| Citation | van de Hulst is proposed for Foreign Membership primarily for his prediction (in 1944) of radiation from the 21cm, hyperfine transition in atomic hydrogen in the galaxy and for his subsequent contributions to the study of the structure of our own galaxy and others through observations of that radiation. The 21 cm. radiation was first detected by Ewen and Purcell in 1951 and shortly afterwards in the same year the first publication of the Netherlands group, appeared. Subsequently, 21cm. observations have enabled the density, temperature and motions of neutral atomic hydrogen to be studied in great detail, with fundamental advances in the knowledge of the structure and dynamics of our own and other galaxies, 21cm. observations gave the first unambiguous objective picture of the spiral structure of our own galaxy and while many observatories joined in 21cm studies, van de Hulst himself, along with Dutch colleagues, continued to work in the field for many years. Not only was the structure of our own galaxy unravelled, but that of other galaxies especially the nearer ones such as the Andromeda nebula were also elucidated. Furthermore, it was the observations of the 21cm radiation that led to the search for other radio line spectra, the first to be discovered being absorption in the 18cm lines of hydroxyl, followed by the observation of maser radiation in those lines. These have been among the most important advances in astronomical knowledge in the last forty years, they were initiated by van de Hulst's original insight and he has contributed greatly to subsequent exploitation of his prediction. van de Hulst is however, an astronomer of wide range. He has worked for many years on fundamental problems of the scattering of electromagnetic radiation by small particles, and radiation transfer in clouds of small particles, and his books on that subject, Light Scattering by Small Particles (1957) and Multiple Light Scattering (1979) are authoritative. He has, in many papers, applied his fundamental work to scattering in the interstellar medium, planetary atmospheres, the solar corona and the zodiacal light.
van der Hulst has had responsible positions in various international astronomical organisations, in particular, he was the first president of COSPAR (1958-62), vice-president of the Preparatory Commission of ESRO (1960-64), Chairman of the Council of ESRO (1968-70) and of the Launch Programme Advisory Committee (1972-75). He has been the Netherlands delegate to the ESA Science Programme Committee since 1974. He was Chairman of the Steering Committee for the COS-B Satellite and is currently chairman of the Instrument Science Team for the Faint Object Camera of the Hubble Space Telescope. He also holds many official scientific positions in the Netherlands and in Europe. His achievements have been recognised by many awards, including in the United Kingdom, Associate (Foreign Member) and Eddington Medallist of the Royal Astronomical, and Rumford Medallist of the Royal Society. He is a Knight of the Nederlandse Leeuw. |