989 Schwassmannia

989 Schwassmannia (prov. designation: or ) is a stony background asteroid and a slow rotator from the central regions of the asteroid belt, approximately 12.5 km in diameter. It was discovered on 18 November 1922, by astronomer Friedrich Karl Arnold Schwassmann at the Bergedorf Observatory in Hamburg, Germany. The bright S/T-type asteroid has a long rotation period of 107.9 hours. It was named after the discoverer himself.

Orbit and classification
Schwassmannia 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 central main-belt at a distance of 2.0–3.3 AU once every 4 years and 4 months (1,584 days; semi-major axis of 2.66 AU). Its orbit has an eccentricity of 0.25 and an inclination of 15° with respect to the ecliptic. The body's observation arc begins at the Heidelberg Observatory on 12 November 1922, just 6 days prior to its official discovery observation at the Bergedorf Observatory in Hamburg.

Naming
This minor planet was named after its discoverer, German astronomer Arnold Schwassmann (1870–1964), who discovered four comets and 22 asteroids in total (including this one). Schwassmann worked at the Potsdam (Berlin) and Hamburg–Bergedorf observatories. The official was mentioned in The Names of the Minor Planets by Paul Herget in 1955 (H 94).

Physical characteristics
In the Tholen- and SMASS-like taxonomy of the Small Solar System Objects Spectroscopic Survey (S3OS2), Schwassmannia is an S-type and T-type asteroid, respectively.

Rotation period
In November 2013, a rotational lightcurve of Schwassmannia was obtained from photometric observations by astronomers Vladimir Benishek at Sopot Astronomical Observatory, Serbia, American Frederick Pilcher at his Organ Mesa Observatory , New Mexico, and Luis Martinez at Lenomiya Observatory at Casa Grande in Arizona. Lightcurve analysis gave a well-defined rotation period of $12.2 km$ hours with a brightness amplitude of $12.63 km$ magnitude (U=3). Alternative observations with a lower rated quality by Robert Stephens at the Center for Solar System Studies in September 2013 gave a period of $12.86 km$ hours (U=2). The results supersede an earlier, tentative observation by Italian Federico Manzini at the Sozzago Astronomical Station from October 2004, with an incorrect period 4.5 hours (U=1).

Diameter and albedo
According to the surveys carried out by the Infrared Astronomical Satellite IRAS, the Japanese Akari satellite and the NEOWISE mission of NASA's WISE telescope, Schwassmannia measures between $107.85 h$ and $0.204$ kilometers in diameter and its surface has a high albedo between 0.20 and 0.31. The Collaborative Asteroid Lightcurve Link adopts the result from IRAS, that is, an albedo of 0.2037 and a diameter of 12.86 kilometers based on an absolute magnitude of 11.8.