After the tragic loss of the space shuttle Challenger, NASA decided to cancel another flight of the Egg. But subsequent cosmic ray experiments continued to fly in high-altitude balloons; Müller then worked on the HEAT (High Energy Anti-matter Telescope), with Professor Simon Swordy, who made several balloon flights from New Mexico and Canada in 1994 and 1995. HEAT measured the abundances and spectra of energy of positrons and antiprotons, as well as measurements of the isotopic abundances of cosmic ray nuclei. These revealed the first hints of an excess of positrons, a mystery that has yet to be fully solved, but has been speculated to be related to pulsars or even dark matter.
In 2003, Müller oversaw the design and launch of TRACER (Transition Radiation Array for Cosmic Energetic Radiation), which flew over Antarctica and was specifically designed to measure heavy elements in cosmic rays. He also worked on Wakely’s Cosmic Ray Electron Synchrotron Telescope (CREST) program, launched in 2011 to search for cosmic ray electrons at extremely high energies.
One of Müller’s main contributions to the field was the development of a technique to use transition radiation in these experiments to measure the energies of cosmic rays in order to distinguish them. This technique is still used in experiments today, including the large Alpha Magnetic Spectrometer currently aboard the International Space Station.
“He had impeccable judgment,” said Gregory Tarlé, a University of Michigan physics professor and longtime friend and colleague of Müller. “He was a most valuable colleague because if you made a mistake he would find it.”
Colleagues recalled Müller’s dedication to scientific integrity: “He felt it was your duty as a scientist to be scrupulously honest. Rather than trying to persuade people of an outcome, he would just present all the information and let them make their own judgments,” Tarlé said.
Another distinguishing characteristic was his imperturbability – particularly important in his chosen field, as scientists watch years of work disappear into the upper atmosphere. “He could be counted on to be calm and cheerful under intense conditions, even if the experiments didn’t go as planned,” said Tom Prince, Ira S. Bowen Professor Emeritus of Physics at Caltech and director of WM Keck. Institute. for Space Studies, who was one of Müller’s first graduate students. “I was also impressed with his willingness to nurture new ideas, which might or might not work, but if they did they might lead to new directions.”
Astronaut John Grunsfeld, a former associate administrator of NASA’s Science Mission Directorate, worked with Müller on the Chicago Egg experiment. “What I remember most about him is that he found joy – I mean real joy – in solving problems in experimental physics. It was so fun to see his enthusiasm for the job,” said Grunsfeld, PhD’88. “I thought of Dietrich’s universe. He was always very supportive of my career and the work I was doing in space, and I think that goes back to his genuine love of mentoring and seeing students succeed.
Müller was director of the Enrico Fermi Institute from 1986 to 1992 and a fellow of the American Physical Society. After his retirement in 2006, he continued to remain active in research. In 2009, he received the Yodh Prize from the International Union of Pure and Applied Physics, which recognizes scientists whose research has had a major impact on the understanding of cosmic rays.
In addition to his scholarly activities, Müller was a great lover of classical music and art, especially German Expressionism, and was a dedicated traveller. He was active in his community, including serving as president of the Jackson Park Highlands Homeowners Association, where he lived for many years. He was also an avid gardener, taking great pride in his roses, restoring native prairie and forest in Wisconsin, and planting hundreds of trees.
He is survived by his wife of 53 years, Renate; his children, Georg and his wife Kathy, Michael and his wife Karessa, and Agnes and his wife Scott; his grandchildren Oskar, Alex, Natalie, Monica, Amalia and Krista; as well as sister Gisela and his wife Dieter, and brother Volker and his wife Angela.