YANNICK MELLIER (1958-2025)
Photography of Yannick Mellier taken at IAP in June 2009, for the CNRS Silver Medal.Credit: Jean Mouette /IAP-CNRS-SU
Our colleague Yannick Mellier passed away on December 19, 2025, at the age of 67. The scientific legacy he leaves behind is immense. At the time of his passing, he was one of the emblematic figures of the IAP and had been directing the international Euclid Consortium since 2011.
Yannick earned his PhD from the University of Toulouse III, Paul Sabatier, in 1987 under the supervision of Bernard Fort, and as part of his team contributed to the discovery of a gravitational arc in the galaxy cluster Abell 370. This work paved the way for demonstrating, on the international stage, the potential of gravitational lensing for the study and mapping of the mysterious dark matter of the Universe.
It marked the beginning of a scientific and human adventure spanning more than 30 years, of which Yannick was both the driving force and the visionary.
The adventure therefore began in Toulouse, with the very first CCD cameras—the 8K and the 12K—installed on the Canada–France–Hawaii Telescope, which at the time provided ground-based images of exceptional quality (better than 0.5 arcseconds of resolution under the best observing conditions). The journey continued in Paris in the mid-1990s, when Yannick joined the IAP, with the launch of the Megacam project and the establishment of the Terapix data center for the reduction and exploitation of these large images. Yannick was the central pillar of this effort, the tireless architect behind the development of these tools. The impact of the scientific projects they delivered to the community was considerable.
Guided by a clear vision, Yannick succeeded in elevating our community to the highest international level. In 2000, his team was among those who first demonstrated the effects of gravitational lensing by the large-scale structures of the Universe.
Group photograph from the Euclid France symposium, with Yannick Mellier (center), taken in front of the Paris Observatory on November 30, 2017.
Credit: Jean Mouette /IAP-CNRS-SU
In 2011, Yannick took the helm of the Euclid Consortium. He was already its spiritual father. He led the consortium during its establishment, overseeing the final designs of the two onboard instruments, the design of the scientific ground segment, and the overall organization of the consortium. He also supervised the creation of the consortium’s scientific teams, a structure bringing together more than a thousand scientists to transform the mission’s exceptional capabilities into scientific reality. Finally, he was at the heart of the project’s strategy, working with ESA and all of our international partners to deliver the ground-based observational data required for Euclid’s core science.
Yet, even though he had become the embodiment of the Euclid project, his scientific legacy and his impact on the community extend far beyond it. A member of dozens of national and international scientific committees throughout his career—within the CNRS, CNES, ESA, and ESO—Yannick was always deeply committed to supporting projects he considered promising, whatever they were and wherever they came from. He devoted himself tirelessly to our community without ever seeking personal recognition; science was literally his raison d’être.
And while he was one of the most visible scientific figures in our community, Yannick was disarmingly generous. Warm and collegial with fellow scientists and collaborators, he made it a point of honor to highlight the merit of each person’s contributions; he was deeply committed to ensuring that others were recognized and appreciated. He was a source of support for all who knew him and, for many of us, a friend.
Photograph of Yannick Mellier taken at the Paris Observatory in November 2006 during the presentation of the SFP Jean Ricard Prize.
Credit: Jean Mouette /IAP-CNRS-SU
Yannick Mellier received the Jean Ricard Prize from the French Physical Society (SFP) in 2005, the Franco-German Gay-Lussac–Humboldt Prize in 2006, and the CNRS Silver Medal in 2009.
Links
Those who knew him can pay tribute to him on the site created for this occasion.
These messages can be found on that website.
Conferences and films
Public conference (in French) at IHP, November 2015, “The Large Structures of the Universe” (Ideas in Science).
Public conference (in French) at IAP, April 2015, “Dark matter and dark energy: what will Euclid teach us?”.
Seminar at Collège de France, February 2015, “Dark matter and gravitational lenses” (Chaire Galaxies et cosmologie - Françoise Combes (2014-2015).
Film “Les 75 ans de l'IAP” (in French, 2013).
Video interview (in French, English subtitles, 3min 54s, unpublished, 2013).
Film (in French, ARTE, 2012), “The mystery of dark matter” (by Cécile Denjean).
Public conference (in French) at IAP, January 2004, “From large surveys to virtual observatories” (introduction by Ludovic Van Waerbeke).
December 2025