896 Sphinx

Lore & Background

896 Sphinx orbits the Sun in the inner asteroid belt at a distance of 1.9–2.7 AU once every 3 years and 6 months. Its orbit has an eccentricity of 0.16 and an inclination of 8° with respect to the ecliptic. The body's observation arc began at Heidelberg Observatory on 9 October 1918, two months after its official discovery observation. It is a non-family asteroid of the main belt's background population when applying the hierarchical clustering method to its proper orbital elements.

Reader's Guide

896 Sphinx is notable as one of the few low-numbered asteroids for which no spectral type has been determined, despite its relatively high albedo suggesting it may be a common, stony S-type asteroid. Its rotation period has been measured with some uncertainty: photometric observations in June 2018 gave a period of 21.038 hours, though an alternative solution of 10.541 hours is also possible, and a modeled lightcurve from 2018 gave a divergent sidereal period of 12.95209 hours. Diameter estimates vary across surveys, ranging from about 11.97 km to 14.45 km, with corresponding albedos from 0.163 to 0.242. The asteroid's name, referencing the Sphinx of mythology, was cited in Paul Herget's 1955 work The Names of the Minor Planets. Its legacy lies in illustrating the ongoing challenges in characterizing even relatively bright, low-numbered asteroids.

Did You Know?

• 896 Sphinx was discovered by Max Wolf at the Heidelberg-Königstuhl State Observatory on 1 August 1918.
• It is one of few low-numbered asteroids for which no spectral type has been determined.
• The asteroid's rotation period has multiple possible solutions, including 21.038 hours and 10.541 hours.
• Diameter estimates from different surveys range from 11.974 km to 14.45 km.

Discovery and Orbital Path

896 Sphinx first entered the astronomical record on 1 August 1918, when German astronomer Max Wolf identified it from the Heidelberg-Königstuhl State Observatory in southwest Germany. The body's formal observation arc, however, does not begin until 9 October 1918, roughly two months after that initial detection. Classified as a non-family member of the main belt's background population—determined by applying hierarchical clustering to its proper orbital elements—Sphinx occupies the inner reaches of the asteroid belt. It completes one circuit of the Sun every 1,262 days, or about three and a half years, at a semi-major axis of 2.29 astronomical units. Its orbital path stretches between 1.9 and 2.7 AU from the Sun, with a moderate eccentricity of 0.16 and an inclination of just 8 degrees relative to the ecliptic plane. These parameters place it firmly among the inner-belt objects, a region that tends to host stony, silicate-rich bodies rather than the carbonaceous material more common in the outer belt.

Mythological Namesake

The designation 'Sphinx' links this modest inner-belt asteroid to one of the most enduring figures in ancient mythology. In both Greek and Egyptian traditions, the Sphinx is depicted as a female creature combining the head of a woman, the haunches of a lion, and the wings of a bird. According to legend, this monster posed a deadly riddle to travelers, killing anyone unable to provide the correct answer. The asteroid received this name as a formal citation recorded in Paul Herget's reference work The Names of the Minor Planets, published in 1955 under entry H 87. The choice of name is fitting in a symbolic sense: just as the mythological Sphinx guards a threshold with an enigmatic challenge, 896 Sphinx remains, in a scientific sense, partially veiled. Despite its relatively low number in the catalog of minor planets, it is one of the few objects in that early range for which no definitive spectral type has ever been established, leaving its true surface composition an open question for astronomers.

Size, Albedo, and Surface Composition

Estimating the physical dimensions of a distant asteroid like 896 Sphinx requires triangulating data from multiple space-based surveys. NASA's NEOWISE mission, the Infrared Astronomical Satellite IRAS, and Japan's Akari satellite each produced slightly different diameter and albedo figures. NEOWISE yielded a diameter of approximately 11.97 kilometers with an albedo near 0.242; IRAS gave 13.07 kilometers and an albedo of roughly 0.197; Akari reported 14.45 kilometers with an albedo around 0.163. The Collaborative Asteroid Lightcurve Link, working from an absolute magnitude of 11.6, derives a diameter of 13.17 kilometers and an albedo of 0.2332. Because no spectral type has been formally assigned—a notable gap for such a low-numbered body—scientists infer from the relatively high reflectivity and the asteroid's inner-belt location that it is likely a common stony S-type object, composed predominantly of silicate minerals and metallic iron.

Rotation and Photometric Studies

Understanding how fast 896 Sphinx spins on its axis has proven more complicated than a single measurement might suggest. In June 2018, astronomer Tom Polakis conducted photometric observations from the Command Module Observatory in Arizona, deriving a rotation period of 21.038 hours with a brightness swing of 0.16 magnitudes. Yet an alternative mathematical solution of 10.541 hours, with a similar amplitude, also fits the data. Both of these results superseded a tentative period first proposed by Laurent Bernasconi in September 2001. A separate 2018 modeled lightcurve, built from photometric data in the Lowell Photometric Database and infrared observations from the Wide-field Infrared Survey Explorer, produced yet another figure: a sidereal period of 12.95 hours, along with two possible spin-axis orientations in ecliptic coordinates. The spread among these values reflects the genuine difficulty of pinning down the rotation of a small, distant, and spectrally unclassified body.