42355 Typhon (; provisional designation ) is a scattered disc object that was discovered on February 5, 2002, by the NEAT program. It measures in diameter, and is named after Typhon, a monster in Greek mythology. Typhon is the first known binary centaur, using an extended definition of a centaur as an object on a non-resonant (unstable) orbit with the perihelion inside the orbit of Neptune.
Planetary symbols are no longer much used in astronomy, so Typhon never received a symbol in the astronomical literature. There is no standard symbol for Typhon used by astrologers either. A hurricane symbol () has been used, which might be identified with .
Measurements of its thermal radiation led to an equivalent diameter of about either or , with the latter value preferable. The diameter of the central body (Typhon) is in the latter case . Due to its small size, it is unlikely to be classified as a dwarf planet. As of 2021, no rotational lightcurve has been analyzed and the body's actual shape remain unknown.
Typhon has a BR taxonomic class, with a blue surface. It has a resulting BâÂÂR color difference of about 1.3, while VâÂÂI color difference is about unity. The spectra of Typhon show the presence of water ice and possibly of water altered silicates.
A large moon was identified in 2006. It is named Echidna (formal designation (42355) Typhon I), after the monstrous mate of Typhon. It orbits Typhon at the distance of , completing one orbit in days. Its diameter is estimated at . Echidna has the same color as Typhon.