Riccardo Giacconi, the “Father of X-ray Astronomy,” Nobel Prize winner and one of the most influential figures of modern astrophysics, died in December at the age of 87. Over a career spanning more than five decades in time and 10 orders of magnitude in wavelength, Riccardo opened up new windows for observing the universe, and revolutionized the way "big astronomy" is done at major observatories, from the Hubble Space Telescope to the Atacama Large Millimeter/submillimeter Array (ALMA). In 2002, Riccardo was awarded half of the Nobel Prize in Physics for “pioneering contributions to astrophysics, which have led to the discovery of cosmic x-ray sources.”
Riccardo was well-versed in the classics, and often spoke of being driven, like Odysseus, to pursue virtue and knowledge. We three were privileged to accompany him in one way or another on his epic journey as colleagues and friends beginning in the late 1960s when, still in our 20s, we came together at American Science and Engineering (AS&E) in Cambridge, Mass.
By the time we arrived at AS&E, Riccardo had been there for 10 years, rising to the position of executive vice president. He had led the way in founding the field of x-ray astronomy, successfully seeking funding in 1961 from the Air Force Cambridge Research Laboratory to launch an x-ray detector into space. Although the ostensible goal was to detect fluorescent x-rays from of the moon, Riccardo was on the hunt for bigger game.
Born in Genoa, Italy on October 6, 1931, Riccardo spent most of his first 25 years in Milan, where he obtained a PhD in physics from the University of Milan, working under the direction of noted cosmic ray physicist Giuseppe Occhialini. At that time, the only practical way to study high-energy nuclear reactions was through the detection and analysis of the interaction of high-energy particles, primarily protons, with atomic nuclei in the atmosphere. Although he learned much about the conception, design and building of detectors, Riccardo was frustrated by the lack of “action”: he spent about two years with his cosmic-ray detector in an Alpine Quonset hut and obtained 80 high-energy cosmic-ray detection events.
In 1956 Riccardo was awarded a Fulbright fellowship and emigrated to the United States, where he worked as a postdoctoral researcher at Indiana University and then Princeton. There he met and worked with Herbert Gursky on cosmic-ray experiments. According to Giacconi, “We built equipment, worked like fiends, analyzed data and declared failure.” When his Fulbright fellowship expired, Riccardo moved to AS&E, a start-up formed by Martin Annis, an ex-student of Bruno Rossi, a professor at the Massachusetts Institute of Technology (MIT).
At a party at his home, Rossi suggested that Riccardo look into the possibility of developing a program in x-ray astronomy. A group led by Herbert Friedman of the Naval Research Laboratory had previously observed x-rays from the sun, but based on the strength of the solar x-rays, it seemed unlikely that x-rays could be detected from the much more distant stars.
Riccardo was unconvinced, suspecting that the problem lay not in the stars but in the efficiency of the x-ray detectors and a lack of imagination as to what the universe could provide. Working with Frank Paolini and Gursky, who had moved from Princeton to AS&E, they built a detector with a much wider field of view and about 50 times more sensitivity than ones flown previously.
On June 12, 1962, after two previous launches had ended in failure, Riccardo and his group achieved success. The rocket spent five minutes above the atmosphere. In that time, it detected a strong source in the direction of the constellation Scorpius, which they named Scorpius X-1, as well as an all-pervasive x-ray background radiation. The field of x-ray astronomy had been born!
Giacconi moved quickly to use this new window for the exploration of the universe. In 1963, he and Gursky led an effort to lay out a bold program for the future of x-ray astronomy that included more rocket flights, an x-ray satellite to survey the entire sky, and within a few years, an x-ray telescope.
The proposed schedule was overly ambitious, but the vision was spot-on, and the results astounding. With funding from NASA, Giacconi’s group developed and operated the first x-ray satellite, Uhuru, which was launched in December 1970. For this project Riccardo refined what he termed a systems engineering approach to science. Engineers and scientists worked side-by-side to establish requirements, develop a design, construct and test the hardware, and plan the operations for the satellite. Riccardo himself was familiar with all of the subsystems—knowing which were critical, which could provide back-up capabilities for others, how the science operations would be carried out, and the like. He pressed NASA to deliver 20 percent of the data back to AS&E within 24 hours of acquisition and directed the team to develop a software system to analyze the data as soon we received it. This was a radical departure from previous NASA science missions where data tapes were delivered by mail several weeks after acquisition.
The quick turnaround on the data enabled Riccardo and the team to make important discoveries almost immediately and to rearrange the observing schedule and satellite configuration to follow up and exploit those discoveries. On a personal level, although junior scientists by university standards and relatively new to the project, we three and several colleagues received assignments on the project that challenged us to our limits while providing opportunities to develop technical, management, scientific and communications skills from the very beginning.
