810 Atossa (prov. designation: or ) is a bright and elongated background asteroid from the region of the Flora family, located in the inner portion of the asteroid belt. It was discovered on 8 September 1915, by German astronomer Max Wolf at the Heidelberg-Königstuhl State Observatory in southern Germany. The presumed S-type asteroid has a rotation period of 4.4 hours and measures approximately in diameter. It was named after the ancient Persian queen Atossa (550–475 BC). Orbit and classification Atossa is a non-family asteroid of the main belt's background population when applying the synthetic hierarchical clustering method (HCM) by Nesvorný to its proper orbital elements. However, in an older HCM-analysis by Zappalà from 1995, this asteroid is considered a member of the Flora family (), a giant asteroid family and the largest family of stony asteroids in the main-belt. In a third HCM-analysis by Milani and Knežević (AstDyS), it is also a background asteroid, as this analysis does not recognize the Flora asteroid clan as a proper family. Atossa orbits the Sun in the inner asteroid belt at a distance of 1.8–2.6 AU once every 3 years and 3 months (1,174 days; semi-major axis of 2.18 AU). Its orbit has an eccentricity of 0.18 and an inclination of 3° with respect to the ecliptic. The body's observation arc begins at Heidelberg Observatory with its official discovery observation on 8 September 1915. Naming This minor planet was named after Atossa (550–475 BC), an ancient Persian queen, daughter of Cyrus, wife of Darius. The was also mentioned in The Names of the Minor Planets by Paul Herget in 1955 (). The asteroids 7209 Cyrus and 7210 Darius were named after her father and husband, respectively. Physical characteristics Atossa is an assumed, stony S-type asteroid, based on its high albedo (see below) and its proximity or potential membership to the stony Flora family. Rotation period In August 2005, a rotational lightcurve of Atossa was obtained from photometric observations by French amateur astronomer Philippe Baudoin. Lightcurve analysis gave a well-defined rotation period of hours with a high brightness variation of magnitude, indicative of a non-spherical, elongated shape (). In 2011, a modeled lightcurve using data from the Uppsala Asteroid Photometric Catalogue (UAPC) and other sources gave a sidereal period hours, as well as two spin axes at (12.0°, 67.0°) and (188.0°, 69.0°) in ecliptic coordinates (λ, β). Diameter and albedo According to the survey