1691 Oort, provisional designation 1956 RB, is a rare-type carbonaceous Themistian asteroid from the outer region of the asteroid belt, approximately 33 kilometers in diameter. It was discovered on 9 September 1956, by German astronomer Karl Reinmuth and Dutch astronomer Ingrid van Houten-Groeneveld at Heidelberg Observatory in south-west Germany.
The dark C-type asteroid, classified as a rare intermediate CU-type in the Tholen taxonomy, is a member of the Themis family, a dynamical family of outer-belt asteroids with nearly coplanar ecliptical orbits. Oort orbits the Sun at a distance of 2.6–3.7 AU once every 5 years and 7 months (2,054 days). Its orbit has an eccentricity of 0.17 and an inclination of 1° with respect to the ecliptic. It was first identified as 1945 TD at Turku in 1945, extending body's observation arc by 14 years prior to its official discovery observation. Information about an earlier 1917-identification, A917 TD, is not available.
In February 2009, a rotational light-curve of Oort was obtained from photometric observations taken by French amateur astronomer René Roy. It gave a well-defined rotation period of 10.2705 hours with a brightness variation of 0.38 magnitude (U=3). An international study from 2013, published a concurring, modeled period of 10.2684 hours (n.a.).
According to the surveys carried out by the Japanese Akari satellite and NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, Oort measures 33.64 and 37.37 kilometers in diameter, and its surface has an albedo of 0.065 and 0.053, respectively. The Collaborative Asteroid Lightcurve Link assumes an albedo of 0.10 and calculates a diameter of 27.13 kilometers based on an absolute magnitude of 10.95.
It was named in honor of Dutch astronomer Jan Oort (1900–1992), director of the Leiden Observatory (1945–1970), president of the International Astronomical Union (1958–1961), and a well-known authority on stellar statistics and galactic structure. He overturned the idea that the Sun was at the center of the Milky Way. The Oort cloud, the outermost gravitationally bound region of the Solar System, was also named after him. Naming citation was published before November 1977 (M.P.C. 3023).