Unlike NASA, which has considerably greater financial resources, theEuropean Space AgencyEuropean Space Agency (ESA) must demonstrate pragmatism in the choice of its missions to explore and observe the Universe. However, throughout its history, its choices have proven successful in many areas. Space exploration bears witness to these successes, notably with the incredible adventure of Rosetta and its Philae lander, after that of GiottoGiotto and its flyby of Halley's comet and while waiting for the ExoMars rover, but also with the technological and scientific feat achieved in January 2015: the landing of the Huygens probe on Titan, the largest moon of SaturnSaturn then very poorly known.
This landmark event in the field of exploration roboticsrobotics World Day is celebrated regularly, and today we commemorate the 20e anniversary of this mission. On January 14, 2005, after a journey of more than 3 billion kilometers that began on October 15, 1997, Huygens landed on the surface of Titan.
Did you know?
Titan, Saturn's largest moon, is shrouded in a thick layer of haze that not only reflects sunlight, but also makes it impossible to observe its surface in visible wavelengths.
A feat as much technological as scientific
Huygens' feat goes beyond technology. Although the probe, built by Thales Alenia Space, was designed to withstand the extreme conditions of TitanTitanwith temperatures around -200°C, no one on Earth could predict, the day before landing, whether the mission would be a success. However, everything went perfectly: from waking up from hibernation to separationseparation with Cassini, its descent through theatmosphereatmosphere of Titan and its landing on terrain with unknown characteristics and nature of its soil (lake, rock, ice, etc.), the probe landed gently and operated for more than three hours after its arrival, collecting and transmitting previously unpublished data.
The atmospheric conditions of early Earth resembled those of modern-day Titan!
Remarkably, mission managers thought Huygens would only be able to operate for about 30 minutes on the ground. Yet it survived its descent and landing so well that it continued to transmit information for more than three hours. Unfortunately, not all the data could be transmitted to the Cassini probe which, from a distance of 60,000 kilometers, relayed them to Earth. Contact with the probe was lost when Cassini passed below the horizon. Damage.
Huygens' feat is also of great scientific value. Despite the loss of data acquired from the ground, those collected throughout the Cassini-Huygens mission revealed that the surface of Titan is a world shaped by cryovolcanism as well as by precipitationprecipitation methane and other hydrocarbonshydrocarbons. The measurements of conductivityconductivity atmospheric carried out by Huygens, combined with the information transmitted by Cassini, highlighted lightlight a rich landscape: lakes and seas of methane and ethane, enormous dunes of sablesablerivers, slabs of ice, as well as an ocean of ammonia water under a crustcrust of ice, all bathed in an atmosphere rich in argonargon and propylene.
Descent of the Huygens probe on Titan.
The floor is given to Patrice Couzin (Thales Alenia Space), head of avionics from the Huygens call for tenders in 1990 until the firing in 1997, then system manager from 1997 to 2005.
Patrice Couzin: Only one atmospheric model was specified – the Lellouch-Hunten model – derived from observations of the Voyager-1 probe during its hoverhover of Titan in 1980, and it involved surviving only 3 minutes on the ground after 150 minutes of atmospheric descent.
The European Space Agency recognized this “lightness” of the specifications with the contract signed with Aerospatiale (now Thales Alenia Space). The scientists then worked throughout the program to enrich the specifications of Titan's environment as seen by Huygens, in mattermatter icing, lightninglightningtriboelectricity, bursts of ventventsurface reflectivity…
“A note first of all, Huygens was a “Descent Probe”, not a “lander””
Futura: What safety margins were put in place to ensure the success of Huygens' landing on Titan?
Patrice Couzin: A remark first of all, Huygens was a “ Descent Probe “, not a ” landerlander ».
The unknown of what the surface might be like, other than a temperature of -180°C, made it simply impossible to specify a “landing.” Of course, all the mechano-thermal elements and the parachutesparachutes were designed for even more extreme cases, in terms of temperature, aerothermodynamic flow, pressionpression dynamic… The most critical events of the mission – awakening of the probe after 22 days of hibernation, detection of the moment of deployment of the parachute – were based on triple redundancies and included a back-up. In addition, all commands were sent by two computers operating in parallel (active hot redundancy). An adaptation to the environment encountered was carried out throughaccelerometersaccelerometersmeasurements of the rotation of the probe and the altitude relative to the surface.
Futura: Would Huygens have been able to float if he had landed on a hydrocarbon lake or any other liquid surface on Titan?
Patrice Couzin: Yes ! A study during development showed that it was possible for a while, even though the probe was never designed to!
Futura: What technologies were specifically developed for the Huygens probe and subsequently reused in other space missions?
Patrice Couzin: These are essentially technologies related to thermal protection during atmospheric reentry, and parachutes.
Concerning the thermal shield, the experience gained on Huygens has made it possible to adapt another thermal protection for exploration activities. She flew on Beagle-2, and on Schiaparelli of ExoMars 2016 (on this mission, we know that she did her job well!).
As for the Huygens parachute, it was also used on Beagle-2, with a similar parachute design and manufactured by the same company. ExoMars also has the same design. The tools digitaldigital predictions have evolved, but they all come from Huygens' developments. The experience also made it possible to define the choices of necessary materials and the aging tests for a trip into space, particularly for nylonsnylons et KevlarKevlar.
As for the Huygens structure, the sizing criteria were reused to ExoMarsExoMars.
“This international collaboration enabled the most grandiose interplanetary mission today”
Futura: What are the main legacies and contributions of the Huygens mission to space exploration
Patrice Couzin: Huygens' legacy is a new and spectacular demonstration of the extension of the limits of space exploration. It is above all a reference, a model in terms of high-level collaboration between the major space agencies: ESA, NASA and also the ASI on Science. This collaboration enabled the most grandiose interplanetary mission today. ESA and also NASA have been “dreaming” of repeating this collaboration for 20 years!
Can life different from ours exist on Titan? NASA's Dragonfly mission will tell us!
Futura: Are there plans for a Huygens 2 mission? What could it look like and what objectives could it pursue?
Patrice Couzin: A Huygens 2 should of course “land”, but also be able to “traverse” Titan, because Cassini-Huygens has shown the great diversity of the surface made up of dunes, craters, hydrocarbon lakes, plains, reliefs, river networks , in order to resolve the many outstanding questions concerning this surface: What is the composition of the surface elements? What chemistrychemistry prebioticprebiotic takes place there? What is the composition of the lakes? How old is the surface?
This vehicle, to survive on Titan, should be equipped with RTG and be able to transmit its data, either directly or through a orbiterorbiter around Titan.
Futura: Could NASA’s Dragonfly be this “Huygens 2”?
Patrice Couzin: Indeed, to a certain extent, we can say yes. The Dragonfly mission, which is currently in development, meets many of the criteria that could be attributed to a “Huygens 2”, as I described previously. That said, it is also relevant to consider the possibilities that a “Huygens 3” mission could offer!
