Titan, Saturn's largest moon, is the only Solar System planetary body other than Earth with a thick nitrogen atmosphere. The Voyager spacecraft confirmed that methane was the second-most abundant atmospheric constituent in Titan's atmosphere, and revealed a rich organic chemistry, but its cameras could not see through the thick organic haze. After a seven-year interplanetary journey on board the Cassini orbiter, the Huygens probe was released on 25 December 2004. It reached the upper layer of Titan's atmosphere on 14 January and landed softly after a parachute descent of almost 2.5 hours. Here we report an overview of the Huygens mission, which enabled studies of the atmosphere and surface, including in situ sampling of the organic chemistry, and revealed an Earth-like landscape. The probe descended over the boundary between a bright icy terrain eroded by fluvial activity—probably due to methane—and a darker area that looked like a river- or lake-bed. Post-landing images showed centimetre-sized surface details.
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The Cassini-Huygens mission is the result of an exemplary international collaboration in space exploration. Huygens involved more than 40 European industries and two US suppliers. The work of the members of the Cassini and Huygens teams from ESA, NASA/JPL, the Huygens industrial consortium led by Alcatel, and all Cassini-Huygens investigators is acknowledged. We especially acknowledge the Cassini orbiter teams that made their early observations available in advance to Huygens, and to R. Yelle for his leadership in coordinating the Titan Atmosphere Model Working Group. Special acknowledgements to B. Smeds for his work in designing and managing the Huygens link test that allowed the Doppler problem to be detected and solved, to L. Popken for his modelling of the Huygens Digital Radio Receiver and to the whole Huygens recovery task force led by K. Clausen and L. Deutsch. We thank K. van't Klooster for his efforts to initiate and promote the Huygens VLBI experiment, and J. Louet for his support. The Earth-Based Huygens Doppler tracking experiment is led by W. Folkner. We appreciated the support provided by the National Radio Astronomy Observatory (NRAO), operated by Associated Universities Inc., under a cooperative agreement with the NSF, and the one provided by the Australia Telescope National Facility (ATNF) managed by CSIRO. We also thank M. Bird, R. Lorenz, R. A. Preston and J. C. Zarnecki for a careful reading of various versions of the manuscript and for providing comments.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
This figure shows an exploded view of the Huygens probe. (PDF 47 kb)
This figure shows the accelerometer measurements (engineering sensors) during the whole descent. (PDF 79 kb)
This figure shows the evolution of the probe internal temperature measurement during the descent and on the surface. (PDF 60 kb)
This figure illustrates the overall variation of the communication radio link parameter during the mission and provides a comparison between the predicted signal envelope and the measured signal strength, both during the descent (probe spinning) and on the ground. (PDF 65 kb)
This figure shows the location of all the radio telescopes that were used to receive the Huygens carrier signal. The telescope configuration is shown as seen from Titan at the beginning (left panel) and at the end (right panel) of the observation. (PDF 48 kb)
This figure shows the path of the direct signal transmitted by Huygens and of the one reflected by the ground. The two signals produces an interference pattern that was detected on board Cassini. (PDF 47 kb)
This includes Supplementary Table 1 (details about the six Huygens investigations), Supplementary Table 2 (a list of 17 radio telescopes that were part of the Huygens radio astronomy segment of the mission), Supplementary Figure Legends and technical information about the probe that was not appropriate to include in the printed version. It also describes the technical details of the radio astronomy segment of the Huygens mission. (DOC 65 kb)
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Lebreton, J., Witasse, O., Sollazzo, C. et al. An overview of the descent and landing of the Huygens probe on Titan. Nature 438, 758–764 (2005). https://doi.org/10.1038/nature04347
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