1536 Pielinen, provisional designation 1939 SE, is a stony Florian asteroid from the inner regions of the asteroid belt, approximately 7.8 kilometers in diameter. It was discovered on 18 September 1939, by Finnish astronomer Yrjö Väisälä at Turku Observatory, Southwest Finland.^[10] It was later named for Finnish lake Pielinen.^[2]

Pielinen is a member of the Flora family, a large group of stony S-type asteroids. It orbits the Sun in the inner main-belt at a distance of 1.8–2.6 AU once every 3 years and 3 months (1,195 days). Its orbit has an eccentricity of 0.20 and an inclination of 2° with respect to the ecliptic.^[1] Pielinen was first identified as A903 SF at Heidelberg in 1903, extending the body's observation arc by 36 years prior to its official discovery observation.^[10]

From September to November 2011, four rotational lightcurves of Pielinen were obtained from photometric observations. One lightcurve analysis gave a rotation period of 66.2 hours,^{[lower-alpha 3]} which is significantly longer than for most minor planets, that spin every 2 to 20 hours around their axis. However, slow rotators have periods typically above 100 hours. Photometric observations were taken by Petr Pravec (66.22 hours, Δ0.85 mag, U=3),^{[lower-alpha 1]} Robert D. Stephens (66.34 hours, Δ0.80 mag, U=3-),^{[lower-alpha 2]} Giovanni Casalnuovo (66.1 hours, Δ0.75 mag, U=2+),^[7] and Silvano Casulli (67.43 hours, Δ0.81 mag, U=2).^[8]

According to the space-based survey carried out by NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, Pielinen measures between 7.38 and 7.975 kilometers in diameter and its surface has an albedo between 0.253 and 0.30.^[4][5][6] The Collaborative Asteroid Lightcurve Link assumes an albedo of 0.24 – derived from 8 Flora, the largest member and namesake of this family – and calculates a diameter of 7.82 kilometers, with an absolute magnitude of 12.7.^[3]

This minor planet is named after Pielinen, Finland's fourth largest lake in Finnish Karelia. The Koli National Park is located on its western shores.^[2] The official naming citation was published by the Minor Planet Center on 20 February 1976 (M.P.C. 3930).^[11]