Neil Gehrels (PhD '82), 1952–2017

Neil Gehrels, a Caltech Distinguished Alumnus who earned his PhD in physics in 1982, passed away on February 6, 2017, at the age of 64.

Gehrels, a friend and colleague to many scientists at Caltech, was a pioneer in the study of gamma-ray bursts, which are blasts of high-energy radiation that come from deep space. Based at NASA's Goddard Space Flight Center in Greenbelt, Maryland, Gehrels was the principal investigator for NASA's Swift Gamma-Ray Burst Mission, which has solved many mysteries about gamma-ray bursts. He was also the project scientist for NASA's upcoming Wide Field Infrared Survey Telescope (WFIRST), a large infrared-based space telescope that will search for planets beyond our sun as well as study the mysterious repulsive force in our universe dubbed dark energy. Additionally, Gehrels was a member of the LIGO Scientific Collaboration, the group that directly observed, for the first time, ripples in space and time called gravitational waves.

"Neil was a pioneering astronomer, a great instrumentalist, and the leader of astrophysics missions spanning from gamma rays to the infrared," says Fiona Harrison, the Benjamin M. Rosen Professor of Physics and the Kent and Joyce Kresa Leadership Chair of the Division of Physics, Mathematics and Astronomy at Caltech. "He was also a mentor and friend to many, including myself. The entire astronomical community is mourning his loss."

In a statement from Goddard, Chris Scolese, the center's director, said, "Our center has lost a dear friend and astronomy pioneer, and his spirit will always live on in our work. Those of us who were fortunate to work with Neil know of his unwavering enthusiasm for science and unselfish generosity in mentoring others."

Gehrels' early passion was music. He played clarinet, guitar, and piano. As an undergraduate at the University of Arizona, he studied to be a classical-music composer. Later, he decided to add an undergraduate physics degree, and ultimately followed his newfound passion to Caltech. As a graduate student, Gehrels worked under physicists Robbie Vogt, the R. Stanton Avery Distinguished Service Professor and Professor of Physics, Emeritus; and Ed Stone, the David Morrisroe Professor of Physics and the project scientist for NASA's Voyager mission. One of Gehrels' first projects was to calibrate a cosmic-ray instrument on Voyager. Much later, in 2012, Voyager 1 entered the uncharted territory of interstellar space, where this instrument detected the full intensity of cosmic rays. In 1979, when the Voyager 1 and 2 spacecraft flew past Jupiter, Gehrels discovered speeding particles of oxygen and sulfur, the origins of which turned out to be volcanoes on Jupiter's moon Io. The discovery remains one of Gehrels most-cited papers and was the topic of his Caltech PhD.

While at Caltech, Gehrels met his wife Ellen Williams (PhD '82), who serves as the director of the Department of Energy's Advanced Research Projects Agency-Energy (ARPA-E) and as a professor of physics at the University of Maryland. Gehrels and Williams—who were both named as Caltech Distinguished Alumni in 2016—were married in Caltech's Dabney Gardens in 1980 and later moved to Maryland, where Gehrels began work at Goddard.

It was at Goddard that Gehrels started studying highly energetic gamma rays that come from space. In the late 1980s, he developed balloon experiments to study gamma rays from the center of our Milky Way galaxy and from supernovas in other galaxies. From 1991 to 2000, he was the project scientist for the Compton Gamma-Ray Observatory, which began unraveling the mysteries of gamma-ray bursts. He served as the principal investigator for Swift, the successor of Compton, from 1999 until his death. Swift revealed that gamma-ray bursts likely come from tremendous supernova explosions as well as collisions between neutron stars.

"Neil built great missions so that astronomers like us can make discoveries about our universe," says Shrinivas (Shri) Kulkarni, the John D. and Catherine T. MacArthur Professor of Astronomy and Planetary Science and director of Caltech's Optical Observatories. "Neil was an exceptional leader. He led in a harmonious fashion; he was everyone's friend and yet at the same time saw missions go from concept to very productive facilities. Swift is one of most heavily used missions and made fundamental discoveries." Kulkarni and Gehrels recently shared the prestigious Dan David prize along with Andrzej Udalski of Warsaw University Astronomical Observatory.

Gehrels was also the recipient of the NASA Exceptional Scientific Achievement Medal, the NASA Outstanding Leadership Medal, and Goddard's John C. Lindsay Memorial Award. He and the Swift team received the 2007 Rossi Prize from the American Astronomical Society and the 2009 Henry Draper Medal from the National Academy of Sciences. He was elected to the National Academy of Sciences in 2010.

In his free time, Gehrels liked to climb mountains; in 2006 and 2015, he climbed the Nose route on El Capitan in Yosemite in six-day solo ascents. He and his family were active volunteers in disadvantaged communities around Goddard. In 2005, he helped develop an internship program that allowed local high school students with hardships to work in his labs.

He is survived by his wife Ellen and two children, Thomas and Emily.