28978 Ixion (// ik-SY-ən), provisional designation 2001 KX76, is a plutino (an object that has a 2:3 orbital resonance with Neptune). Brown and Tancredi calculate that it is very likely to be a dwarf planet, although the IAU has not officially classified it as such. It was discovered on May 22, 2001 by the Cerro Tololo Inter-American Observatory. It is named after Ixion, a figure from Greek mythology.
|Discovered by||Deep Ecliptic Srvy.|
|Discovery site||Cerro Tololo Obs.|
|Discovery date||22 May 2001|
|MPC designation||(28978) Ixion|
|TNO  · plutino|
|Orbital characteristics |
|Epoch 4 September 2017 (JD 2458000.5)|
|Uncertainty parameter 3|
|Observation arc||31.94 yr (11,665 days)|
|Earliest precovery date||17 July 1982|
|249.80 yr (91,240 days)|
|0° 0m 14.04s / day|
−20 km 
|B–V = 1.009±0.051|
V–R = 0.610±0.030
V–I = 1.146±0.086
|3.6 · 3.828±0.039|
Analysis of the lightcurve's brightness variation shows only small deviations, which suggests that Ixion is a spheroid with small albedo spots and hence a dwarf planet. It has a diameter of approximately 650 km, making it about the fifth-largest plutino. It is moderately red in visible light and has a surface made of a mixture of tholin and water ice.
Other than Pluto, Ixion was the first trans-Neptunian object (TNO) discovered that was originally estimated to be larger than dwarf planet Ceres, Even in 2002, a year after its discovery, Ixion was still believed to be more than 1000 km in diameter, though the 2002 estimate was a result of a spurious detection at 250 GHz that was not confirmed by later observations. More recent estimates suggest that Ixion has a high albedo and is smaller than Ceres. Observations of Ixion by Herschel Space Telescope and Spitzer Space Telescope in the far-infrared part of the spectrum revealed that its size is about 620 km.
Ixion is moderately red (slightly redder than 50000 Quaoar) in the visible light. It also 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. In the near-infrared the spectrum of Ixion is flat and featureless. Water ice absorption bands at 1.5 and 2 μm are absent. This is in contrast to Varuna, which has a red spectral slope in the near-infrared as well as prominent water absorption bands. 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.
Orbit and rotationEdit
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 below the ecliptic 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. Ixion does demonstrate some regular changes in brightness, which are thought to be caused by its rotation.
In May 2010, a rotational lightcurve of Ixion was obtained from photometric observations. Lightcurve analysis gave a rotation period of 12.4±0.3 hours with a small brightness variation confirming the body's supposed spheroidal shape (U=n.a.).
This minor planet was named after Ixion, king of the Lapiths from Greek mythology. Ixion desired Zeus's wife, Hera. Zeus found out about his intentions and created the cloud Nephele in the shape of Hera, and tricked Ixion into coupling with it, fathering the race of Centaurs. For his crimes, Ixion was expelled from Olympus, blasted with a thunderbolt, and bound to a burning solar wheel in the underworld for all eternity. The official naming citation was published on 28 March 2002 (M.P.C. 45236).
A study published in 2012 determined that Ixion and Huya were the most feasible out of seven possible TNO targets for an orbiter mission that would launch on an Atlas V 551 or Delta IV HLV and use a Jupiter flyby for a gravity assist. A mission launched on 11 November 2039 would arrive at Ixion after 17 years cruise time.
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