Michael F. A'Hearn died of pancreatic cancer on 29 May 2017 after its diagnosis a few months earlier. He was 76. Mike, or Mike A., was known for his career-long contributions elucidating the physical and chemical behavior of comets and serving both the University of Maryland and the international astronomical community as an eminent scientist, distinguished professor, leader, and advisor. In the course of his career, he was honored by the naming of asteroid 3192 A'Hearn in 1986, elected fellow of the American Association for the Advancement of Science (1989), appointed Distinguished University Professor (2000), and received 5 awards and medals for his scientific contributions including the AAS-DPS Gerard P. Kuiper award (2008), American Institute of Aeronautics and Astronautics Space Science Award (2008), two NASA medals for exceptional scientific achievement (2006, 2012), and NASA’s Exceptional Public Service Award, awarded posthumously in 2017.
Mike was born 17 November 1940 in Wilmington, Delaware. His father was with the Internal Revenue Service and his mother was an elementary school teacher. Died-in-the-wool New Englanders, the family moved back to Massachusetts when Mike, an only child, was 5 years old. Mike was always involved in sports as a boy. The family spent part of their summers at Lake Boone in Massachusetts where his love of all things water-based was fostered. He loved sailing, playing ice hockey on the neighborhood pond, ice skating, canoeing, kayaking, or just being near a body of water. Swimming remained a lifelong favored activity and his lifelong friendships were water-based in one way or another.
He attended Boston College High School, where he most likely got into the habit of doing multiplication in his head by converting to logarithms, adding them and calculating the anti-log. Later, as a scientist, he would recite a product of rational numbers with the appropriate number of significant figures before many of us could muster an estimate. He also ran track and lettered in football. His undergraduate degree was from Boston College (1961), where he completed his degree in three years, graduating cum laude. He received his PhD from the University of Wisconsin (1966) with a dissertation entitled The Polarization of Venus.
Mike met his wife, Maxine, in Madison, Wisconsin, where they both were in grad school at the University of Wisconsin. They belonged to the same eating co-op held in the basement of St. Paul’s Chapel. “Mike relished the quality of the food—and the quantity,” reports Maxine. “We were both crew captains and our shifts were often spent paring 50+ pounds of potatoes!” They raised their family of 3 boys, Brian, Kevin, and Patrick, mostly in University Park, Maryland, and had 5 grandchildren at the time of Mike’s passing.
After receiving his PhD in 1966, Mike joined the faculty of the Physics and Astronomy program at U. Maryland, College Park, with which he was affiliated for his 45-year career. He enjoyed teaching, including a class on celestial navigation that ended with a night sail. In all, he mentored more than 15 grad students and just as many, if not more, postdocs. Some of his professional contributions are noted below.
In the course of his career, he held visiting appointments at Lowell Observatory, Flagstaff, AZ (1977), Max-Planck-Institut für Aeronomie, Katlenburg-Lindau Germany (1992), Max-Planck-Institut für Kernphysik Heidelberg Germany (1992), Institute for Nuclear and Particle Physics, Budapest Hungary (1992), Laboratoire d'Astronomie Spatiale, Marseilles France (1995), Vatican Observatory Summer School, Castel Gandolfo, Vatican (1997) and in 2015 he was Gauss Professor at Akademie der Wissenschaften zu Göttingen. In 2000, he became a Distinguished University Professor, with the honor of Emeritus Professor bestowed upon him in 2011.
Mike’s contributions to planetary sciences began with his study of the C2 densities in Comet Mrkos. Analyzing published photometry using a 4710 Å filter at the peak of C2 emission, he calculated molecular densities as a function of heliocentric distance and compared them to other comets. The results showed that Mrkos had the highest C2 density of any comet measured at that time and this molecular abundance varied by a factor of 10 among previously observed comets. Mike maintained a comet focus and went on to show from observing close to 100 comets (with colleagues R. Millis, and others) that C-bearing species can form the basis of a classification system of comets, that the percentage of active regions of comet nuclei varies from comet to comet, and that cometary nuclei bear a complex history of the solar system’s formation and evolution.
Mike pursued scientific research in response to the appearance of bright comets, for example Comets West, Bradfield, and Schwassman-Wachmann 1 in the 1970s and early 1980s. With collegial collaborations developed at telescopes both on the ground and in space, he, his graduate students and his post-docs studied the temporal variations of active cometary molecules and ions, explaining the observed brightness distribution of comae and tails as comets traveled through the inner solar system. His ground-based photometric studies of comets with Bob Millis at Lowell Observatory sprang from their friendship established while in graduate school. Millis went to Lowell Observatory, a private facility that provided abundant observing time, and together they garnered the necessary photometric measurements of fluorescent emission in comets to determine abundance ratios of spectrally active elements in cometary comae.
