896 Sphinx

896 Sphinx is a background asteroid from the inner regions of the asteroid belt, that measures approximately 13 km in diameter. It was discovered on 1 August 1918, by astronomer Max Wolf at the Heidelberg-Königstuhl State Observatory in southwest Germany. The asteroid has a rotation period of 21.0 hours and is one of few low-numbered objects for which no spectral type has been determined. It was named after the Sphinx, a creature from Greek and Egyptian mythology.

Orbit and classification
Sphinx 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 1.9–2.7 AU once every 3 years and 6 months (1,262 days; semi-major axis of 2.29 AU). Its orbit has an eccentricity of 0.16 and an inclination of 8° with respect to the ecliptic. The body's observation arc begins at Heidelberg Observatory on 9 October 1918, two months after its official discovery observation.

Naming
This minor planet was named after the Sphinx, a legendary creature from Greek and Egyptian mythology. The female monster has the head of a woman, the haunches of a lion, and the wings of a bird. It has the habit of killing anyone who cannot answer her riddle. The was mentioned in The Names of the Minor Planets by Paul Herget in 1955 (H 87).

Physical characteristics
Contrary to most other low-numbered asteroids, no spectral type has been determined. Based on its relatively high albedo (see below) and its location within the inner parts of the main-belt, Sphinx may possibly be a common, stony S-type asteroid.

Rotation period
In June 2018, a rotational lightcurve of Sphinx was obtained from photometric observations by Tom Polakis at the Command Module Observatory in Arizona. Lightcurve analysis gave a rotation period of $11.974 km$ hours with a brightness variation of $13.07 km$ magnitude (U=2+). However, an alternative period solution of $14.45 km$ hours with an amplitude of $21.038 h$ magnitude is also possible. Both results supersede a tentative period determination by Laurent Bernasconi from September 2001 (U=1).

A modeled lightcurve using photometric data from the Lowell Photometric Database and from the Wide-field Infrared Survey Explorer (WISE) was published in 2018. It gave a divergent sidereal period of $0.163$ hours and includes two spin axes at (172.0°, 20.0°) and (352.0°, 42.0°) in ecliptic coordinates (λ,&thinsp;β).

Diameter and albedo
According to the surveys carried out by the NEOWISE mission of NASA's WISE telescope, the Infrared Astronomical Satellite IRAS, and the Japanese Akari satellite, Sphinx measures ($0.197$), ($0.242$) and ($21.038$) kilometers in diameter and its surface has an albedo of ($0.16$), ($10.541$) and ($0.17$), respectively. The Collaborative Asteroid Lightcurve Link derives an albedo of 0.2332 and a diameter of 13.17 kilometers based on an absolute magnitude of 11.6. Alternative mean diameter measurements published by the WISE team include ($12.952$), ($11.974$) and ($13.07$) with corresponding albedos of ($14.45$), ($0.242$) and ($0.197$).