Nix (moon)

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Nix

Artist conception of Hydra (foreground), Pluto & Charon (background), and Nix (bright dot center left)
Discovery
Discovered by	Hubble Space Telescope Pluto Companion Search Team
Discovered in	June 2005
Orbital characteristics
Semi-major axis	48 675 ± 120 km
Eccentricity	0.002 ± 0.002
Orbital period	24.856 ± 0.001 d
Inclinationa	0.04° ± 0.22°
Parent body	Pluto
Physical characteristics
Mean diameterb	max 137 km ( miles)
Mass	18 kg Based on the range of diameters from Weaver et al (2006), and densities ranging from 1.0 g/cm³ (ice) to 2.0 g/cm³ (Pluto).
Mean density	unknown
Rotation period	unknown
Axial tilt	unknown
Albedo	0.04 − 0.35 (assumed)
Surface temp.	33-55 K
Atmosphere	none
a Relative to parent body's equator. b Based on assumed albedo.

Nix (formerly known as S/2005 P 2), is a natural satellite of Pluto. It is one of two discovered in June 2005 by the Hubble Space Telescope Pluto Companion Search Team, composed of Hal A. Weaver, S. 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 first spotted by Max J. Mutchler on June 15, 2005, and the discoveries were announced on October 31, 2005, after confirmation by other observations and precoveries from 2002 [IAU Circular No. 8625] describing the discovery.

The moon has a circular orbit in the same plane as Charon, at a distance of about 50 000 km. It orbits the barycenter of the system every 24.9 days, which misses being a 1:4 orbital resonance with Charon by 2.7%. It is thought that this near-resonance originated in the outward migration of Charon after the formation of all three moons, and is maintained by a periodic local fluctuation of 15% in the Pluto-Charon gravitational field.

Discovery images of S/2005 P 2.

Although its size has not been directly measured, the moon is calculated have a diameter of between 46 km (29 miles), if its reflectivity is similar to Charon's 35%, and 137 km (85 miles), if it has a reflectivity of 4%, like the darkest KBOs. All of Pluto's moons are spectrally neutral, however, so the lower end of the range seems likely. Nix is about as bright as Hydra, suggesting that they are approximately equal in size. The moon is to be visited along with Pluto by the New Horizons mission in 2015.

S/2005 P 2 was nicknamed "Boulder," after the city in which the Hubble's instruments were built. The formal name "Nix" was announced on June 21, 2006 on IAU Circular 8723, the designation Pluto II is also given. Together with Hydra, Pluto's third moon, the initials are those of the New Horizons probe. The initial proposition was Nyx (the Greek goddess, called Nox by the Romans), but to avoid confusion with the asteroid 3908 Nyx, the spelling was changed to a transliteration of the form used during Egypt's late period.

External links

• [IAU Circular No. 8625] describing the discovery
• [Background Information Regarding Our Two Newly Discovered Satellites of Pluto] – The discoverers' website
• [NASA's Hubble Reveals Possible New Moons Around Pluto] – Hubble press release
• [Two More Moons Discovered Orbiting Pluto] (SPACE.com)
• [Pluto's Newest Moons Named Hydra and Nix] (SPACE.com)

References

* 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., Spencer J.R. (2005), New Constraints on Additional Satellites of the Pluto System, Astronomical Journal, submitted ([preprint])

* Buie M.W., Grundy W.M., Young, E.F., Young L.A., Stern S.A. (2005), Orbits and photometry of Pluto's satellites: Charon, S/2005 P1 and S/2005 P2, submitted ([preprint])