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Benjamin H. Zellner, III (1942–2021)

Zellner was among the first to use the Hubble Space Telescope to image Main Belt asteroids and several planetary satellites.

Published onDec 08, 2022
Benjamin H. Zellner, III (1942–2021)

Photo credit: A.V. Holm.

Asteroid researcher Benjamin Holmes Zellner III passed away in Culloden, Georgia on Monday December 6, 2021. He was 79.

Benjamin Holmes Zellner III was born on his paternal grandfather’s farm in Forsyth, Georgia on April 16, 1942; the farm has been in the family for nearly 200 years. His parents were B. H. Zellner II, an electrical wiring contractor, and Euna Virginia Dumas, a third-grade teacher for over 30 years. Ben went to Mary Person High School in Forsyth, graduating in 1960 as the class president, and to Georgia Tech where he majored in physics and obtained his B.S. in 1964. He earned his Ph.D. at the University of Arizona in 1970 with the thesis titled “Polarization in Reflection Nebulae”; his adviser was Tom Gehrels.

Ben stayed on as a research associate at the Lunar and Planetary Laboratory (LPL) of the University of Arizona until the mid-1980’s when he moved to the Space Telescope Science Institute (STScI) in Baltimore, Maryland. He was a dedicated observer and prolific writer, authoring or co-authoring over 100 articles while at LPL. His early papers were on the polarization and grain properties in several novae, but by 1973 and thereafter he focused entirely on polarization and photometry of asteroids and planetary satellites. An early highly-cited paper (with C. R. Chapman and D. Morrison) was “Surface Properties of Asteroids: A Synthesis of Polarimetry, Radiometry, and Spectrophotometry” [1] for 110 asteroids, finding that:

“More than 90% of the minor planets fall into two broad compositional groups, defined by several optical parameters, designated by the symbols C and S. Comparisons with meteorite spectral albedo curves suggest that the two groups are compositionally similar to carbonaceous and stony-metallic meteorites, respectively. C-type asteroids predominate in the belt, especially in the outer half. An unusual distribution of compositions is found between 2.77 and 3.0 AU. Many S-type objects have diameters of 100-200 km; C-type objects are much more common at both larger and smaller sizes. Vesta is unique, being apparently the only differentiated asteroid remaining intact in the belt.”

His most-cited paper (with co-authors D. J. Tholen and E. F. Tedesco) was “The eight-color asteroid survey: Results for 589 minor planets” [2]. The main finding was:

“The general evolution of predominant compositional type from S to C to D with increasing heliocentric distance is evident, as is the spectral homogeneity of the Eos, Koronis, Nysa, and Themis families.”

His most-cited, sole-authored paper was “The polarization of Titan” [3] in which he showed his observations were consistent only with an opaque cloud layer with a strongly UV-absorbing component. Another well-cited paper was “Physical properties of asteroid 433 Eros” [4] where he marshalled many types of observations to determine the dimensions, albedo, rotation and surface properties of Eros making it probably identifiable with H-type ordinary chondrites.

Another of Ben’s major contributions during this period was the creation and maintenance of the Tucson Revised Index of Asteroid Data (TRIAD), which contained all reliable orbital and physical parameters for minor planets including near-infrared, thermal-radiometric and eight-color results.

In July 1985 he left LPL to accept a position as operations astronomer with the Computer Sciences Corporation, a contractor to AURA at STScI in Baltimore, where he provided support for the Hubble Space Telescope. He worked in the Observation Support System (OSS) branch developing and testing software to observe Solar System objects — ‘moving targets’ in the lingo. This was a few years before the 1990 launch, and the time was well spent in developing a robust system. Other minor planet astronomers in the group were Eddie Wells and Charles Kowal. After launch, they and others in OSS monitored the instruments and data stream in real time on a 24/7 basis to ensure successful observations. Ben was at the console when the first images came in after the COSTAR correcting mirrors were installed during the first servicing mission in December 1993.

Zellner and others were eager to use HST to image asteroids, and well before launch he, Wells, C. Chapman and D. Cruikshank wrote:

“The Hubble Space Telescope will provide asteroid observers with exceptional opportunities to make observations not possible from the ground. It will provide expanded wavelength coverage in the ultraviolet, plus high spatial resolution permitting observations of body shape, configuration and compositional variations at geophysically and cosmochemically relevant scales for the nearest and largest objects.” [5]

He led teams that obtained telescope time in several early cycles discovering, e.g., a large impact crater with central peak at Vesta’s south pole; the paper was published in Science with the image on its cover. In 2011 the orbiting Dawn spacecraft resolved it into two, nearly overlapping craters of different ages. Proposals with Alex Storrs of AURA and Towson University to search for companions to several asteroids found one in 2001 orbiting 107 Camilla. Observations with the Faint Object Spectrograph of Phobos and Deimos showed that they were analogous to the D-type asteroids. And successful proposals with Dan Pascu of the U.S. Naval Observatory led to improved orbits for the inner satellites of Uranus and Neptune discovered by Voyager 2.

In 1994 Ben fulfilled a desire to teach and to return to his native state by accepting a position as professor of astronomy and planetarium director at Georgia Southern University in Statesboro. He was very active and popular with his students and initiated a series of lectures and observing sessions for the public. As one former student assistant wrote:

“His students loved him. He was very passionate about his students learning astronomy, and he brought it to life in class and in his labs. All lab students learned how to operate basic 8-inch Newtonian reflectors with finder scopes on the rooftop of the Math/Physics Building, no computers involved to guide the scope. They were in awe when those photons of light from Jupiter or the Andromeda Galaxy hit their own retinas through the eyepiece.”

He retired in June 2004 after also serving the professional astronomy community for many years through participation on various NSF and NASA review panels and in refereeing papers.

In retirement he remained active in his church and taught Sunday School. A favorite hobby throughout his life was wood-working and building furniture. His wife, Ida Abercrombie Zellner, predeceased him by many years. He is survived by their daughter, Susan Z. Mundy of Churchville, Virginia and son George Andrew Zellner of Bridgeport, Alabama and grandchildren Alex, Beth, and Samantha Mundy.

Acknowledgements: It is a pleasure to thank B. Lowder, D. Pascu, J. Secrest, A. Storrs, E. Wells and G. A. Zellner for their input.

See also Zellner’s AstroGen entry.

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