Hydra is a moon of Pluto and is Pluto's second largest moon, and was discovered along with Nix by the Hubble Space Telescope at the Pluto Companion Search Team in June 2005. Since the discovery, it was a small dot until New Horizons photographed it in July 2015.
Discovery[]
Hydra was discovered by New Horizons team at the Pluto Companion Search Team. The New Horizons team had used the Hubble Space Telescope to observe faint prevously undiscovered. Discovery images was transmitted on early of 15 May 2005 and discovered by Max J. Mutchler on 15 June 2005. The discovery was officially annouced on 31 October 2005, after precovering previous Hubble space images from 2002.
Originality[]
Pluto's small moons were theoried to be formed after debris ejected after a massive collision with Pluto and another Kuiper belt object. Then the derbis ejected forms into the moons of Pluto. In this case, the small moons, including Hydra would have migrated outwards with Charon into their current orbits around the Pluto-Charon tidal lock system. Although the tidal damping with Charon interaction, Hydra's orbit around the Pluto-Charon barycenter gradually became more circular over time. Hydra is believed to have formed from two smaller objects merging into one bigger object from several newspapers. Hydra is also thought to have formed in an icy debris disc produced when water-rich mantles were stripped from the two bodies that collided to form the Pluto-Charon binary four billion years ago.
Physical characteristics[]
Hydra is an irregular-shaped object, measuring 50.9 km along its longest axis and its shortest axis measuring 30.9 km across. The surface is dominated with crystalline water ice making it highly reflective, which NASA suggested that the planet's icy shell is cleaner than the ice found on Pluto's other moons. The surface also displays colour of water similar to Pluto's other moons though Hydra appears slightly bluer. One explanation suggests Hydra’s deep water bands and high reflectance imply relatively little contamination by darker material that has accumulated on Charon's surface over time. Scientists say the reason to Hydra's clear water ice micrometeorite impacts continually refresh the surface of Hydra by blasting off contaminants.
Rotation[]
Hydra is not tidal locked, therefore does not rotate chaotically, its rotational period and axial tilt vary quickly over astronomical timescales, to the point that its rotational axis regularly flips over. Hydra's rotation period was recorded by New Horizon's flyby as aproximately 10 hours, at a tilt of 110 degrees to its orbit, rotating sideways when New Horizon's passed by. Hydra has a quick rotation compared to other moons, where the rotations take more than one day. If Hydra increases it's rotation rates higher, the surface material could get ejected out.
Orbit[]
Hydra orbits the Pluto-Charon barycenter at a distance of 64,738 km and is the outermost moon of Pluto. Similar to all of Pluto's moons, Hydra's orbit is nearly circular and gets circular over time and is coplanar to Charon's orbit; all of Pluto's moons have very low orbital inclinations to Pluto's equator.
The nearly circular and coplanar orbits of Pluto's moons suggest that they may have gone through tidal evolutions since their formation. At the time of the formation of Pluto's smaller moons, Hydra may have had a more eccentric orbit around the Pluto-Charon barycenter. The present circular orbit of Hydra may have been caused by Charon's tidal damping of the eccentricity of Hydra's orbit, through tidal interactions. The mutual tidal interactions of Charon on Hydra's orbit would cause Hydra to transfer its orbital eccentricity to Charon, thus causing the orbit of Hydra to gradually become more circular over time. Hydra has an orbital period of approximately 38.2 days and is resonant with other moons of Pluto.
Hydra's orbit is close to a 1:6 orbital resonance with Charon, with a timing discrepancy of 0.3%. A hypothesis explaination explains the near-resonance suggests that the resonance originated before the outward migration of Charon after the formation of all five known moons, and is maintained by the periodic local fluctuation of 5% in the Pluto–Charon gravitational field strength.