Naiad (moon)

Naiad

Naiad or Thalassa as seen by Voyager 2 (elongation is due to smearing)
Discovery
Discovered by	Voyager Imaging Team
Discovery date	September 1989
Orbital characteristics
Epoch 18 August 1989
Semi-major axis	48 227 ± 1 km
Eccentricity	0.0004 ± 0.0003
Orbital period	0.2943958 ± 0.0000002 d
Inclination	4.75 ± 0.03° (to Neptune equator) 4.75° (to local Laplace plane)
Satellite of	Neptune
Physical characteristics
Dimensions	96×60×52 km
Mean radius	33 ± 3 km
Volume	~1.5×105km³
Mass	~1.9×1017 kg (based on assumed density)
Mean density	~1.3 g/cm³ (estimate)
Surface gravity	~0.012 m/s2
Escape velocity	~0.028 km/s
Rotation period	synchronous
Axial tilt	zero
Albedo	0.07
Temperature	~51 K mean (estimate)
Apparent magnitude	23.9

Naiad (/ˈneɪ.əd/ NAY-əd or /ˈnaɪ.əd/ NY-əd; Greek: Ναϊάδ-ες), also known as Neptune III, is the innermost satellite of Neptune, named after the Naiads of Greek legend.

History

Naiad was discovered sometime before mid-September 1989 from the images taken by the Voyager 2 probe. The last moon to be discovered during the flyby, it was designated S/1989 N 6. The discovery was announced on September 29, 1989, in the IAU Circular No. 4867, but the text only talks of "25 frames taken over 11 days", giving a discovery date of sometime before September 18. The name was given on 16 September 1991.

Physical characteristics

A simulated view of Naiad orbiting Neptune with the Sun in the distance.

Naiad is irregularly shaped and probably has not been modified by any internal geological processes since its formation. It is likely that it is a rubble pile re-accreted from fragments of Neptune's original satellites, which were smashed up by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit.

Orbital characteristics

Naiad orbits about 23,500 km above Neptune's cloud tops. Since this is below the synchronous orbit radius, its orbit is slowly decaying due to tidal deceleration and it may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching. Naiad orbits Neptune well within its fluid Roche limit, and its density is expected to be low enough that it may be very close to its actual Roche limit already.

Exploration

Since the Voyager 2 flyby, the Neptune system has been extensively studied from ground-based observatories and the Hubble Space Telescope as well. In 2002–03 the Keck telescope observed the system using adaptive optics and detected easily the largest four inner satellites. Thalassa was found with some image processing, but Naiad was not located. Hubble has the ability to detect all the known satellites and possible new satellites even dimmer than those found by Voyager 2. On October 8, 2013 the SETI Institute announced that Naiad had been located in archived Hubble imagery from 2004. The suspicion that the loss of positioning was due to considerable errors in Naiad's ephemeris proved correct as Naiad was ultimately located 80 degrees from its expected position.

External links

• Naiad Profile by NASA's Solar System Exploration
• Neptune's Known Satellites (by Scott S. Sheppard)
• Archival Hubble Images Reveal Neptune's “Lost” Inner Moon (SETI : October 8, 2013)