28978 Ixion (IPA pronunciation: [ɪk'saɪ.ɒn]) 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 second largest plutino. It is named after Ixion, a figure from Greek mythology; it previously had the provisional designation 2001 KX76.
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 absobtion 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.
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.
- H. Boehnhardt, S. Bagnulo, K. Muinonen, M. A. Barucci, L. Kolokolova, E. Dotto and G. P. Tozzi (2003). "Surface characterization of 28978 Ixion (2001 KX76)". Astronomy & Astrophysics 415: L17-L19. Pre-print about Ixion's surface from the Planetary Systems Research group of the University of Helsinki
- J. Licandro, F. Ghinassi, L. Testi (2002). "Infrared spectroscopy of the largest known trans-neptunian object 2001 KX76". Astronomy & Astrophysics. Pre-print on arXiv
- W. J. Altenhoff, F. Bertoldi and K. M. Menten (2004). "Size estimates of some optically bright KBOs". Astronomy & Astrophysics 415.
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