Despina (moon)

Despina /dɛˈspaɪnə/, also known as Neptune V, is the third-closest inner moon of Neptune. It is named after the Greek mythological character Despoina, a Goddess who was a daughter of Poseidon and Demeter.

Despina

Despina as seen by Voyager 2 (smeared horizontally)
Discovery[1]
Discovered by	Stephen P. Synnott and Voyager Imaging Team
Discovery date	July 1989
Designations
Designation	Neptune V
Pronunciation	/dəˈspaɪnə, dəˈspiːnə, dɛ-/
Named after	Δέσποινα Despœna
Adjectives	Despinian
Orbital characteristics[2][3]
Epoch 18 August 1989
Semi-major axis	52 525.95 km
Eccentricity	0.00038 ± 0.00016
Orbital period (sidereal)	0.33465551 ± 0.00000001 d
Inclination	0.216 ± 0.014° (to Neptune equator) 0.06° (to local Laplace plane)
Satellite of	Neptune
Physical characteristics
Dimensions	(180±6) × (148±12) × (128±6) km[4]
Mean radius	75±3 km[4]
Volume	~1.8×106 km3[a]
Mass	~(0.71–1.4)×1018 kg[b]
Mean density	0.4–0.8 g/cm3[5]
Surface gravity	~0.006–0.023 m/s2[c]
Escape velocity	~0.032–0.054 km/s[d]
Synodic rotation period	synchronous
Axial tilt	zero
Albedo	0.09[4][6]
Temperature	~51 K mean (estimate)
Apparent magnitude	22.0[6]

Discovery

Despina was discovered in late July 1989 from the images taken by the Voyager 2 probe. It was given the temporary designation S/1989 N 3.^[7] The discovery was announced (IAUC 4824) on 2 August 1989, and mentions "10 frames taken over 5 days", implying a discovery date of sometime before July 28. The name was given on 16 September 1991.^[8]

Physical characteristics

Despina's diameter is approximately 150 kilometres (93 mi).^[4] Despina is irregularly shaped and shows no sign of any geological modification. It is likely that it is a rubble pile re-accreted from fragments of Neptune's original satellites, which were disrupted by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit.^[9]

Compositionally, Despina appears to be similar to other small inner Neptunian satellites, with a deep 3.0 micron feature attributed to water ice or hydrated silicate minerals. It has a 0.09 albedo at 1.4 microns, 0.1 albedo at 2.0 microns, dropping to 0.03 at 3.0 microns, and increasing to 0.07 at 4.6 microns.^[10]

Orbit

Despina's orbit lies close to but outside of the orbit of Thalassa and just inside the Le Verrier ring and acts as its shepherd moon.^[11] As it is also below Neptune's synchronous orbit radius, it is slowly spiralling inward due to tidal deceleration and may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching.

A simulated view of Despina orbiting Neptune

Notes

1. Volume derived from the long axis A, the medium axis B and the short axis C: ${\displaystyle {\frac {\pi }{6}}ABC}$
2. Mass derived from density ρ and the volume V: ${\displaystyle V\rho }$
3. Surface gravity derived from the mass m, the gravitational constant G and the radius r: ${\displaystyle {\frac {Gm}{r^{2}}}}$
4. Escape velocity derived from the mass m, the gravitational constant G and the radius r: ${\displaystyle {\sqrt {\frac {2Gm}{r}}}}$