Highly productive scientific collaborations with Paul Feldman, William M. Jackson, Michel Festou, B. Donn, J. Rahe, and H.U. Keller, came about through a programmatic mandate after all of them wrote similar proposals to observe Comet Bradfield with the International Ultraviolet Explorer (IUE) satellite in 1979. Thus began their study of OH, CO2, CO+, CO, C, S and S2. The result was a decade of advances in understanding of the chemistry and structure of cometary comae using both ground- and space-based telescopes including: IUE, later the Hubble Space Telescope, the InfraRed Telescope Facility (IRTF) in Hawaii, and mm and radio telescopes in California and Hawaii. Mike first participated in cometary missions when the International Comet Explorer flew past Comet Giacobini-Zinner in 1985 and he led near-simultaneous observations with IUE and other astronomical telescopes then available. A steady stream of observable comets appeared over the next three decades. With his students and post-docs, he performed theoretical calculations of fluorescence in cometary species (Dave Schleicher), studied the morphology of CN jets in Comet Halley (Susan Hoban, J. J. Klavetter), and explored the complex rotational states of comets (Nalin Samarasinha). The behavior of other molecular species were studied; CO+ (Magnani), NH (S. Kim, A. Cochran), sulfur molecules: S2 (S. Kim, S. Larson), SO and SO2 (S. Kim) and SI (R. Meier).
In 1984, with programmatic support from NASA’s Planetary Astronomy program, Mike conceived a coordinated, world-wide observing campaign supporting the apparition of the famous Comet Halley in 1986. Scientists coordinated their observations optimizing the scientific return after completing its 76-year revolution around the Sun. From those observations, he and his collaborators observed that cometary activity was not uniform, though that had been suspected for some time; jetting activity could be mapped; and the impact of the non-gravitational forces could be separated from measurements of its rotational period, to name a few discoveries.
A second campaign of coordinated observing led by A’Hearn and L. McFadden was conducted for the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994. This event of planetary accretion showed dramatically the effect of tidal forces on a comet’s nucleus and the short-lived effects on the upper atmosphere and ionosphere of Jupiter. The images from telescopes from around the world contributed to a media sensation for a couple of weeks in July of 1994 as 26 fragments of comet Shoemaker-Levy 9 crashed into Jupiter’s upper atmosphere with repeated splashes that temporarily left orange-brown splotches in Jupiter’s upper atmosphere.
The results of systematic telescopic observations culminated in a milestone publication, The ensemble of properties of comets: Results from narrowband photometry of 85 comets, 1976-1992, which was published in 1995. In this paper, A'Hearn, R. L. Millis, D. G. Schleicher, D. J. Osip and P. V. Birch demonstrated that most comets are similar in chemical composition and that the dust-to-gas ratio does not vary with the dynamical age of the comet. There is no evidence of differentiation among cometary nuclei and there are significant compositional groupings related to place of origin within the solar system. Kuiper Belt (short period) comets are depleted in carbon-chain molecules, C2 and C3, their dust is redder than long-period comets and they exhibit brightness asymmetries about perihelion implying a smaller active fraction of their surface than long period comets.
In the late 1990s, significant comets passed through the inner solar system and Mike participated in their study (i.e., Comets Hale-Bopp, Hyakutake, 46P/Wirtanen – then the target of European Space Agency’s Rosetta mission – and Encke). Most of his time between 1998 and 2005 was spent leading the effort to win funding for a Discovery mission that was selected in 1999. He was Principal Investigator of the Deep Impact mission that successfully sent a spacecraft to Comet Tempel 1, penetrating the nucleus with a 500-kg impactor that allowed us to explore beneath the comet’s crust. The mission, which culminated on July 4, 2005, was a smashing success. An international observing campaign (led by K. Meech) was carried out to provide context for the mission’s target comet. The impact was stunning to say the least, and the team’s scientific expectations were exceeded. We witnessed multiple and frequent outbursts of gas and dust as the comet approached perihelion via observations of near-continuous observing from space. Gas and dust were ejected in a spectacular plume that allowed NASA’s Spitzer space telescope to detect many molecular species, indicating that the comet collected material from a wide range of temperature regimes in the solar system (led by C. Lisse). The coma was mapped with respect to its chemical composition of water and CO2 and their uncorrelated release from the nucleus (collaboration with L. Feaga). Additionally, the nucleus’s surface was observed to have a vast range of terrain types. Relationships between insolation, temperature, roughness and thermal inertia were untangled with colleagues O. Groussin and B. Davidsson. With P. Schultz, the porosity was constrained, and the composition of the ejecta showed no difference with time and, hence, depth. A region of water ice was found on the surface where thermodynamic expectations precluded it (with J. Sunshine).
