William (Bill) R. Ward passed away on September 20, 2018, at his home in Prescott, Arizona, after a battle with brain cancer. Ward was a preeminent theoretician that made many seminal contributions to our understanding of planetary dynamics and solar system formation. With his thesis advisor, Peter Goldreich, Ward proposed that planetesimals were formed via local gravitational instability in the protostellar disk. In 1973, Ward was the first to recognize that the obliquity of Mars undergoes large oscillations, and with Alastair Cameron in 1976, he was one of the original proposers of the giant impact theory for the origin of the Moon. Ward was a pioneer in the study of gravitational interactions between planets and their precursor gas disk, and how these may cause large-scale changes in planetary orbits. His many papers on this topic elucidated the nature of Type I vs. Type II migration, central to our understanding of planet formation in our solar system and in exoplanetary systems. Ward also contributed greatly to our understanding of satellite formation and dynamical evolution.

Ward earned bachelor’s degrees in Mathematics and in Physics (with highest distinction) at the University of Missouri, Kansas City, in 1968. He completed his Ph.D. in Planetary Sciences at the California Institute of Technology (Caltech) in 1972, then worked as a post-doc at the Harvard Center for Astrophysics before moving to the Jet Propulsion Laboratory. He joined the Southwest Research Institute (SwRI) in Boulder, Colorado, 1998, and retired from SwRI as an Institute Scientist in 2014.

Ward was the recipient of numerous honors and awards during his career, including being named a Fellow of the American Academy of Arts and Sciences (2012), a Fellow of the American Geophysical Union (2005), a Fellow of the American Association for the Advancement of Science (2006), and an Alumnus of Distinction from the University of Missouri, Kansas City (2006). He also received the Kuiper Prize from the Division of Planetary Sciences of the American Astronomical Society (2011), the Brouwer Award from the Division of Dynamical Astronomy of the American Astronomical Society (2004), and had the honor of having an asteroid named after him, (7812) Billward = 1984 UT (1999).

— Portions of text courtesy of Robin Canup, Southwest Research Institute