January 17, 2020 : Some Researchers May Have Solved The Mystery Of The Reversed Spin Of The Huygens Probe During Its Atmospheric Descent
About 15 years ago, on January 14, 2005, the Huygens probe released from the Cassini orbiter performed an atmospheric descent and landed on the surface of Saturn's largest moon Titan for the first time in history. The Huygens probe which was proposed by ESA and ASI had separated from the Cassini spacecraft proposed by NASA at the end of December 2004. During the atmospheric plunge of January 14, 2005, the Huygens probe started to spin the wrong way and engineers and researchers had not anticipated that configuration. That mystery may have been solved thanks to recent tests or simulations. In fact, during the atmospheric journey, some instruments of the probe may have generated an unexpected torque opposite to that engendered by the 36 angled vanes of the Huygens probe. The vanes played a key role since they were used to control the rotation of the probe during its descent inside the deep and thick atmosphere of the giant moon. The atmosphere of Titan like the atmosphere of Venus or like the atmosphere of Mars makes any parachute useful for any probe that dives to the surface.
Researchers have been in a position to determine that two devices of the Huygens probe, the Separation Subsystem (SEPS) and the Radar Altimeter (RA) antennae, actually engendered an unexpected torque opposite to that generated by the multiple vanes. That phenomenon was strengthened due to the alteration of the gas flow around the atmospheric probe by the vanes which led to a rise in the amplitude of the « negative torque ». Let's point out that the negative torque corresponds to the effect that made the Huygens module flip its direction of rotation. The unexpected effect turned out to exceed the influence of the vanes. It is crucial for engineers to clearly understand the mechanisms that led to the unexpected configuration in the prospect of the development of future probes sent to extraterrestrial atmospheres like the atmosphere of Venus, Mars, Titan or the four Gas Giants. The behavior of any atmospheric probe can engender scientific failures. That's why engineers and scientists must be in a position to anticipate any scenario during the complex phase of the atmospheric descent.
The touchdown of the Huygens probe at a low latitude in the area of Adiri and Shangri-La on January 14, 2005 is the outcome of a particularly long journey starting in 1997 with the launch of the Cassini-Huygens spacecraft. The Cassini-Huygens spacecraft had reached the Saturn System in mid-2004. That was the first time that a spacecraft entered into orbit around the Ringed Planet Saturn. At the start of the mission in 2004, we did not expect the Cassini-Huygens mission to last so long. The Huygens probe had gathered and sent back to the orbiter a huge amount of data regarding the chemistry and the physical properties of Titan's atmosphere or Titan's surface during a limited amount of time but the Cassini spacecraft continued its mission until its planned crash into Saturn on September 15, 2017. Thus, the Cassini mission has collected data upon Saturn and its numerous moons for more than 13 years. The in-depth study of Saturn and Titan was the major goal of the mission at the beginning but other worlds orbiting Saturn have captured our attention and in particular Enceladus where geysers can be found in the fractures of its south polar region.
The perilous atmospheric descent of the Huygens probe on January 14, 2005 lasted for 2 hours and 27 minutes. The eye of the probe acquired impressive aerial views of the atmosphere and the landscape during the delicate plunge. The probe captured the first in situ measurements of the atmosphere of the Opaque Moon, determining the pressure profile, the density profile and the temperature profile in particular, from an altitude of 1400 km down to the soil. The atmospheric pressure on Titan's surface was recorded at 1467 Millibars and the environmental temperature at the level of the surface was recorded at about minus 179 degrees Celsius, minus 290 degrees Fahrenheit or 94 Kelvin. The Doppler Wind Experiment (DWE) of the module identified strong east-west winds in the atmosphere of the Hazy Moon and some of the winds rotated faster than the celestial body itself. We also gathered data regarding the chemical composition of the atmosphere which is dominated by molecular nitrogen and which contains a relatively significant concentration of methane. The haze of Titan's atmosphere appears to be composed of tiny aerosols and is rich in organics or hydrocarbons.
