60558 Echeclus

60558 Echeclus is a centaur, approximately 84 km in diameter, located in the outer Solar System. It was discovered by Spacewatch in 2000 and initially classified as a minor planet with provisional designation (also written 2000 EC98). Research in 2001 by Rousselot and Petit at the Besançon observatory in France indicated that it was not a comet, but in December 2005 a cometary coma was detected. In early 2006 the Committee on Small Bodies Nomenclature (CSBN) gave it the cometary designation 174P/Echeclus. It last came to perihelion in April 2015, and was expected to reach about apparent magnitude 16.7 near opposition in September 2015.

Name
Echeclus is a centaur in Greek mythology.

60558 Echeclus is only the second comet (after Chiron) that was named as a minor planet, rather than after the name of its discoverer. Chiron is also a centaur; other centaurs are being observed for signs of a cometary coma.

Besides Echeclus, eight other objects are cross-listed as both comets and numbered minor planets: 2060 Chiron (95P/Chiron), 4015 Wilson–Harrington (107P/Wilson–Harrington), 7968 Elst–Pizarro (133P/Elst–Pizarro), 118401 LINEAR (176P/LINEAR), (282P/2003 BM80),  (288P/2006 VW139),  (362P/2008 GO98), and  (433P/2005 QN173).

Fragmentation
On 30 December 2005, when 13.1 AU from the Sun, a large chunk of Echeclus was observed to break off, causing a great cloud of dust. Astronomers have speculated this could have been caused by an impact or by an explosive release of volatile substances.

Outbursts
Echeclus appears to have outburst again around June 2011 when it was 8.5 AU from the Sun. On 24 June 2011, follow up imaging with the 2 meter Haleakala-Faulkes Telescope South showed the coma of Echeclus to be very close to the sky background limit.

Echeclus outburst again around 7 December 2017 when it was 7.3 AU from the Sun, and was 4 magnitudes brighter than expected.

Presence of gas
In 2016, carbon monoxide was detected in Echeclus in very small amounts, and the derived CO production rate was calculated to be sufficient to account for the observed coma. The calculated CO production rate from Echeclus is substantially lower than what is typically observed for 29P/Schwassmann–Wachmann, another distantly active comet often classified as a centaur.

Orbit
Echeclus came to perihelion in April 2015.

Centaurs have short dynamical lives due to strong interactions with the giant planets. Echeclus is estimated to have an orbital half-life of about 610,000 years.