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28978 Ixion

28978 Ixion
28978-ixion hst.jpg
Hubble Space Telescope image of Ixion taken in 2006
Discovered byDeep Ecliptic Survey
Discovery siteCerro Tololo Obs.
Discovery date22 May 2001
MPC designation(28978) Ixion
Pronunciation/ɪkˈs.ən/ (ik-SY-ən)[a]
Named after
2001 KX76
TNO · plutino[2] · distant[3]
Orbital characteristics[1]
Epoch 27 April 2019 (JD 2458600.5)
Uncertainty parameter 3
Observation arc35.93 yr (13,122 days)
Earliest precovery date17 July 1982
Aphelion49.479 AU
Perihelion30.103 AU
39.791 AU
251.01 yr (91,240 days)
0° 0m 14.148s / day
Physical characteristics
Mean diameter
12.4±0.3 h[5]
IR (moderately red)[6]
3.6 (assumed)[1][3]

28978 Ixion (/ɪkˈs.ən/ 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,[9][10] although the International Astronomical Union has not officially classified it as such. It was discovered on 22 May 2001 by the Cerro Tololo Inter-American Observatory.[3] It is named after Ixion, a figure from Greek mythology.[11]

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.[12] It has a diameter of approximately 617 km (383 mi), making it about the fifth-largest plutino. It is moderately red in visible light and has a surface made of a mixture of tholins and water ice.



Ixion was discovered with the Víctor M. Blanco Telescope at the Cerro Tololo Observatory

Ixion was discovered on 22 May 2001 by a team of American astronomers at the Cerro Tololo Inter-American Observatory in Chile.[3][13] The discovery formed part of the Deep Ecliptic Survey, a survey conducted by American astronomer Robert Millis to search for Kuiper belt objects located near the ecliptic plane using telescopes at the facilities of the National Optical Astronomy Observatory.[14][13] On the night of 22 May 2001, American astronomers James Elliot and Lawrence Wasserman identified Ixion in digital images of the southern sky taken with the 4-meter Víctor M. Blanco Telescope at Cerro Tololo.[15][13] Ixion was first noted by Elliot while compiling two images taken approximately two hours apart,[16][13] which revealed Ixion's slow motion relative to the background stars.[b] At the time of discovery, Ixion was located in the constellation of Scorpius.[c]

The discoverers of Ixion noted that it appeared relatively bright for a distant object, implying that it might be very large in size.[13][18] The discovery alluded to the possibility of a significant extent of undiscovered trans-Neptunian objects that may be as large as Pluto.[13][19] Since Ixion's discovery, numerous large trans-Neptunian objects including the dwarf planets Haumea, Eris, Makemake have been discovered.[9] The discovery of Ixion was formally announced by the Minor Planet Center in a Minor Planet Electronic Circular on 1 July 2000.[15] It was given the provisional designation 2001 KX76, indicating that it was discovered in the second half of May 2001.[20][21] Ixion was the 1,923rd object discovered in the latter half of May, as indicated by the last letter and numbers in its provisional designation.[20]

At the time of disovery, Ixion was thought to be among the largest trans-Neptunian objects in the Solar System, as implied by its high intrinsic brightness.[13][18] These characteristics of Ixion prompted follow-up observations in order to ascertain its orbit, which would in turn improve the certainty of later size estimates of Ixion.[22][19] In August 2001, a team of astronomers used the European Southern Observatory's Astrovirtel virtual observatory to automatically scan through archival precovery photographs obtained from various observatories.[19] The team had obtained nine precovery images of Ixion, with the earliest taken by the Siding Spring Observatory on 17 July 1982.[23][21] These precovery images along with subsequent follow-up observations with the La Silla Observatory's 2.2-meter MPG/ESO telescope in 2001 extended Ixion's observation arc by over 18 years, sufficient for its orbit to be accurately determined.[19][21] Hence, the Minor Planet Center later assigned the minor planet number 28978 to Ixion on 2 September 2001.[24]


Ixion imaged by the MPG/ESO telescope's Wide Field Imager at the La Silla Observatory in 2001[19]

Ixion is named after the eponymous Greek mythological figure Ixion, in accordance with the International Astronomical Union's (IAU's) naming convention which requires plutinos (objects in a 3:2 orbital resonance with Neptune) to be named after mythological figures associated with the underworld.[25] In Greek mythology, Ixion was the king of the legendary Lapiths of Thessaly and had married Dia, a daughter of Deioneus (or Eioneus), whom Ixion promised to give valuable bridal gifts.[26] Ixion invited Deioneus to a banquet but instead pushed him into a pitfall of burning coals and wood, killing Deioneus. Although the lesser gods despised his actions, Zeus pitied Ixion and invited him to a banquet with other gods.[26] Rather than being grateful, Ixion became lustful toward's 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.[26] 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.[11]

The name for Ixion was suggested by James Elliot, who was involved in its discovery by the Deep Ecliptic Survey team.[1][11] The naming citation was published by the Minor Planet Center on 28 March 2002.[27]

Physical characteristics

Size and brightness

Size estimates for Ixion
Year Diameter (km) Method Refs
2002 1055±165 radiometry [28]
2003 <804 photometry [29]
2005 <822 thermal [30]
2005 475±75 thermal [31]
2005 600+152
thermal [32]
2007 ~446.3 thermal (Spitzer 1-Band) [33]
2007 573.1+141.9
thermal (Spitzer 2-Band) [33]
2013 ~549 best-fit albedo [6]
2013 617+19
thermal [4]
Different diameters for Ixion depending on its albedo