The aerial views of Titan's landscape during the atmospheric plunge had revealed remarkable landscape features with two types of terrain, bright hills unveiling a network of dark sinuous channels and a dark or red plain which may represent an ancient sea. The numerous images obtained from the Huygens probe during the atmospheric descent and from the surface have fuelled our imagination and our hypotheses or theories. The dark channels found in the bright hills are apparently drainage channels related to meteorological phenomena. However, they may also be related to cryovolcanism or tectonic activity on Saturn's largest moon. There may be fractures related to the instability of the crust within the bright area. Like the tiny moon Enceladus, Titan may contain a subsurface ocean. That ocean hidden beneath the external crust may be rich in liquid water or even liquid methane. Many icy moons or icy worlds of the Outer Solar System may contain a subsurface ocean dominated by liquid water. That's the case for Europa, Enceladus, Titan, Triton, Pluto or Charon. Cryovolcanism is likely to tell us a lot regarding the internal structure of the world.
The landing site of the Huygens probe had revealed eroded stones or pebbles implying that the module may have landed on an ancient brook or river. Those exotic pebbles photographed by the lander may be rich in water ice. The Huygens probe had also recorded a relatively significant release of methane during the landing phase. One can assume that there are seasonal rainfall events in the area. The rain on Titan may be composed of methane which can appear in its liquid form in the harsh environment of the giant moon. Ethane and propane can also appear in their liquid form in that type of environment where there are the right combinations of atmospheric pressure and temperature. The radar data as well as the infrared or near-infrared data obtained from the Cassini orbiter during its long mission in the Saturn System have clearly shown that the dark areas of the low or mid-latitudes are dominated by Seif dunes or linear and parallel dunes whose shape is influenced by prevailing winds. Erosion exerted by winds and liquid methane or liquid ethane may have played a key role in the development of the dunes observed at low or mid-latitudes.
Engineers and researchers had studied the potential behavior of the probe during its atmospheric descent in order to design the devices in the right way or to produce the right settings of the instruments or softwares. In reality, the behavior of the probe in the Titanian atmosphere has surprised engineers and researchers. The rotation of the module turned out to be in the wrong direction or in the direction that had not been expected. The Huygens probe had separated from the Cassini orbiter so that it rotated anti-clockwise at a rate of 7.5 spins per minute. Thanks to the design of the module, the spin rate allowed the probe to remain stable in the first step during the three weeks of the journey in outer space toward Titan. Obviously, the penetration of the probe into Titan's atmosphere was likely to reduce the stability of the probe. Engineers had anticipated the behavior of the probe in the first step. However, during the atmospheric descent, the rotation rate of the probe decreased much more rapidly than anticipated, before reversing after about 10 minutes to take a clockwise direction.
The Huygens module continued to rotate this way for the remaining 2 hours and 15 minutes of atmospheric journey. Fortunately, the magnitude of the reversed rotation was at the same level as that anticipated by the engineers or scientists, implying that the unexpected flip triggered some bad effects on the timing of the planned observations. However, the unexpected flip did not significantly affect the quality of the data collected from the Huygens probe. The new conclusion on the potential cause of the unexpected behavior of the probe is based on recent subsonic wind tunnel testing at the PRISME Laboratory at the University of Orléans located in France. The research work was performed from 2017 to 2019 under a contract between the European Space Agency and LPC2E/CNRS-University of Orléans. In the past, other studies had investigated that unexpected behavior. One can mention for instance a research work led by Vorticity in 2014-2015. Moreover, during the atmospheric descent, there were also signs that the Huygens Atmospheric Structure Instrument (HASI) booms might have not been completely deployed during the descent process in the atmosphere. That's why particular tests were carried out in three different configurations (stowed, deployed and half-deployed). They concluded that a "negative torque" can take shape under a non-symmetric deployment. New studies on that effect are developing. The preliminary hypotheses or assumptions of the new research work on the unexpected reversed rotation of the probe were presented at the International Planetary Probe Workshop conferences in 2018 and 2019.
The image above represents a mosaic of aerial views of Titan's landscape obtained from the DISR of the Huygens probe during its atmospheric plunge on January 14, 2005. The views were acquired at an altitude of approximately 8 km and the resolution of the landscape features is about 20 meters per pixel. One can clearly notice a sharp contrast between bright hills composed of a network of dark channels and a dark plain which seems quite uniform. The Cassini-Huygens mission represented the outcome of a collaboration between ESA, NASA and ASI. Image credit: ESA/NASA/JPL/University of Arizona.
- To get further information on that news, go to: https://www.esa.int/Science_Exploration/Space_Science/Cassini-Huygens/Huygens_landing_spin_mystery_solved.