Chief of the Astroparticle Physics Laboratory, NASA's Goddard Space Flight Center
For his scientific leadership in the study of gamma-ray bursts as well as for his significant contributions to high-energy astrophysics, infrared astronomy, and instrument development.
Every day or so, unseen by your eyes, a bright burst of light explodes in the sky. These bursts shine in gamma rays, the most energetic kind of light that's way beyond the visible part of the spectrum. Among the most explosive and violent events in the universe, these gamma-ray bursts produce as much energy in a few seconds as the sun will during its entire 10-billion-year life.
And for decades, Neil Gehrels has been a pioneer in understanding these bursts and in exploring the gamma-ray universe. He’s helped lead teams of researchers on multiple projects and missions, including as the principal investigator of NASA’s Swift Gamma-Ray Burst Mission, which has solved long-standing mysteries about the powerful blasts.
“Neil is among the most distinguished leaders in space astrophysics who has made a number of important scientific contributions,” said Fiona Harrison, the Kent and Joyce Kresa Leadership Chair of the Division of Physics, Mathematics and Astronomy and the Benjamin M. Rosen Professor of Physics at Caltech.
But Gehrels wasn’t always sure science was his calling. Despite being the son of an astronomer, and having spent much of his youth in and around observatories, his early passion was music. He played the clarinet, guitar, and piano. When he went to the University of Arizona as an undergraduate, he studied to be a classical-music composer.
It wasn’t until his junior year that he decided to add a physics degree, after he found work in a lab studying high-energy particles from space called cosmic rays. “It wasn’t just a summer job,” he said. “It was the place I went every afternoon after I went to class. I just really enjoyed the research.”
Gehrels took his newfound passion to graduate school at Caltech, where he worked under noted physicists Robbie Vogt and Ed Stone. One of Gehrels’ first assignments was to help calibrate the cosmic-ray instrument that would launch onboard the Voyager spacecraft. The primary goal of the instrument was to detect particles coming from interstellar space when the spacecraft exited the solar system—an event that wouldn’t happen until 2013.
As Voyager flew past Jupiter in 1979, however, Gehrels got an opportunity to use the instrument and analyze the particles around the planet—most likely run-of-the-mill particles like protons and electrons. If anything, the task would be good experience for the grad student.
But then Gehrels discovered something unusual: speeding particles of oxygen and sulfur. “It was completely unexpected and not something that’d been seen at any other planet,” he said. The particles, it turned out, were being expelled from volcanoes on Jupiter’s moon Io. This discovery, one of the most dramatic to come from the Voyager mission, remains one of Gehrels’ most-referenced papers.
While at Caltech, Gehrels also met his wife, Ellen Williams (PhD ’82). (Williams is also a Caltech Distinguished Alumna this year.). The two were married in a ceremony held in Dabney Gardens in 1980. “Neil was—and is—a remarkable individual,” Williams says. “He has an infectious enthusiasm for science that spans a range of interests. I am lucky to count him as a colleague and my partner.”
After earning their degrees, Gehrels and Williams moved to Maryland, where Gehrels joined NASA’s Goddard Space Flight Center. He’s been there ever since, focusing on the field that has defined his career: gamma-ray astronomy.
The universe produces a lot of gamma rays, from things like solar flares, exploding stars, and the hot disks of gas and dust that spiral into black holes. Then there are gamma-ray bursts. Although astronomers have observed these curious explosions since 1967, their origins were a long-standing mystery.
To understand these bursts and other gamma-ray sources, Gehrels started work on a balloon experiment, launching gamma-ray instruments 40 kilometers above Earth, beyond much of the gamma-ray-absorbing atmosphere. The balloon instruments were paving the way for NASA’s ultimate goal: gamma-ray space telescopes. When NASA went ahead with its satellite, the Compton Gamma-Ray Observatory, it made Gehrels the project scientist.
The space telescope was the first to explore and map the gamma-ray universe with high sensitivity, and it detected thousands of new bursts and hundreds of new sources. One of Compton’s biggest discoveries was that gamma-ray bursts come in at least two types: short blasts that last fewer than two seconds, and longer-lasting ones. The mission would occupy Gehrels for about a decade. “Those were some of the best times in my professional life,” he says.
When a faulty gyroscope finally forced NASA to retire Compton (by plunging it into the Indian Ocean), Gehrels would lead a new spacecraft called the Swift Gamma-Ray Burst Mission to continue where Compton left off.
Thanks to Swift, scientists learned that the long and short bursts have different origins. The long bursts come from the explosive deaths of massive stars as they collapse into black holes. The short-duration bursts come from collisions between stellar corpses called neutron stars.
Gehrels is also a part of other space missions, including the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) and NASA’s Fermi gamma-ray telescope. These missions study other aspects of the universe. “The field of gamma-ray astronomy has progressed in the span of my career from detection of a few sources to detailed studies of thousands,” he says.
Gehrels’ leadership extends to numerous projects beyond gamma rays. He is the project scientist for the Wide-Field Infrared Survey Telescope (WFIRST), an upcoming large space telescope that will explore everything from alien worlds to dark energy, and was also a member of the LIGO Scientific Collaboration, working on the Laser Interferometer Gravitational-Wave Observatory, which recently made the groundbreaking discovery of detecting gravitational waves for the first time.
“Neil’s calm, deliberative style together with his careful approach makes him particularly effective at directing or participating in a number of large, important missions,” Harrison says.
Gehrels sees his career reflective of a larger movement in astronomy as new types of observatories are brought into service.
“This whole area of looking at the universe in different ways other than the light we see with our eyes has become the way that astronomy works,” Gehrels says. “From gamma rays, which was a new window to the universe when I started, to the extraordinary breakthroughs with gravitational waves, we have incredible tools at our disposal to probe the mysteries of the universe. I find it very fulfilling to plunge into the unknown.”
by Marcus Woo
photo credit: Scott Council