Orbit and classification
The S-type asteroid is a member of the Flora family, one of the largest populations of stony asteroids in the main-belt. It orbits the Sun at a distance of 2.0–2.5 AU once every 3 years and 4 months (1,229 days). Its orbit has an eccentricity of 0.10 and an inclination of 5 ° with respect to the ecliptic. ''Sillanpää'' was first identified as at Simeiz Observatory in 1935, while its observation arc begins with its official discovery observation at Turku in 1938.Lightcurves
In March 2009, Czech astronomer Petr Pravec obtained a rotational light-curve from photometric observations at Ondřejov Observatory. It gave a well-defined rotation period of 9.6602 hours with a brightness variation of 0.55 magnitude (). One month later, a concurring period of 9.659 hours with an amplitude of 0.71 magnitude was obtained by Adrián Galád at Modra Observatory (). Photometric observations at the Palomar Transient Factory in December 2011. gave a 9.6597 hours and Δ0.59 in magnitude (). A modeled light-curve using data from the ''Uppsala Asteroid Photometric Catalogue'' and other data sources, gave a period of 9.65855 hours, as well as a spin axis of (129.0°, 76.0°) in ecliptic coordinates ().Diameter and albedo
According to the survey carried out by NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, ''Sillanpää'' measures between 7.35 and 8.76 kilometers in diameter, and its surface has an albedo between 0.21 and 0.327. The ''Collaborative Asteroid Lightcurve Link'' assumes an intermediate albedo of 0.24 – derived from 8 Flora, the largest member and namesake of this asteroid family – and calculates a larger diameter of 8.19 kilometers using an absolute magnitude of 12.6.Naming
This minor planet was named after one of the most famous Finnish writers, Frans Eemil Sillanpää (1888–1964), first Finnish writer to receive the Nobel Prize in Literature in 1939 ''(also see List of Laureates since 1901)''. The official was published by the Minor Planet Center on 20 February 1976 ().References
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