scientific discovery,” Jared Cole, NASA JPL’s Odyssey mission manager, said in the release. “You’re exploring a system of engineering that you don’t know what you’ll find. The longer you look, the more you find what you didn’t expect.”
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