After the spectacular flyby, the spacecraft, was still functional and NASA awarded funding for an extended mission named EPOXI to flyby another comet, 103P/Hartley 2, and also observe and characterize extra-solar planets. The flyby of Hartley 2 was accomplished in November 2010 and allowed a second comet nucleus to be studied with the same instruments (and the fifth to be observed via spacecraft) to examine the diversity of morphology, topography, and volatile production among comets. No impact was possible the second time around. Mike was actively inspiring investigations, while his protégés were leading the investigations in cometary photometry (J. Y. Li, T. Farnham), spectroscopy (L. Feaga, J. Sunshine), infrared thermal imaging (O. Groussin), and investigations of dust particles (M. S. Kelley, S. Protopapa). And with his last student, A. Gersch, Mike pursued modeling comets in the regime of an optically dense medium, a challenge he took up from Deep Impact data.
The European Space Agency’s Rosetta mission reached its target comet, 67/P Churyumov-Gerasimenko in 2014 and Mike was a team member of the mission’s visible camera (OSIRIS) camera and UV imaging spectrograph (named Alice). He was an active collaborator in the analysis of coma activity as the spacecraft approached, and studied its rotation state, structure of the nucleus, morphology of its coma and jets, and the distribution of dust and its UV composition. His publication list is replete with 30 papers related to Comet Churyumov-Gerasimenko after it arrived at the comet; he always contributed to data analysis, interpretation and offered deep thoughts about the implications of the results.
Most recently, Mike explored how cometary volatiles yield implications on solar system origins. He considered mixing, snow lines, formational history in the disk, and how these processes are interrelated. He also explored whether the cometary families showed different volatile trends. The Rosetta ROSINA results on noble gases and molecular O and N were of great interest to him. He continued to engage in spaceflight missions, research and authoring papers related to comets right up to the time of his death.
Mike A’Hearn led the pioneering efforts to establish the Small Bodies Node (SBN) of the Planetary Data System (PDS) through a NASA competitive selection in 1990. He hired the right people (E. Grayzeck, L. Kolokolova, A. Raugh) to form the core group assembled at the University of Maryland with well-managed subnodes at the Planetary Science Institute, led by D. Davis, and the University of Arizona, led by M. Sykes. He understood the value of a data archive through his professional experience, beginning with his participation in NASA’s International Halley Watch, which gathered data and preserved it for the future. Mike made sure that PDS efforts were open to the global planetary community. He married knowledge as a ground-based observer to his efforts in spacecraft missions. He was instrumental in keeping the focus on science and science users. This was evident by his role as senior member of the PDS Management Council that evolved over 28 years under Mike’s leadership and included the concept of data reviews to make sure the users of the future could understand the associated metadata. Mike had the vision to have the SBN assemble and manage disparate comet and asteroid data to provide useful databases for current researchers and mission planners. His strong leadership resulted in the SBN being an active data archive in the self-governing model of the PDS. Mike would memorably keep PDS meetings in order with a scientific focus by brandishing his cane (after hip replacement surgery). His passion and enthusiasm for SBN and PDS will be missed.
While walking at a good clip across University of Maryland campus one late-afternoon in the late 1990’s, all of a sudden Mike stopped and put his hand across his heart, turning to the flagpole. It took me (L. McFadden) a few steps to both break my momentum and to realize what he was doing. The flag was being lowered, and when I commented that I hadn't noticed it, he said, “It's from my years as a cub scout master.” Other evidence of good citizenship in Mike's professional world include being a Shapley lecturer (1985-89) for the American Astronomical Society; serving as the elected Vice Chair, Chair and Past-Chair of the Division for Planetary Science of the AAS (1992-94); and appointment as AAS Council representative to the U.S. National Committee to the International Astronomical Union (IAU). His longest service to the AAS was an eight-year stint on the Publications Board, first as a member (2001-2004), then as chair (2005-2008). During his tenure, he played a key role analyzing the various proposals received from many publishers that led to the positive outcome of selecting IOP Publishing, notes Kevin Marvel, AAS Executive Director. He also served on 5 National Academy panels, task groups, or Committees between 1989-2009. He was a member of the Association of Universities for Research in Astronomy's Board of Directors after serving on task groups for Hubble Space Telescope and for the Observatories Council as Vice-Chair and then Chair, serving for eight years in that capacity, too. For the IAU, he was a member of 7 working groups related to naming of comets, small body nomenclature, and cartographic coordinates, and 3 officer positions to Commission 15 (Working Group on Minor Planets and President of Division III, Planetary Systems Science). In doing all of this he set an example for his colleagues and students who also serve their communities to this day.
Mike made every single one of us feel special and loved. Those at University of Maryland can still hear his keys clanging at the rate of his gait, as if he were still walking down the hall to talk about comets, departmental issues, basketball or sailing.
Obituary Written By: Lucy McFadden (NASA Goddard Space Flight Center, Emerita) with thanks to Maxine A’Hearn, J. Trasco, V. Trimble, J. Cowan, P. Feldman, M. Belton, R. Millis, E. Grayzeck, L. Woodney, N. Samarasinha, L. Feaga, J. Sunshine.