BornBrooklyn, New York, USA, 21 June 1916
DiedArlington, Virginia, USA, 9 September 2000
Herbert Friedman pioneered X-ray astronomy using V-2 and Aerobee rockets during the late 1940s and 1950s. He and his team were the first to find X-ray emission from the Sun, discover the second X-ray source outside the Solar System – the Crab Nebula – and demonstrate that X-rays come from the nebula as a whole, not just from a central star as Friedman had hoped.
Friedman, the son of fine-arts dealer Samuel and Rebecca (née Seligson) Friedman, entered Brooklyn College as an art major. Under the influence of physicist Bernhard Kurrelmeyer, Friedman graduated in physics and spent the summer of 1936 looking for a job until Kurrelmeyer, a graduate of Johns Hopkins University, arranged for Friedman to be given a student instructorship there.
At Hopkins University, Friedman started laboratory work directly under Nobel Prize winner James Franck, the head of the physics department. When Franck left for Chicago, Friedman stayed at Hopkins University, working with the X-ray spectroscopist Joyce A. Bearden. Friedman used the fine structure of X-ray absorption edges to explore the structure of metals. Under Bearden’s direction, Friedman built improved Geiger counters with thin entrance windows that increased the path length of soft X-rays through gas. Using Bearden’s X-ray spectrometer with this improved detector, Friedman contributed to a better understanding of the nature of the transition metals, receiving a Ph.D. in 1940.
As a Jew, Friedman found a permanent position at a major university unattainable, though he evidently tried hard. He stayed on as an instructor at Hopkins University for a year, during which time Hopkins physicist Alfred Pfund helped him secure civil service employment at the Naval Research Laboratory [NRL]. The permanent job made it possible for Friedman to marry Gertrude Miller, an instructor at Brooklyn College.
At NRL, Friedman applied his knowledge of X-ray spectroscopy and gamma-ray radiography to diagnostic analysis of critical metals. He soon moved into the Optics Division under Edward Hulburt to create an entirely new electron optics branch. It was there that Friedman developed a fast and efficient technique for precisely cutting and tuning radio-frequency crystals, controlling the process by examining the Bragg reflections from the crystals. In 1945 he won the Navy’s Distinguished Civilian Service Award for this work. Friedman applied improved X-ray-sensitive Geiger counters to the study of thin films and X-ray fluorescence analysis, developed reliable gamma-ray Geiger counters for radiation exposure surveys of Hiroshima and Nagasaki, and participated in setting up the radiation-monitoring systems that detected Soviet nuclear tests in 1949.
Hulburt drew Friedman into his group that was performing ultraviolet solar and atmospheric studies with captured German V-2 rockets at White Sands, New Mexico. By the end of the decade, they had still not solved major scientific problems regarding the high-energy spectrum of the Sun and the source of ionizing radiation in the Earth’s upper atmosphere. Applying his electronic detector expertise, Friedman started flying banks of counters on V-2 rockets in 1949 and soon provided a more detailed understanding of how the solar ultraviolet and X-ray spectrum influenced different layers in the Earth’s high atmosphere. Friedman’s electronic detectors also solved the data retrieval problem because the information could be transmitted by radio during flight and did not require physical retrieval. This new application of his expertise appealed to Friedman; he concentrated on space science for the rest of his career.
During the 1950s, Friedman’s group continued solar and atmospheric research. A 1952 flight on the Navy’s Viking rocket using counters sensitive to extreme ultraviolet and X-ray confirmed Friedman’s model of the solar source of ionization in the E layer. After that flight, Friedman relied heavily on a cheaper balloon-launched rocket system called a “Rockoon” and coordinated a series of shipboard rocket launches to study solar X-rays from widely differing parts of the Earth. More than anyone else, Friedman developed the instrumentation expertise that would prove to be important in the Sputnik era. The work of his core staff, men like E. T. Byram, Talbot Chubb, and Robert Kreplin, set the stage for scientific research with sounding rockets and satellites in the 1960s. In addition to devising counters that worked reliably and honestly in a very hostile environment, they devised a simple way to produce an X-ray image of the Sun using a pinhole camera, developed a rugged Bragg crystal spectrometer for measuring hard X-rays, and eventually provided the detectors for the Navy’s SOLRAD satellites dedicated to long-term monitoring of the high-energy radiation from the Sun.
From the mid-1950s Friedman’s group developed larger detector systems and small telescopic devices intended to detect nonsolar astronomical X-ray sources. After some initially confusing results, which created a schism in the group, they decided that they had evidence of emission from diffuse sources within the Milky Way. But in 1962, Riccardo Giacconi’s team from American Science and Engineering – see Bruno Rossi – became the first to unambiguously detect a nonsolar X-ray point source in the region of Scorpius. NRL rocket flights in 1963 improved the measured position of Sco X-1, confirmed the existence of an X-ray backround, and found the second compact source at the position of the 1054 supernova remnant (Crab Nebula). Friedman hoped that this might be the glow of a cooling neutron star left behind by the explosion, as had been suggested in 1933/1934 by Walter Baade and Fritz Zwicky . Friedman’s group knew that the limb of the Moon would pass directly across the nebula in 1964. They managed to time a flight so that the 5 min of flight data acquisition covered precisely the 5 min when the Moon was moving across the nebula. They expected the source to disappear suddenly, when the neutron star was occulted. Instead, it faded gradually, meaning that the source was the extended body of the nebula, not the neutron star. The X-ray radiation from a 1,000-year-old remnant required continuous energy input, accounted for by the discovery of a neutron star (pulsar) in the Nebula 4 years later.
In 1958, Friedman was made superintendent of a new atmospheric and astrophysics division at NRL. After reorganization in 1963, he became superintendent of the Space Science Division and Chief Scientist in the E. O. Hulburt Center for Space Research, positions he held until 1980.
Friedman’s staff won an important role in the High Energy Astrophysical Observatory [HEAO] satellite series created by the National Aeronautics and Space Administration [NASA]. Encouraged by NASA, he conceptualized using leftover Apollo hardware to create a large man-tended X-ray telescope in orbit that eventually evolved to the unmanned HEAO concept. Friedman designed a huge bank of seven tray-like thin window X-ray proportional counters that were intended to produce a sensitive map of the X-ray sky that included spectrum, intensity, and time variations. Launched on HEAO A-1 in August 1977, the Large Area Sky Survey Experiment observed until January 1979 and cataloged a wealth of data on particular sources and source classes that supported further studies of X-ray emission from clusters of galaxies. Evidence for a continuous X-ray background was also strengthened.
By the mid-1970s Friedman was writing popular books on astronomy that have received wide appreciation. He also acted as a spokesperson and arbitrator for science and science policy in Washington. Friedman was honored with a long list of awards and prizes including honorary doctorates (Tübingen and Michigan); election to the National Academy of Sciences; and medals from the Royal Society (London), Royal Astronomical Society, and others. He was a National Medal of Science recipient in 1968 and won the Wolf Foundation Prize in Physics (1987). Richard Nixon appointed Friedman to the President’s Science Advisory Committee [PSAC]. Friedman also advised the Atomic Energy Commission and was a member of the Space Science Board of the National Academy of Science.
Herb and Gertrude Friedman raised two sons, Paul and Jon. He died of cancer at his home.