54598 Bienor

54598 Bienor (provisional designation 2000 QC₂₄₃) is a centaur orbiting the Sun between Saturn and Uranus in the outer Solar System. Named after the mythological centaur Bienor, it was discovered on 27 August 2000 by the Deep Ecliptic Survey at Cerro Tololo Inter-American Observatory in Chile. Bienor has a highly elongated shape that spans up to 254 km (158 mi) across its longest dimension to 90 km (56 mi) across its shortest. It is one of the largest centaurs with a known size and is the fourth centaur whose stellar occultation was detected by multiple people simultaneously, after 10199 Chariklo, 2060 Chiron, and (95626) 2002 GZ32.

Like other centaurs, Bienor is believed to have originated from the region beyond Neptune in the outer Solar System, where the trans-Neptunian objects reside. The present-day orbit of Bienor is strongly influenced by the gravity of the giant planets, which makes it unstable and subject to ejection within a few million years. Observations have shown that the surface of Bienor is dark, colored gray, and contains some water ice, with potentially large amounts of organic compounds. Bienor's brightness periodically fluctuates as it rotates every 9.17 hours, although it exhibits several unusual behaviors such as gradual brightening in absolute magnitude (intrinsic brightness), which are less easily explained. The unusual behavior of Bienor's brightness alongside other anomalies such as radiometric overestimates of Bienor's diameter have led some astronomers to hypothesize that Bienor might either have an icy ring system, a natural satellite, or albedo variations across its surface.