|Date of discovery||May 15, 2005| |Name of discoverer||Hubble
Space Telescope Pluto Companion Search Team| |Name
origin||Mythical 9-headed monster; second initial of "New
Horizons" mission| |Order from primary||3| |Semi-major
axis||64,780 km| |Sidereal month||38.206 da| |Inclination||0.22°
to Pluto's equator| |Equatorial radius||37 km| |Mean
temperature||44 K| The Hubble Space Telescope Pluto Companion
Search Team examined Pluto and its already-known moon Charon
using the Hubble Space Telescope's Advanced Camera for Surveys.
The stated discovery date of May 15, 2005 is the date that the
discovery images were taken; the discovery was announced only
after confirmatory comparison with previous images of the
Plutonian system. In Greek mythology, the Hydra was a nine-
headed monster who was eventually destroyed by Hercules. The
names given to Hydra and the middle moon Nix are also the
initials of the NASA mission New Horizons, launched in 2006. Nix
and Hydra, like Charon, revolve around Pluto in the direction of
Pl