Todd J. Barber, Cassini lead propulsion engineer
Happy 2012 from Cassini and the ringed planet! Our intrepid explorer continues its quest to understand Saturn, its rings, icy moons, large moon Titan and magnetosphere. Despite what you might conclude from the paucity of these columns, I continue working on Cassini as the lead propulsion engineer—although a certain Mars rover en route to the red planet has also been vying for my attention (and will continue to do so until landing in August). When we last chatted in November, I had the honor and privilege of celebrating the life and career of Jean-Pierre Lebreton, the Huygens project scientist. I interviewed him upon the occasion of his retirement from the European Space Agency (ESA) last September, and despite a lengthy column a few months ago, we were only able to cover his career and contributions up to and including Cassini-Huygens’ launch to Saturn in 1997. In this second of two columns, I will talk about Dr. Lebreton’s post-launch contributions to the mission through his well-earned retirement last year.
During Cassini’s lengthy cruise to Saturn, Jean-Pierre’s primary role was preparing for the probe mission in late 2004 and assessing probe engineering checkouts, which occurred twice a year. One test not planned before launch was championed by Dr. Lebreton—and it ended up being of the utmost importance for the success of the Huygens mission. Perhaps on a whim (or more likely from a decades-long career and prescient intuition), Jean-Pierre suggested a test to calibrate and validate the data path from Huygens to Cassini. Naturally, with Huygens mated to Cassini, this was only possible by thinking outside the box. After two years of building, designing, testing, and convincing, a Deep Space Network (DSN) station on Earth simulated a Huygens signal and transmitted it to the Cassini orbiter for interpretation. This first test in February of 2000 (made possible because the spacecraft was finally leaving the inner solar system and thus could point its high gain antenna directly towards Earth instead of the sun) was not thought by some to be of great use, but it identified that the Doppler shift in the probe’s signal during its descent was not properly accounted for, which would have caused most of the probe’s data to be lost. It took another year to understand this “subtle and nasty” problem, including modeling efforts, working with Jerry Jones and the Cassini Navigation team at JPL, and testing a Huygens radio model in the European Space Operations Center (ESOC), Darmstadt, Germany. Dr. Lebreton had nothing but praise for all the folks at ESA and NASA who found a way to work around this issue and to restore the probe link performance.
Dr. Lebreton came to JPL in June of 2004 for Saturn Orbit Insertion (SOI), a necessary step for the success of the Huygens mission. He vividly remembers watching the Doppler plot and the “great moment” when the actual Doppler data fell into place, one point at a time, on top of the predicted curve. He also opined it was a “bold step” to switch to the low gain antenna during SOI to obtain data during the burn, but that was definitely the right thing to do. The final probe checkout was in September of 2004; he said one probe principal investigator could not believe we would not be able to talk to his instrument anymore. As Jean-Pierre so eloquently replied, at a certain stage it is time to listen, not talk. He said he remembers feeling very emotional at the moment—decades of work were finally coming to fruition!
On Christmas Eve, 2004, Dr. Lebreton again found himself at JPL, this time for an engineering event — Huygens probe release! We enjoyed a catered holiday dinner, perhaps to thumb our noses at Isaac Newton’s orbital mechanics dictate of a Christmas Eve probe release. After probe release was successful, Jean-Pierre enjoyed a few days on holiday before returning to Europe. During the next three weeks, Huygens was in free fall to Titan, radio silent by design to preserve battery life for the Titan mission. There was “nothing he could do” during this time, but he said he also felt confident they did they best they could to ensure mission success.
The moment of truth came on January 14, 2005, and Dr. Lebreton remembers seeing “lots of journalists” at ESOC, ESA’s Operations center in Darmstadt. One of the most interesting anecdotes he relayed was regarding a Huygens party—he and his colleagues decided to have a big party the night before the Huygens mission commenced! His rationale seems sound to me—he said if the mission failed, they would be too sad to have a party, but if the mission succeeded, they would be too busy to celebrate! Thankfully, success was mere hours away at that point. There were actually two key moments during the brief probe mission; two large radio telescopes on Earth (Green Bank in West Virginia in the U.S., and Parkes in Australia) trained their sensitive “ears” directly towards Titan to see if they could pick up the faint whisper of a signal directly from Huygens, without relying on the relay through the Cassini orbiter at all! Jean-Pierre had the great pleasure of announcing to the world the Huygens signal was picked up by Green Bank, which was followed by cries of elation. He noted “things got weird” in the Doppler signal around 80 kilometers in altitude—this turned out to be buffeting from the winds of Titan! How exciting! After a break of 15-20 minutes, Parkes saw a direct radio signal from Huygens, too. This was announced by ESA colleagues, interrupting a press conference with the good news. The radio antenna at Parkes even detected the Huygens soft landing, 2 1/2 hours after atmospheric entry, when the Doppler data flattened out but kept being received! There was no design requirement to survive landing on the surface, but Jean-Pierre and his team were hopeful it might happen. Chalk up one to excellent European engineering!
The second part of the Huygens data return, Cassini’s playback of relayed probe science and engineering data, had a bit more of a rocky start. There was a few minutes delay in receiving the data, accompanied by “real anxiety.” Once data were received, there was a “real explosion of happiness” and it took only a few hours to complete the initial analysis of the data. Shortly after, the first three descent and surface images were released, including a descent image that resembled the coastline of the French Riviera (complete with a network of dried river channels) and a sublime surface image with smooth rounded rocks. One Huygens principal investigator said that the image taken after the probe was sitting on the surface was worth the entire trip to Saturn! Dr. Lebreton recalled that all teams worked well into the night to prepare for a press conference, with emphasis on coordinating science results among the instruments. He said it was a particular challenge given the intermixed data set. One investigator likened the impact data from landing like touching down on “crème brulee”! Interestingly enough, the probability of landing in liquid methane or ethane was thought to be very high (near 100 percent) early at the beginning of the development of the Cassini-Huygens mission in the early 90’s, but this value decreased monotonically with time. I think Vegas odds makers actually placed the chances of landing in liquid at 30 to 40 percent shortly before landing!
Dr. Lebreton kept working on Cassini/Huygens until he retired from his Huygens Project Scientist job a few months before he left ESA. After leaving ESA he decided to move back into basic research (including on Huygens data) and became associated with Centre National de la Recherche Scientifique [National Center for Scientific Research] (CNRS)/University of Orleans and the Observatory of Paris. I was also astounded to learn of his work on the DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions), an Earth-orbiting ionospheric satellite launched just a few days before SOI! He provided the Langmuir probe for this French Centre National d’Etudes Spatiales (CNES) mission, and among its duties were to detect ionospheric effects induced by human activities (e.g., power lines) and natural phenomena (earthquakes, volcanoes, etc.). Despite this amazing part-time work, I’m quite confident Cassini-Huygens will always be his “baby.” He called the amount of Cassini-Huygens data and its diversity “incredible” and referred to the mission as a “great voyage.” Jean-Pierre, as a current member of the flight team, I can only say “thank you”—our great voyage would have never been possible without decades of your very heart and soul. May you enjoy your well-earned retirement.
Todd J. Barber, Cassini lead propulsion engineer