|Discovered by||Deep Ecliptic Survey
Cerro Tololo (807)
|Discovery date||22 May 2001|
|MPC designation||28978 Ixion|
|Pronunciation||// ik-SY-ən [note 1]|
|Alternative names||2001 KX76|
|Minor planet category||TNO (plutino)|
|Epoch December 31, 2006 (JD 2 454 100.5)|
|Aphelion||49.269 AU (7,370.503 Gm)|
|Perihelion||30.091 AU (4,501.495 Gm)|
|Semi-major axis||39.680 AU (5,935.999 Gm)|
|Orbital period||249.95 a (91,295.847 d)|
|Longitude of ascending node||71.028°|
|Argument of perihelion||298.779°|
|Spectral type||B-V=1.03 ± 0.03
V-R=0.61 ± 0.03
|Apparent magnitude||19.6 (opposition) |
|Absolute magnitude (H)||3.86 ± 0.04|
28978 Ixion is a plutino (an object that has a 2:3 orbital resonance with Neptune). It is considered very likely to be a dwarf planet, although the IAU has not officially classified it as such. Light-curve-amplitude analysis shows only small deviations, which suggests that Ixion is a spheroid with small albedo spots and hence a dwarf planet. Tancredi (2010) thinks that Ixion is probably a dwarf planet. Its diameter of 650 km estimated by Spitzer makes 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 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, 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 Spitzer Space Telescope in the far-infrared part of the spectrum revealed that its size is about 650 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.
Ixion may have occulted a relatively bright apparent magnitude 11.5 star in the constellation of Ophiuchus on July 19, 2010. This event may have been visible from North America. The RA and DE for this event was in a star-rich region at 16 54 25.8620 −24 24 22.856. The occultation could have lasted up to 45 seconds, but the predicted occultation path was likely to miss the Earth. An occultation detection could help refine the diameter of this possible dwarf planet.
Orbit and rotation
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. As of 2010, however, the rotation period of Ixion remains undetermined.
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