Even though Riccardo had multiple responsibilities as a senior executive at AS&E, he set aside part of each day to work with the other scientists to analyze the latest Uhuru data. Within a few months, the team had discovered erratic, sub-second variability in the source known as Cygnus X-1 and a regular periodicity of 4.84 seconds in Centaurus X-3. With intensive follow-ups of Cen X-3, we determined that it orbits in a binary system with a period of 2.087 days. Further observations confirmed that Cen X-3 is powered by the gravitational energy released by matter falling from the companion star towards and onto a neutron star. Subsequently, Giacconi and the Uhuru team, along with a number of other observers and theorists, determined that Cygnus X-1 is most probably a black hole (with a mass later established as approximately 15 times the mass of our sun), orbiting in a binary system and powered by matter falling towards the black hole. This finding constitutes the first confirmation of the existence of black holes.
During the Uhuru period, Riccardo met every Friday afternoon with the Uhuru science group. In these often-stormy sessions, there was wide-open give and take regarding what we were seeing, what we thought we understood, what we were clueless about and what we wanted to do next. Ideas, however wild, were floated with abandon and shot down remorselessly. There was respect for one and all, but no one was sacrosanct and everyone had to defend their ideas based on logic and scientific merit. Uhuru meetings left most of the participants drained but also built confidence—in our results, in our plans for moving ahead and in one another.
In 1973, Giacconi’s core group moved to the newly organized Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, where it formed the High Energy Astrophysics Division. It was there that one of Riccardo’s earliest visions was fulfilled, with the development and launch, in 1978, of the Einstein X-ray Observatory, the first imaging x-ray telescope for extrasolar sources. Einstein demonstrated beyond a doubt the importance of x-ray imaging, finding that essentially all types of astronomical objects and systems, from nearby stars to distant quasars, emit x-rays. Innovations on this mission included the solicitation of proposals from the general community and the development of procedures and techniques to plan, schedule and archive the Einstein observations, thereby opening access to the observatory for the broader astronomical community. This model, new at the time, has now been adopted by essentially all NASA astrophysics missions and most ground-based observatories.
In 1976, cognizant of the limited lifetime projected for Einstein and confident of the prospects it would engender, Giacconi and Harvey Tananbaum proposed Einstein’s successor, the Chandra X-ray Observatory. Launched in 1999 and now in its 20th year of operation, Chandra remains without peer for its ability to produce sub-arcsecond x-ray images and has established itself as one of the most productive observatories ever. More than 7,000 refereed papers have been published using Chandra data on topics as diverse as exoplanets, neutron stars, black holes, clusters of galaxies, dark matter and dark energy.
Although Riccardo would move on to new challenges, he remained involved in Chandra in several capacities, including his role as team lead for the first few years of Chandra observations on the Chandra Deep Field South. He also remained involved “in spirit” as many of the people he had recruited and trained played key roles in making Chandra a reality, including, but not limited to, Leon Van Speybroeck as telescope scientist, Stephen Murray as principal investigator for the High Resolution Camera, and Harvey Tananbaum as first director of the Chandra X-ray Center.
In 1981, Riccardo became the first director of the Space Telescope Science Institute (STScI). The scientific community insisted that the scientific operations of a large, unique and expensive new facility—the first major international optical observatory in space, later to be christened Hubble—be managed by the community itself, and not by a NASA center. He insisted on leaving the CfA group intact, but, recognizing the need to transfer the scientific operations philosophy from the x-ray group to the optical community, he recruited Ethan Schreier to Baltimore to oversee the Hubble operations and data system.
Riccardo proceeded to create from scratch an entirely new institution, based on many of the same principles that had been so successful with his path-breaking x-ray astronomy satellites. He assembled a core staff with expertise in the operational, engineering and especially the scientific disciplines necessary to operate Hubble, taking into account all aspects of what it would take to make Hubble scientifically useful: planning and scheduling, satellite commanding and operations, a process to guide development of the scientific program, and an ecosystem to help the community use Hubble.
Other innovations, now standard for large astronomy projects, were introduced by Riccardo and his staff for Hubble. These included a formal data archive, the distribution and archiving of calibrated data, a formal archival data analysis program, an AI-based planning and scheduling system, reserved time for large and “key” programs, and freely distributed portable data analysis software. The first internet-based networking between astronomy facilities was also developed under the auspices of Hubble. He instituted the first community-operated grants program to support Hubble users and a “Hubble Fellows” program to support young astronomers, which became models for other missions and disciplines.