918 Itha

918 Itha (prov. designation: or ) is a stony asteroid and the namesake as well as the principal body of the Itha family, located in the outer region of the asteroid belt. It was discovered by German astronomer Karl Reinmuth at the Heidelberg Observatory on 22 August 1919. The S-type asteroid has a notably short rotation period of only 3.5 hours and measures approximately 21 km in diameter. It was named "Itha", a female name taken from the almanac Lahrer Hinkender Bote, unrelated to the discoverer's contemporaries.

Orbit and classification
When applying the synthetic hierarchical clustering method (HCM) by Nesvorný, Itha is the principal body and namesake of the Itha family (633), a small family of stony asteroids with only a few dozen known members. Other larger and low-numbered members of this family include 968 Petunia, 1067 Lunaria, 3787 Aivazovskij, 4119 Miles and 5232 Jordaens. However, according to another HCM-analysis by Milani and Knežević (AstDys), Itha is a background asteroid as it is not a legitimate family in their analysis. It orbits the Sun in the outer asteroid belt at a distance of 2.3–3.4 AU once every 4 years and 10 months (1,770 days; semi-major axis of 2.86 AU). Its orbit has an eccentricity of 0.19 and an inclination of 12° with respect to the ecliptic. Itha was first observed as  at the U.S. Taunton Observatory  in Massachusetts. The body's observation arc begins at Heidelberg Observatory on 23 August 1919, the night after its official discovery observation.

Naming
This minor planet was named "Itha", picked from the Lahrer Hinkender Bote, an almanac which was published in Lahr, southern Germany. Especially in the alemannic-speaking region, a Hinkender Bote (lit. "limping messenger") was very popular from the late 17th throughout the early 20th century. The calendar section contains feast days, the dates of important fairs and astronomical ephemerides. For 15 January, the calendar gives "Itha" as the German name day analogue next to Maurus and Habakuk, the protestant and catholic entries in the calendar of saints, likely referring to Saint Maurus and prophet Habakkuk.

Reinmuth's calendar names
As with 22 other asteroids – starting with 913 Otila, and ending with 1144 Oda – Reinmuth selected names from this calendar due to his many asteroid discoveries that he had trouble thinking of proper names. These names are not related to the discoverer's contemporaries. Lutz Schmadel, the author of the Dictionary of Minor Planet Names learned about Reinmuth's source of inspiration from private communications with Dutch astronomer Ingrid van Houten-Groeneveld, who worked as a young astronomer at Heidelberg.

Physical characteristics
Itha is an S-type asteroid in the SMASS-I classification by Xu (1995). This agrees with the overall spectral type for members of the Itha family.

Rotation period
In June 2011, a rotational lightcurve of Itha was obtained from photometric observations by Julian Oey at Kingsgrove Observatory, Australia, in collaboration with three Argentinian astronomers. Lightcurve analysis gave a well-defined rotation period of $20.44 km$ hours with a brightness variation of $21.37 km$ magnitude (U=3). Two modeled lightcurves using photometric data from the Lowell Photometric Database and from the BlueEye600 robotic telescope at Ondřejov Observatory gave a sidereal period of $21.561 km$ and $3.474 h$, respectively. The modelling of the former gave two poles at (59.0°, −59.0°) and (249.0°, −72.0°), while the one of the latter gave only one pole at (72.0°, −54.0°) in ecliptic coordinates (λ,&thinsp;β).

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 Wide-field Infrared Survey Explorer (WISE), Itha measures ($0.203$), ($0.222$) and ($0.224$) kilometers in diameter, and its surface has an albedo of ($3.474$), ($0.3$) and ($3.474$), respectively. The Collaborative Asteroid Lightcurve Link derives an albedo of 0.2412 and a diameter of 20.53 km based on an absolute magnitude of 10.6. Other published mean-diameters by the WISE team include ($3.474$) and ($20.44$) with corresponding albedos of ($21.37$) and ($21.561$).