Space

ESA solves the riddle of why the Huygens probe spun the wrong way

ESA solves the riddle of why t...
Replica of Huygens undergoing wind tunnel tests
Replica of Huygens undergoing wind tunnel tests
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Mockup of the Huygens probe used in wind tunnel tests
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Mockup of the Huygens probe used in wind tunnel tests
Predicted and actual spin rates of the Huygens probe
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Predicted and actual spin rates of the Huygens probe
Replica of Huygens undergoing wind tunnel tests
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Replica of Huygens undergoing wind tunnel tests
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ESA engineers have determined why the space agency's Huygens probe suddenly began spinning the wrong way 15 years ago as it descended to the surface of Saturn's largest moon, Titan. The first spacecraft to land on a world of the outer solar system, the reversal of its spin could have implications for future space missions.

In many ways, the Huygens mission was an unqualified success. It was launched in 1997 as a ride-along of the NASA/ESA/ASI Cassini-Huygens mission with the task of descending by parachute into the methane atmosphere of Titan and, if it survived, sending back data from the moon's surface. It did this but there was one major glitch that remained a mystery.

While the unmanned Cassini probe was in orbit around Saturn, Huygens was released to coast on its own for three weeks before landing on Titan on January 14, 2005. As it approached its target, the lander spun in an anti-clockwise direction at a rate of 7.5 RPM – a standard practice for stabilizing a spacecraft by effectively turning into one big gyroscope.

Artist's impression of Huygens landing on Titan (Image: ESA)
Artist's impression of Huygens landing on Titan (Image: ESA)

The idea was that as the probe encountered the outer atmosphere of Titan, 36 angled vanes aboard would slow and control the spin. But as Huygens began its descent, the spin slowed down more than anticipated and within 10 minutes it was rotating in the opposite direction. It continued doing this for the next two hours and 15 minutes of the descent, and though this affected the timing of some observations, the spin rate was such that it didn't interfere with the mission too much.

However, the question remained. Why did Huygens reverse its spin? This was of more than academic interest because trivial mistakes can have catastrophic consequences, like when NASA's Mars Climate Orbiter burned up in the Martian atmosphere because of a mix up between Imperial and metric measurements.

According to ESA, recent subsonic wind tunnel tests at the PRISME Laboratory at the University of Orléans, France, have uncovered the culprits, these being Huygens' Separation Subsystem (SEPS) and the Radar Altimeter (RA) antennae. When these were deployed they acted like airfoils and forced the probe in the opposite direction until they overcame the control vanes and the spin flipped direction.

Predicted and actual spin rates of the Huygens probe
Predicted and actual spin rates of the Huygens probe

The space agency is continuing to study the incident, especially the effect of the Huygens Atmospheric Structure Instrument (HASI) booms, which may not have deployed fully during descent, to better understand exactly what happened and use this knowledge for improving the chances of future planetary missions.

The video below shows a replica of Huygens undergoing wind tunnel tests.

Huygens wind tunnel test

Source: ESA

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3 comments
ChairmanLMAO
That wind tunnel test is like studying the leaves on the ground to figure out which branch they came from. Put a spin on it then maybe they'll get to the cause of the mystery.
Gene Preston
Looks like the effect of the antenna was not studied in advance.
Gregg Eshelman
Sounds like they did the design and tested their part of the design. Got the result they expected, but then failed to test what would happen when other parts were stuck out into the wind.

Assumptions like that are what has caused most of the problems with various spacecraft. Cassini's main antenna was rendered useless because of two false assumptions. 1, that deploying it soon after starting the probe on its way would leave the antenna potentially vulnerable to damage 2, that the deployment mechanism would still function properly after years exposed to conditions in the vacuum of space.

The first Ariane 5 rocket was destroyed due to incorrectly assuming that a component from the Ariane 4 design could not only be left in as part of the electronics, it could be turned on and allowed to operate - despite its output not being used, yet given access to the data bus - and ignored. Since it was assumed to be safe to do that (very stupid) thing a full up simulated launch what that component connected and operating was never done. When the rocket reached its gravity turn altitude before the unused component was set to shut down, it essentially vomited garbage onto the data bus, crashing the backup guidance system, then the primary.

The Therac 25 was a costly case of assuming safeguards could be removed from a flawed system used in previous Therac models, because "The new software makes it impossible to be unsafe." The new software just made things worse, and without the mechanical safeties of circuit breakers and fuses, it made the problems *deadly*.