Huygens Probe Delivers Data From Saturnian Moon

The call from Titan is clear, technical problems were minor

18 January 2005—European and American scientists last Saturday morning unveiled the very first scientific results sent back from the Huygens Titan probe following its triumphant landing on Saturn’s largest moon. Though bleary-eyed from their all-night vigil, the scientists gave a presentation that delighted a crowd of journalists and staff assembled at the European Space Agency’s Space Operations Centre in Darmstadt, Germany.

A joint project of NASA, the European Space Agency (ESA), and the Italian Space Agency, the Cassini-Huygens orbiter-lander mission was conceived 25 years ago to explore Saturn and its moons. Piggybacked on the Cassini orbiter, the Huygens lander, named for the 17th-century astronomer who discovered Saturn’s rings and Titan, was the European part of the mission. On Friday, the 3-meter-wide flying-saucer-shaped probe plunged through Titan’s thousand kilometer thick atmosphere, braving extremes of heat and turbulence before coming to rest intact on the moon’s surface.

Images of Titan had previously been taken by Cassini earlier in its mission and before it by the Voyager probes in the early 1980s. But the pictures frustrated scientists because of how hard it is to discern the surface through Titan’s thick, hazy atmosphere. In contrast, the data sent back from Huygens reveal with remarkable clarity features that evoke things like riverbeds, rocks, and ground fog.

“We are the first visitors of Titan, and the scientific data that we are collecting now shall unveil the secrets of this new world,” exulted ESA chief, Jean-Jacques Dordain.

“It’s almost impossible to resist the speculation that this flat, dark material is some kind of drainage channel,” said Marty Tomasko, team leader for the Huygens imaging component, about one image, “that we’re seeing some kind of shoreline, that we don’t know whether this still has liquid in it, or whether the liquid has drained away or drained into the surface.” Indeed, experiments by the surface science team indicate that the Huygens’s landing site may have the consistency of “wet sand or clay” topped by a thin crust, leading one scientist to draw an analogy to créme brûlée.

Scientists will spend years poring over the trove of data returned during the few hours of transmission from Huygens as it descended through the atmosphere and landed on the surface, mirroring the years it took to get Huygens to Titan.

After about a quarter-century of planning, the lumbering Cassini finally headed into space in 1997 with Huygens strapped to its back. It was heralded as the last of the giant-sized, big-ticket unmanned space missions.

But the seven-year Cassini voyage was not without incident, and the Huygens mission was almost stillborn. In 2000, ESA engineer Boris Smeds discovered a flaw in the hardware onboard Cassini used to receive Huygens’s once-in-a-lifetime transmissions before relaying them back to Earth (see “Titan Calling,” IEEE Spectrum, October 2004). The flaw meant that as the smoggy atmosphere of Titan braked Huygens, the deceleration would cause its transmission frequency to shift due to the Doppler effect, and as built, Cassini’s receiver couldn’t accommodate the shift.

Smeds’s findings started a massive chain reaction in NASA, ESA, and the mission’s contractors to find a fix. The solution involved altering Cassini’s trajectory, so that the orbiter would travel almost parallel to Huygens’s line of descent, reducing Doppler shift to manageable levels.

So even though he was not an official member of the Huygens landing team, it was no surprise to find that Smeds was sitting in the Huygens control room waiting to see if the fix worked and for the data to flow onto their screens.

The control rooms at ESA’s operations center in Darmstadt were off-limits to the scores of journalists who descended on this normally quiet city to cover the event. But at about 5:30 in the afternoon, staff could be seen on ESA’s closed-circuit TV screens breaking into applause, and the happy news was announced formally minutes later. “There will only be one first successful landing on Titan. And this was it,” said a tearful Alphonso Diaz, NASA’s associate administrator for science.

Standing in a hallway afterwards with his wife, Smeds said simply, “Well, I’m very glad… I really wanted the scientists to get their data.”

