308 Polyxo is a main-belt asteroid that was discovered by Alphonse Borrelly on 31 March 1891, in Marseille. It is orbiting the Sun at a distance of 2.75 AU with a low orbital eccentricity (ovalness) of 0.04 and a period of 4.56 yr. The orbital plane is tilted at an angle of 4.36° to the plane of the ecliptic.

308 Polyxo is classified as a rare T-type asteroid, with a spectrum that bears some similarity to the Tagish Lake meteorite. A spectral feature at a wavelength of 3.0 μm suggests aqueous alteration of some surface materials. Photometric measurements reported in 1983 give a rotation period of 12.03 hours and a brightness variation of 0.20 in magnitude. The adaptive optics instrument at the W. M. Keck Observatory shows an oblate object with a diameter of 130 km. The size ratio between the major and minor axes is 1.26 ± 0.11. Light curves for this object suggests it has a very irregular shape.

Stellar occultation events were observed for this asteroid during 2000 and 2004. The resulting chords provided cross-section diameter estimates of 144.4 and 117.1 km, respectively.

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