28978 Ixion is a trans-Neptunian object in the Kuiper belt. It is considered to be a dwarf planet by some astronomers, but the IAU has not formally designated it as such.
Ixion is possibly the 24th largest TNO and 4th largest Plutino currently known.
Discovery
Ixion was discovered on May 22, 2001 by Deep Ecliptic Survey (DES) at the Cerro Tololo Inter-American Observatory in the Atacama Desert of northern Chile.
Other than Pluto, Ixion was the first TNO discovered that was originally estimated to be larger than asteroid Ceres. Even in 2002, a year after its discovery, Ixion was still believed to be more than 1000 km in diameter. More recent estimates suggest that Ixion has a high albedo and is smaller than Ceres. Observations of Ixion by Spitzer Space Telescope in the far-infrared part of the spectrum revealed that its size is about 650 km.
Naming
Ixion the TNO is named after Ixion, a figure from Greek mythology. The name was suggested by E. K. Elliot.
In Greek mythology, Ixion was king of the Lapiths, the most ancient tribe of Thessaly. By killing his father-in-law, Ixion was reckoned the first man guilty of kin-slaying in Greek mythology. That alone would warrant him a terrible punishment.
However, Zeus had pity on Ixion and brought him to Olympus and introduced him at the table of the gods. Instead of being grateful, Ixion grew lustful for Hera, Zeus's wife, a further violation of guest-host relations.
Zeus thwarted this by creating the cloud Nephele, which resembled Hera and by whom Ixion fathered the Centaurs. For his crimes Ixion was bound to a wheel that turns forever in the underworld.
Stats
Estimated Diameter: 650 km (430 – 910 km)
Aphelion: 49.27 AU
Perihelion: 29.73 AU
Semi-major axis: 39.42 AU
Orbital Period: 247.53 years
Rotation period: ? hrs
Date discovered: 2001.5.22
Satellite: 0
Classification: TNO, KBO - Plutino
Orbit
Ixion is a plutino, locked in a 2:3 resonance with Neptune, making two revolutions around the Sun, while Neptune makes three.
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 does demonstrate some regular changes in brightness, which are thought to be caused by its rotation. As of 2010, however, the rotation period of Ixion remains undetermined.
Physical characteristics
Ixion is moderately red in visible light and has a surface made of a mixture of tholin and water ice.
Ixion has a higher albedo (>0.15) than the mid-sized red cubewanos. There may be an absorption feature at the wavelength of 0.8 μm in its spectrum, which is commonly attributed to the alteration of surface materials by water.
Both visible and infrared spectroscopic results indicate that Ixion's surface is a mixture of water ice, dark carbon and tholin, which is a heteropolymer formed by irradiation of clathrates of water and organic compounds.
The Very Large Telescope (VLT) has checked Ixion for cometary activity, but did not detect a coma. Ixion is currently about 41 AU from the Sun, and it is possible that Ixion could develop a coma or temporary atmosphere when it is closer to perihelion.
How big is it?
Ixion is so far away in the outer solar system that we don't know for sure how large it is. Because all we see is a dot of light, which is sunlight reflected off the surface of the TNO. But we don't know if the object is bright because it is large or if it is bright because it is highly reflective or both.
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