The one glitch was the failure of one of two radio channels (called A and B) to receive the experimental data. The purpose of having two channels was to provide redundancy by sending the same data separately. And indeed, because channel B operated flawlessly—“not a single data dropout on channel B,” reported John Zarnecki, head of the mission’s surface science package—most of the mission’s six experiments are unaffected. But for technical reasons, the Doppler wind experiment (DWE), designed to measure the speed of Titan’s winds, relied completely on channel A.

The DWE measures the frequency and signal level between Huygens and Cassini itself. But, explained Robin Dutta-Roy, project manager for the DWE, in an e-mail, the instrument needs a very stable frequency. That means equipping both the transmitter and receiver with ultrastable oscillators (USOs). But because experiments were not permitted to provide redundant equipment (as opposed to providing redundant data), only one channel could be equipped with USOs. “The sequence to be executed by Cassini during the Huygens mission, believe it or not, did not contain the command to switch on the receiver [with the] USO,” said Dutta-Roy, preventing the receiver from locking on the Huygens signal from channel A.

David Southwood, ESA’s science director, announced Saturday that he would be conducting an inquiry into the matter. “The problem went wrong with the way we put the command in,” he told journalists later, declining to speculate how that might have happened. He cautioned strongly against implicating any agency other than ESA. “It’s an ESA issue,” he said, “the command was totally under our control.”

Fortunately, a worldwide network of 18 Earth-based radiotelescopes dotted around the globe was able to track Huygens on its voyage, including taking measurements of the winds in Titan’s atmosphere, with the aim of complementing the DWE. Dutta-Roy said he is confident that all the experiment’s goals can be achieved using those measurements, though it will entail making “a few changes to our software” and will take weeks rather than days.

The failure of channel A also affected the number of images scientists will have to work with. Waiting before the computers in the main control room Friday afternoon, Tomasko could see that the receiver on one channel was off, and at that point there was no indication whatsoever about the state of the other one. Several minutes passed after the data were supposed to arrive during which confidence gave way to dread and then to elation as “the screens lit up and all of a sudden we were getting data on one of the channels,” he told IEEE Spectrum.

Because the number of images that could be transmitted was limited by the bandwidth of Huygens transmitters, early on the team made a calculated decision not to duplicate a nominal set of 350 images but instead to transmit different ones on the different channels. Had both receivers worked, the haul would have been twice as many images. As it is, the scientists have ended up with 350. Tomasko is happy with that amount, though it will mean “holes in the panoramic images.” He believes there is enough overlap to fill those in.

Tomasko also had some kind words for Smeds and his rigorous testing. “If he hadn’t done it,” said Tomasko, “the probe would likely have gone in, [and] we wouldn’t have heard a peep from it on either channel.” Smeds, who insists that his role is to help the scientists, kept a low profile throughout the joyful pandemonium surrounding the events in Darmstadt. But Friday night at an official dinner he could be seen on ESA’s closed-circuit TV accepting a bottle of champagne from colleagues.

Commenting on the success of Huygens, Smeds reflected that it was the urging of a co-worker to ask for two days of testing instead of one, that enabled him to have enough maneuvering room to find the cause of the trouble in 2000. “Knowing we had five years to fix it,” he said, “I was confident that with all the scientists and the experts around me, we would be able to get this going. And today, I see the result. And also, I’m glad to see that we had redundancy. I got two days, and I could use one. And today we had two radio channels, and radio channel [B] was the single channel working.”

Cassini will now complete its orbiter mission around Saturn, including 44 encounters of Titan. Although to everyone’s amazement Huygens was still transmitting when Cassini moved too far below the horizon to track it anymore, it is now dead. But the scientific work on Huygens is just beginning, and the project is a watershed in several respects, not least in that it puts Europe on par with the United States in robotic space projects, and it serves as a model of international collaboration. The Huygens mission is also a shining example of pure exploration. “It’s all about making contact,” Carolyn Porco, head of imaging for Cassini, told IEEE Spectrum. “It is still a kind of a baby step, but it’s a big one.”

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