Hydra (moon)

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For other uses, see Hydra.
Hydra

Artist conception of Hydra (foreground),
Pluto and Charon (background),
and Nix (bright dot center left)
Discovery
Discovered by Hubble Space Telescope
Pluto Companion Search Team
Discovered in June 2005
Orbital characteristics[1]
Semi-major axisa 64 780 ± 90 km
Eccentricity 0.005 ± 0.001
Orbital period 38.206 ± 0.001 d
Inclinationb 0.22° ± 0.12°
Is a satellite of Pluto
Physical characteristics
Mean diameter 61 − 167 km[2]
Mass 5×1016 − 2×1018 kg[3]
Mean density unknown
Rotation period unknown
Axial tilt unknown
Albedo 0.04 − 0.35 (assumed)[4]
Apparent Magnitude 22.9 to 23.3 (measured)[4]
Surface temp. 33-55 K
Atmosphere none

a Relative to the Pluto-Charon barycenter.
b Relative to Pluto's equator (also Charon's orbital plane).

Hydra (pronounced /ˈhaɪdrə/ HYE-drə, Greek Ύδρα) is the outer-most natural satellite of Pluto. It was discovered along with Nix in June, 2005 by the Hubble Space Telescope's Pluto Companion Search Team, which is composed of Hal A. Weaver, Alan Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot F. Young, and Leslie A. Young. The discovery images were taken on May 15, 2005 and May 18, 2005; the moons were independently discovered by Max J. Mutchler on June 15, 2005 and Andrew J. Steffl on August 15, 2005. The discoveries were announced on October 31, 2005, after confirmation by precoveries from 2002. The moons were provisionally designated S/2005 P 1 (Hydra) and S/2005 P 2 (Nix).[5][6]

The satellite orbits the barycenter of the system in the same plane as Charon and Nix, at a distance of about 65,000 km. Unlike other satellites of Pluto, its orbit is only nearly circular; its eccentricity of 0.0052 is small, but significantly non-zero. Its orbital period of 38.2 days is close to a 1:6 orbital resonance with Charon, with the timing discrepancy being 0.3%. Whether this is a true resonance awaits more detailed determinations of its orbit, in particular its rate of precession. If there is no true resonance, a hypothesis to explain the near-resonance is that it originated before the outward migration of Charon following the formation of all three known moons, and is maintained by the periodic local fluctuation of 5% in the Pluto-Charon gravitational field strength.

Discovery images of Hydra.
Discovery images of Hydra.

Although its size has not been directly measured, calculations based on its brightness give it a diameter of between 61 km, if its reflectivity is similar to Charon's 35 percent, and about 167 km, if it has a reflectivity of 4 percent like the darkest Kuiper belt objects (KBOs).[2] At the time of discovery, Hydra was about 25 percent brighter than its sister moon Nix, which led to the assumption that its diameter was some 10 percent larger.[7] Pre-discovery data from Hubble observations in 2002-3 implied that Nix was the brighter moon.[1] However, Hubble observations in 2005-6, specifically targeting the dim moons, once again showed Hydra to be a little brighter.[4] Hydra appears to be spectrally neutral like Charon and Nix, though Pluto is reddish.

Hydra is to be visited along with Pluto by the New Horizons mission in 2015.

The name Hydra was announced on June 21, 2006, in IAU Circular 8723,[6] along with the formal designation Pluto III. It was named after Hydra, the serpent who guarded the waters of the underworld in Greco-Roman Mythology.

[edit] References

  1. ^ a b Buie, M. W.; Grundy, W. M.; Young, E. F.; Young, L. A.; and Stern, S. A. (2006). "Orbits and Photometry of Pluto's satellites: Charon, S/2005 P1, and S/2005 P2 [sic]". Astronomical Journal 132: 290. doi:10.1086/504422. (Final preprint)
  2. ^ a b H. A. Weaver; S. A. Stern, M. J. Mutchler, A. J. Steffl, M. W. Buie, W. J. Merline, J. R. Spencer, E. F. Young and L. A. Young (23 February 2006). "Discovery of two new satellites of Pluto" (subscription required). Nature 439 (7079): 943–945. doi:10.1038/nature04547. 
  3. ^ Based on the range of diameters from Buie et al. (2006), and densities ranging from 1 g/cm³ (ice) to 2 g/cm³ (Pluto).
  4. ^ a b c Stern, S. A.; Mutchler, M. J.; Weaver, H. A.; and Steffl, A. J. (2006). "The Positions, Colors, and Photometric Variability of Pluto's Small Satellites from HST Observations 2005-2006". Astronomical Journal: submitted.  (Final preprint)
  5. ^ IAU Circular No. 8625 describing the discovery
  6. ^ a b IAU Circular No. 8723 naming the moons
  7. ^ Stern, Alan; Hal Weaver (JHU APL), Max Mutchler (STScI), Andrew Steffl (SwRI), Bill Merline (SwRI), Marc Buie (Lowell Observatory), John Spencer (SwRI), Eliot Young (SwRI), and Leslie Young (SwRI) (15 May 2005). Background Information Regarding Our Two Newly Discovered Satellites of Pluto. Planetary Science Directorate (Boulder Office). Retrieved on 2007-11-10.
  1. Steffl, A. J.; Mutchler, M. J.; Weaver, H. A.; Stern, S. A.; Durda, D. D.; Terrell, D.; Merline, W. J.; Young, L. A.; Young, E. F.; Buie, M. W.; and Spencer, J. R. (2006). "New Constraints on Additional Satellites of the Pluto System". The Astronomical Journal 132: 614–619. doi:10.1086/505424. (Final preprint)

[edit] External links

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Charon · Nix · Hydra