Ixion has an absolute magnitude (brightness) of 3.83 and is estimated to have a geometric albedo (reflectivity) of 0.14, corresponding to a diameter of 617 km (383 mi).[4] Compared to Pluto and its moon Charon, Ixion is approximately one-fourth the diameter of Pluto and half the diameter of Charon.[d] Ixion is the intrinsically brightest object discovered by the Deep Ecliptic Survey and was the brightest known Kuiper belt object object found at the time of its discovery.[13][35] Hence, it was thought to be one of the largest Kuiper belt objects discovered due to its high intrinsic brightness.[13][22] Under the assumption of a low albedo, it was presumed to have a diameter of about 1,200 km (750 mi), which would make it larger than the dwarf planet Ceres and comparable in size to Charon.[13][19] Subsequent observations of Ixion with the 2.2-meter MPG/ESO telescope at the La Silla Observatory along with the Astrovirtel virtual observatory in August 2001 concluded a similarly large diameter for Ixion, though under the latter assumption of a low albedo.[19] Radiometric observations with the IRAM 30m telescope in 2002 obtained an albedo of 0.09, corresponding to a slightly smaller diameter of 1,055 km (656 mi), though relatively similar to previous assumptions of its size and albedo.[28]

The uncertain size estimates of Ixion by ground-based thermal measurements are due to Earth's atmosphere.[36] Space-based thermal measurements suggest that Ixion has a higher albedo and is smaller than Ceres.[37] 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 617 km (383 mi).[4]

Spectra and surface

Comparison of sizes, albedo, and colors of various large trans-Neptunian objects. The gray arcs represent uncertainties of the object's size.

Ixion is moderately red (slightly redder than Quaoar) in the visible light.[38] It also has a higher albedo (>0.15) than the mid-sized red cubewanos.[29] 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.[38] 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.[39] Both visible and infrared spectroscopic results indicate that Ixion's surface is a mixture of water ice, amorphous carbon and tholins, which is a heteropolymer formed by irradiation of clathrates of water and organic compounds.[40] The Very Large Telescope (VLT) has checked Ixion for cometary activity, but did not detect a coma.[41]


Ixion displays some regular changes in brightness, which are thought to be caused by its rotation.[40] 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.).[5][42]

Orbit and classification

Polar view of Ixion's orbit (green) along with several other large plutinos
Side view of Ixion's orbit (green) compared to Pluto (red) and Neptune (grey). The perihelion and aphelion dates are shown for both Pluto and Ixion.

Ixion is in a 2:3 mean-motion orbital resonance with Neptune, meaning that Ixion completes two orbits around the Sun for every three orbits completed by Neptune.[2] Due to its 2:3 orbital resonance with Neptune, Ixion is classified as a plutino, a dynamical class of objects with orbits similar to that of Pluto.[43][39] At the time of Ixion's discovery, it was initially thought to be in a 3:4 orbital resonance with Neptune, which would have implied that Ixion orbited closer to the Sun.[13][18] Ixion orbits the Sun at an average distance of 39.8 AU (5.95×109 km), taking 251 years to complete a full orbit.[1] This is characteristic of all plutinos, which have orbital periods around 250 years and semi-major axes around 39 AU.[43]

Like Pluto, Ixion's orbit is elongated and inclined to the ecliptic.[43] Ixion has an orbital eccentricity of 0.24 and an orbital inclination of 19.6 degrees, slightly greater than Pluto's inclination of 17 degrees.[1][43] Over the course of its orbit, Ixion's distance from the Sun varies from 30.1 AU at perihelion (closest distance) to 39.8 AU at perihelion (farthest distance).[1] Although Ixion's orbit is similar to that of Pluto, their orbits are oriented differently: Ixion's perihelion is below the ecliptic whereas Pluto's is above it (see right image). As of 2019, Ixion is approximately 39 AU from the Sun and is currently moving closer, approaching aphelion by 2070.[1] Simulations by the Deep Ecliptic Survey show that Ixion can acquire a perihelion distance (qmin) as small as 27.5 AU over the next 10 million years.[2]


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 rocket and use a Jupiter flyby for a gravity assist.[44] A mission launched on 11 November 2039 would arrive at Ixion after 17 years cruise time.


  1. ^ Or as in Latin: Ixīōn, Ancient Greek: Ιξίων. Sometimes erroneously /ˈɪksiən/ IK-see-ən.
  2. ^ The Minor Planet Electronic Circular published in July 2001 lists two coordinates of Ixion taken from the two recorded observations at Cerro Tololo (observatory code 806) on 22 May 2001. The time between the first and second observations is 0.08127 days, or approximately 1.95 hours. Within this time interval, Ixion has moved about 0.41 arcseconds from its original position first observed by Cerro Tololo.[15]
  3. ^ The given equatorial coordinates of Ixion during 22 May 2001 is  16h 16m 06.12s and −19° 13′ 45.6″,[15][3] which is close to the Scorpius constellation's coordinates around  17h and −40°.[17]
  4. ^ The current estimates of Pluto and Charon's diameters are 2376 km and 1212 km, respectively.[34] One-fourth of Pluto's diameter is 594 km and one-half of Charon's diameter is 606 km, close to the 2013 Herschel estimate of 617+19
    for Ixion's diameter.


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External links