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Passing of Frank Estabrook

With deep sadness we report the passing of our friend and colleague Frank B. Estabrook on October 16, 2019.

Frank was born in Boise, Idaho in 1922. He received a Ph. D. from Caltech in 1950. He came to NASA’s Jet Propulsion Laboratory in 1960, following appointments as a professor at Miami University and as a physicist for North American Aviation and the US Army Office of Ordinance Research. Frank brought to JPL a deep understanding of, and a love for, Einstein’s general relativity theory. In the mid to late 1960s, when few thought relativity relevant to anything practical, he understood that rapidly advancing technology would make relativity’s effects observable in JPL spacecraft tracking and in planetary orbits, and he played a lead role in inspiring his JPL colleagues to incorporate relativity into their work.

At JPL Frank achieved an international reputation for his own work in general relativity, differential geometry, soliton theory, and gravitational waves. With Hugo Wahlquist he reformulated Einstein’s general relativity equations into a “dyadic” form that is closely tied to physical measurements, and used this reformulation to gain new insights into Einstein’s theory. For the Caltech relativity group led by Kip Thorne, he was a valuable source of mathematical inspiration and advice from the 1960s to the 2000s.

On a more practical side, with Hugo Wahlquist he derived the exact response of spacecraft Doppler tracking (electromagnetically-tracked separated test masses) to gravitational waves and thus how Doppler tracking could be used as a detector in the low-frequency band. The Estabrook-Wahlquist response function is also central to the analysis of very-low-frequency pulsar-timing gravitational wave searches, and it is a foundation for deducing the response of the ultra-sensitive LISA (Laser Interferometer Space Antenna) detector to gravitational waves. Frank was co-inventor of “time delay interferometry”, a crucial enabling technology for LISA.

Frank’s theoretical work on detector response, and his analysis of noise and sensitivity, provided the scientific impetus for technical improvements in NASA’s Deep Space Network, leading to ~1000-fold improvement in Doppler tracking sensitivity between 1980-2000. He was a member of the Galileo radio science team (Principal Investigator for its gravitational wave experiment) and a member of the LISA Mission Definition Team. He lectured in general relativity and applied mathematics at Caltech, served on several NASA advisory committees, and was on the editorial board of the Journal of Mathematical Physics.

Frank had extensive scientific interests. In addition to his general relativity and mathematical physics work he was, for example, the first to propose deep sea drilling into the earth’s mantle. He was generous with his time and ideas. On a personal note, he was a man of broad classical culture and a lover of music, especially opera. A quick wit and elegant spirit, he was beloved by his friends and colleagues.

/s/ J. W. Armstrong, J. Kendall, M. Tinto, M. Vallisneri, R. Woo, K. Thorne

Further information on Frank’s life and work can be found in:

– https://resolver.caltech.edu/CaltechOH:OH_Estabrook_F [Caltech oral history project (2007); Frank in his own words]
– Estabrook, F.B. “Geophysical Research Shaft” Science, 124, 686 (1956) [proposed deep sea drilling] – https://dx.doi.org/10.1126/science.124.3224.686
– Estabrook, F. B. and Wahlquist, H. D. “Dyadic Analysis of Space-Time Congruences”, J. Mathematical Physics, 5, 1629 (1964) [Estabrook-Wahlquist reformulation of Einstein’s general relativity equations] – https://doi.org/10.1063/1.1931200
– Estabrook, F. B. and Wahlquist, H. D. “Response of Doppler Spacecraft Tracking to Gravitational Radiation,” General Relativity and Gravitation 6, 439-447 (1975) [Estabrook-Wahlquist response function for electromagnetically-tracked test masses] – https://link.springer.com/article/10.1007/BF00762449
– Estabrook, F. B., Tinto, Massimo, and Armstrong, J. W. “Time-Delay Analysis of LISA Gravitational Wave Data: Elimination of Spacecraft Motion Effects”, Phys. Rev. D, 62, 042002 (2000) [LISA time delay interferometry, including some practical instrumental effects] – https://doi.org/10.1103/PhysRevD.62.042002