28978 Ixion

28978 Ixion (Pronunciation respelling key, sometimes erroneously /ˈɪksiən/, IK-see-ən, from Ixīōn, ) is a Kuiper belt object discovered on May 22, 2001. Ixion is a plutino (an object that has a 2:3 orbital resonance with Neptune); its estimated diameter of 800 km makes it the third largest plutino. It is named after Ixion, a figure from Greek mythology; it previously had the provisional designation .

Physical characteristics

The latest spectroscopic results indicate that Ixion's surface is a mixture of dark carbon and tholin, which is a heteropolymer formed by irradiation of clathrates of water and organic compounds (see TNO spectra). Water ice absorption lines (1.5 and 2μm) were absent (Licandro et al. 2002). Unlike Varuna, Ixion does not show greater reflectivity for longer waves (the so-called red slope) in infrared.

Other than Pluto, Ixion was the first TNO discovered that was originally estimated to be larger than asteroid Ceres. But more recent estimates suggest that Ixion has a high albedo and is smaller than Ceres. It is more likely that 20000 Varuna or 50000 Quaoar may turn out to the first TNO discovered that is larger than Ceres.

The Very Large Telescope (VLT) has checked Ixion for cometary activity, but did not detect a coma.

Orbit

Ixion and Pluto follow similar but differently oriented orbits: Ixion’s perihelion is below the ecliptic whereas Pluto's is above it. Uncharacteristically for bodies locked in resonance with Neptune (such as Orcus), Ixion approaches Pluto with less than 20 degrees of angular separation. Ixion is currently crossing the ecliptic heading below, and will reach its perihelion in 2070. Pluto has passed its perihelion (1989) and is descending toward the ecliptic. Ixion's orbital period is almost 250 Earth years, about 0.5% larger than Pluto's.

See also

• Comparison of colours of trans-Neptunian objects

References