Hendrik "Henk" C. van de Hulst, an honorary member of the AAS, died in Leiden on July 31, 2000, at the age of 81. He was one of the greatest Dutch astronomers of the past 150 years. In 1944 he had predicted that the amount of neutral atomic hydrogen in interstellar space would be so great as to produce a measurable signal at the radio wavelength of 21-centimeters. This prediction led to a breakthrough in astronomical research. Henk van de Hulst was born in Utrecht, The Netherlands, on November 19, 1918. He was one of six children born to W. G. van de Hulst, a well-known writer of children's books, who was the principal of the elementary school where Henk received his first education. In the summer of 1930, when Henk was just 11, he solved all the problems in the high school math books. Clearly, he would go on to study math in college. Astronomy, however, was still only one of his numerous hobbies.
Henk decided to become an astronomer in his second year at the University of Utrecht, where he was strongly influenced by the lectures given by M. Minnaert. Later, Minnaert pointed out a Prize Competition that the University of Leiden had announced in 1941. The subject of the competition centered on the small particles of dust that had been discovered some ten years earlier in interstellar space but that were still largely a mystery. Scattering of light by these particles of "interstellar smoke" determines much of the appearance of the Milky Way. Henk submitted his entry in April 1942 and received one of the two prizes given.
The Prize Competition had two particularly important consequences. Henk became acquainted with Jan Oort, and he became deeply interested in the general problem of light scattering in an astronomical context. This interest was soon reflected in his Doctoral thesis, "Optics of Spherical Particles." In June 1946, with Minnaert as his supervisor, he was awarded a PhD cum laude. The subject of light scattering remained central to Henk's interests for his entire life. He wrote two monographs on the subject. One of these, Light Scattering by Small Particles, first published in 1957, was immediately recognized as a classic.
In the late 1930s Grote Reber, an American engineer and radio amateur, began studying the source of interference that disturbed long-distance communication systems. Reber constructed a steerable antenna to map cosmic radio emission. He observed that the emission was particularly strong in the direction of the center of the Milky Way. In Leiden, Oort learned about Reber's map and the fact that the emission could not be explained in terms of known mechanisms. Oort realized that radiation at radio wavelengths would penetrate the cloudy Dutch sky and create a new field for astronomical research there.
In 1946 after Henk married Wilhelmina Mengerink, the van de Hulsts moved to the United States, where Henk had been awarded a postdoctoral fellowship at the Yerkes Observatory. At Yerkes he developed strong bonds with S. Chandrasekhar and Gerard Kuiper. Kuiper fostered Henk's interest in the solar system and his work on the dust in the zodiacal belt.
Although Henk could have remained in the United States after his postdoctoral studies, Oort convinced him to return to The Netherlands. He was appointed at the University of Leiden, first to the rank of Lector in 1948, then to a professorship in 1952. He remained in Leiden throughout his career and became Professor Emeritus in 1984.
Shortly after Sputnik I had been launched, Henk's career took an unexpected turn. In 1958 Oort arranged for him to attend a meeting in London of the International Council of Scientific Unions (ICSU). When Henk returned home, he was the first president of COSPAR, a new international organization for the peaceful exploration of the Universe. The meeting had been called because the ICSU was concerned that the competition between the United States and the Soviet Union would cause the military aspects of space studies to supersede the scientific ones. Henk referred to the meeting as the time "I was launched into a space career."
One of the highlights in Henk's career was an important congress, organized by COSPAR, at which he presented two astronauts (Glenn from the United States and Titov from the Soviet Union) with a wooden shoe, each cut from the same tree. Everyone immediately understood the symbolism. From 1960 to 1975 Henk was closely involved with the European Space Research Organization (ESRO); and from 1975 to 1986, with its successor, the European Space Agency (ESA). Henk held very important positions on the boards of both ESRO and ESA. He was also one of the pioneers of space research in The Netherlands, supervising much of its prosperous development. In 1959, also at his instigation, a committee for space research was formed within the Royal Academy of Sciences (of which he had become a member before he was 40 years old). He was president of this committee (GROC) until 1984, when it was incorporated into a new Institute, SRON, which now builds major instruments for space research under the auspices of the Dutch national science foundation.
On behalf of Henk's colleagues, friends, and students at the Sterrewacht Leiden, Harm Jan Habing assembled and posted an obituary on the website for Leiden Observatory (http://www.strw.leidenuniv.nl/vdhulst_e.html), from which this abridged version was drawn.
This version contains a correction of an error in the original.
Photo courtesy of the University of Leiden