856 Backlunda

856 Backlunda (prov. designation: or ) is a dark background asteroid from the inner region of the asteroid belt. It was discovered on 3 April 1916, by Russian astronomer Sergey Belyavsky at the Simeiz Observatory on the Crimean peninsula. The carbonaceous C-type asteroid has a rotation period of 12.1 hours and measures approximately 45 km in diameter. It was named after Swedish-Russian astronomer Oskar Backlund (1846–1916).

Orbit and classification
Backlunda is a non-family asteroid of the main belt's background population when applying the hierarchical clustering method to its proper orbital elements. It orbits the Sun in the inner asteroid belt at a distance of 2.1–2.7 AU once every 3 years and 10 months (1,388 days; semi-major axis of 2.43 AU). Its orbit has an eccentricity of 0.12 and an inclination of 14° with respect to the ecliptic. The asteroid was first observed as at Taunton Observatory  in February 1908. The body's observation arc begins at Algiers Observatory in North Africa on 28 February 1931, almost 15 years after to its official discovery observation at Simeiz.

Naming
This minor planet was named after Swedish-born Russian astronomer Oskar Backlund (1846–1916), who is known for studying the orbit of comets, in particular that of Comet Encke. The was mentioned in The Names of the Minor Planets by Paul Herget in 1955 (H 84). The astronomer is also honored by the 75-kilometer lunar crater Backlund.

Physical characteristics
In the Bus–Binzel SMASS classification, Backlunda is a common carbonaceous C-type asteroid. It is also a C-type in the MOVIS catalog of the VISTA Hemisphere Survey conducted with the VISTA telescope at Paranal Observatory in Chile.

Rotation period and poles
In February 1984, a rotational lightcurve of Backlunda was obtained from photometric observations by Richard Binzel. Lightcurve analysis gave a rotation period of $43.43 km$ hours with a brightness variation of $45.449 km$ magnitude (U=2).

In May 2019, an alternative period determination of $12.08 h$ hours with an amplitude of $0.045$ magnitude was made by Tom Polakis at the Command Module Observatory in Arizona (U=2). Additional, tentative lightcurves gave a period of ($0.05$) by French amateur astronomer Laurent Bernasconi in July 2004, ($0.68$) by Jean-Gabriel Bosch and Axel Martin in March 2007, and ($0.411$) by Bruno Christmann, David Augustin and Raoul Behrend in July 2019 (U=2/2/2).

In 2016, a modeled lightcurve gave a concurring sidereal period of $12.08$ hours using data from a large collaboration of individual observers (such as above). The study also determined two spin axes of (42.0°, 44.0°) and (226.0°, 73.0°) in ecliptic coordinates (λ,β).

Diameter and albedo
According to the surveys carried out by the Japanese Akari satellite and the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, Backlunda measures ($0.29$) and ($11.965$) kilometers in diameter and its surface has an albedo of ($0.09$) and ($12.05 h$), respectively. The Collaborative Asteroid Lightcurve Link assumes a standard albedo for carbonaceous asteroids of 0.057 and derives a diameter of 40.51 kilometers based on an absolute magnitude of 10.69. Alternative mean-diameter measurements published by the WISE team include ($12.028 h$), ($12.05 h$), ($12.029$), ($43.43$) and ($45.449$) with corresponding albedos of ($0.05$), ($0.045$), ($42.354 km$), ($45.94 km$) and ($47.09 km$). Between 2003 and 2009, several asteroid occultations of Backlunda were observed. The best-rated observations from March 2003, October 2005 and August 2008, gave a best-fit ellipse dimension of ($49.222 km$), ($67.06 km$) and ($0.039$), respectively. These timed observations are taken when the asteroid passes in front of a distant star.