5145 Pholus

5145 Pholus was the second Centaur to be found, in 1992 by the American astronomer David Rabinowitz (1960–). It has a diameter of about 190 km and an orbit similar to that of the first discovered Centaur, Chiron. Unlike Chiron, however, which is pretty much gray all over, Pholus is very red – in fact, the reddest known object in the Solar System; also unlike Chiron, it doesn't appear to develop a comet-like coma when near perihelion. Its infrared spectrum has a feature in the 2-micron region that had never been seen before and is possibly related to whatever makes Pholus so red. The most likely explanation is that Pholus is covered with organic molecules that started off as simple ices but were converted into more complex, stable molecules by millions of years of bombardment by cosmic rays. According to one theory, it may be a potential giant cometary nucleus that has never been activated.


semimajor axis 20.35 AU
perihelion 8.68 AU (just inside Saturn's orbit)
aphelion 31.91 AU (just outside Neptune's orbit)
eccentricity 0.574
inclination 24.7°
Period 91.8 years