Kaare was born on 25 March 1938 in Kvam in Hardanger and grew up in the same village as Norway’s noted astrophysicist Svein Rosseland. He began graduate studies in astronomy at the University of Oslo during the early days of spaceflight and, with Rosseland as his supervisor, wrote a thesis on satellite orbits. He received his cand.real. degree in 1963 for “On the Motion of a Near‑Earth Satellite Around the Oblate Earth.” After a short period as a research assistant at the Solar Observatory at Harestua, he travelled to the United States, spent time at the Smithsonian Observatory in Massachusetts, and became a doctoral student at Yale University, where he developed new methods for calculating the orbits of satellites and space probes. He defended his Ph.D. in 1969 with the dissertation “A Second Order Solution of an Artificial Earth Satellite Based on an Intermediate Orbit”.
After completing his Ph.D., Kaare spent two years at NASA’s Jet Propulsion Laboratory in California before accepting a permanent position in 1971 at the Harvard–Smithsonian Center for Astrophysics in Massachusetts. At both institutions he refined methods for computing spacecraft trajectories and broadened his research to include natural small bodies in the Solar System, such as moons and asteroids. He showed that precise timing of mutual eclipses and occultations among the larger moons of Jupiter—and similarly for Saturn’s moons—could greatly improve orbital determinations. That work proved crucial for planning the Voyager 1 and 2 flybys of the Jupiter system in 1979 and the Saturn system in 1980–1981. Kaare not only developed the theory but also took part in the observations that enabled the Voyager probes to pass close to the moons.
In 1978 Kaare returned to Norway with his family to join the Norwegian Defence Research Establishment (FFI) as a researcher and later head of the Mathematics Section. At FFI he and colleagues developed the Satellite Doppler Positioning Program, which used Doppler signals from communication satellites to determine ground positions — a method successfully employed by the Joint Rescue Coordination Centre. Kaare led FFI’s work on orbital calculations and on using navigation satellites for precise terrestrial positioning, a capability that also proved vital for Earth‑observation missions. He also headed a project to detect ships using satellite‑based synthetic aperture radar. From 1980 he served as Professor II (adjunct professor) at the University of Tromsø, allowing him to continue part‑time basic research, teaching, and student supervision.
After ten years at FFI, Kaare was appointed professor of astronomy in 1988 at the Institute of Theoretical Astrophysics, University of Oslo, where he continued and expanded his previous research. Together with students he observed mutual eclipses and occultations of Jupiter’s moons—first with the old 30 cm telescope at the Solar Observatory at Harestua and later with the Nordic Optical Telescope in the Canary Islands. With colleagues at FFI and the Norwegian Mapping Authority he further developed methods for extremely precise satellite‑based positioning.
From the early 1990s Kaare and his Ph.D. students focused on a major challenge posed by ESA’s Rosetta mission: how to place and manoeuvre a spacecraft around a small, irregularly shaped, low‑mass comet nucleus. Because a comet’s gravity is weak and uneven and perturbations from other bodies are significant, keeping a probe in orbit without excessive fuel use was far from obvious. This problem became a central research topic for Kaare’s group. Rosetta launched in 2004, reached comet 67P/Churyumov–Gerasimenko in 2014, and remained in orbit for two years before its planned impact; the mission’s orbital calculations proved correct.
During his twenty years as professor at the University of Oslo, Kaare was a highly regarded student supervisor. He guided about 13 students to their cand.scient. or master’s degrees and supervised six Ph.D. candidates to completion.
In 1993 he became chair of the University’s almanac committee and editor of Almanakk for Norge, a role he continued as professor emeritus until 2013. He improved the almanac’s astronomical tables and edited the volume Almanakkens historie (History of the Almanac), published in 2011. From 1991 to 2006 he chaired the International Astronomical Union’s working group responsible for naming Solar System bodies and surface features.
Honors include the naming of asteroid 2067 Aksnes (1978); Yale University’s Brouwer Prize for his doctoral dissertation (1970); a NASA Group Achievement Award for the Voyager trajectory calculations (1981); election to the Norwegian Academy of Science and Letters (1991); and the King’s Medal of Merit in Gold (2006).