891 Gunhild

891 Gunhild (prov. designation: or ) is a large background asteroid from the outer regions of the asteroid belt, that measures approximately 58 km in diameter. It was discovered on 17 May 1918, by astronomer Max Wolf at the Heidelberg-Königstuhl State Observatory in southwest Germany. The dark D-type asteroid has a rotation period of 11.9 hours. Any reference of the asteroid's name to a person is unknown.

Orbit and classification
Gunhild 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 outer asteroid belt at a distance of 2.8–2.9 AU once every 4 years and 10 months (1,769 days; semi-major axis of 2.86 AU). Its orbit has an eccentricity of 0.03 and an inclination of 14° with respect to the ecliptic. The asteroid was first observed as  at Simeiz Observatory in November 1915. The body's observation arc begins at Heidelberg Observatory on 18 May 1918, the night after its official discovery observation.

Naming
This minor planet is named after a Feminine German first name. Any reference of this name to a person or occurrence is unknown.

Unknown meaning
Among the many thousands of named minor planets, Gunhild is one of 120 asteroids, for which no official naming citation has been published. All of these low-numbered asteroids have numbers between and  and were discovered between 1876 and the 1930s, predominantly by astronomers Auguste Charlois, Johann Palisa, Max Wolf and Karl Reinmuth.

Physical characteristics
In both the Tholen- and SMASS-like taxonomy of the Small Solar System Objects Spectroscopic Survey (S3OS2), as well as in the SDSS-based taxonomy, Gunhild is a dark D-type asteroid. This asteroid spectral type is common among outer belt asteroids and very common among the Jupiter trojan population.

Rotation period
In July 2015, a rotational lightcurve of Gunhild was obtained from photometric observations by the Spanish group of asteroid observers, OBAS. Lightcurve analysis gave a rotation period of $51.95 km$ hours with a brightness amplitude of $55.747 km$ magnitude (U=3−). The result supersedes period determinations of ($63.8 km$) by Robert Stephens in 2000, ($11.892 h$) by Laurent Bernasconi in 2005, and ($0.049$) by Janus Kozdon in 2015 (U=2/2/2+).

Diameter and albedo
According to the survey carried out by the Infrared Astronomical Satellite IRAS, the Japanese Akari satellite, and the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer (WISE), Gunhild measures ($0.057$), ($0.072$) and ($11.892$) kilometers in diameter and its surface has an albedo of ($0.36$), ($7.93 h$) and ($11.853 h$), respectively. The Collaborative Asteroid Lightcurve Link derives an albedo of 0.0656 and a diameter of 51.89 kilometers based on an absolute magnitude of 10.0. Alternative mean-diameter measurements published by the WISE team include ($10.556 h$), ($51.95$) and ($55.747$) with corresponding albedos of ($63.8$), ($0.072$) and ($0.057$). Several asteroid occultation observed between 2010 and 2014 gave a best-fit ellipse dimension of 52.0&thinsp;×&thinsp;52.0 kilometers (one observation) and 63.0&thinsp;×&thinsp;63.0 kilometers (three observations). These timed observations are taken when the asteroid passes in front of a distant star.