Was a Belgian physicist, inventor and explorer born in 1906. He was director of the Centre de Physique Nucléaire de l'Université Libre de Bruxelles and involved in Cosmic Rays research.
In 1932, along with Auguste Piccard's he participated in the second FNRS (Fonds de la Recherche Scientifique) balloon stratospheric ascent that break the altitude record reaching 53,152 ft. It was launched from Dübendorf, Switzerland on August 18. For this achievement, he was awarded the Cross of Knight of the Order of Leopold by the Belgian King.
Two years later, on 18 August 1934 Cosyns together with one of his students Nérée Van der Elst piloted the third FNRS balloon ascent to an altitude of 52,952 feet. Following a take off from Hour-Havenne in Belgium, they flew over Germany and Austria before landing near the village of Zenavlje (now in Slovenia). They were unsuccessful in maintaining satisfactory radio communication with ground, but were able to make observations of the currents in the stratosphere as well as investigate the nature of the cosmic rays. They failed to beat the height record, but stated on landing that they were fully satisfied with their discoveries. A large bronze monument in the shape of a balloon was erected in 1997 on the spot of their landing in Zenavlje to commemorate the event.
After the outbreak of Second World War he joined the Resistance against German occupation and was imprisoned in Dachau concentration camp. After the war he was co-director of the FNRS-2 bathyscaphe expedition in Dakar in 1948.
In 1954 Cosyns was led to resign from its position, and to left the University as the aftermath of the death of a French speleologist in an accident during an expedition he directed to the Gouffre de la Pierre-Saint-Martin cave system in the Pyrenees. Apparently the steel cable of the electric hoist snapped during the ascent from the cave and Cosyns -due to his involvement in the design of the winch- was considered responsible for the accident.
Little is known of him after these years, until his death that occured in May 1998.