Despina (moon)

Despina

Despina as seen by Voyager 2 (smeared horizontally)
Discovery
Discovered by	Stephen P. Synnott[1] and Voyager Imaging Team
Discovery date	July 1989
Orbital characteristics[2]
Epoch 18 August 1989
Semi-major axis	52 526 ± 1 km
Eccentricity	0.0002 ± 0.0002
Orbital period	0.33465551 ± 0.00000001 d
Inclination	0.216 ± 0.014° (to Neptune equator) 0.06° (to local Laplace plane)
Satellite of	Neptune
Proper orbital elements
Physical characteristics
Dimensions	180×148×128 km[3][4]
Mean radius	75 ± 3 km[5]
Volume	~1.8×106km³
Mass	~2.2×1018 kg (based on assumed density)
Mean density	~1.2 g/cm³ (estimate)[5]
Equatorial surface gravity	~0.026 m/s2[a]
Escape velocity	~0.063 km/s[b]
Rotation period	synchronous
Axial tilt	zero
Albedo	0.09[3][5]
Temperature	~51 K mean (estimate)
Apparent magnitude	22.0[5]

Despina ( /dɨˈspiːnə/ di-spee-nə or /dɨˈspaɪnə/ di-spy-nə; Latin: Despœna,; Greek: Δέσποινα), also known as Neptune V, is the third closest inner satellite of Neptune. It is named after Despoina, a nymph who was a daughter of Poseidon and Demeter.

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.^[6] The discovery was announced (IAUC 4824) on August 2, 1989, but the text only talks of "10 frames taken over 5 days", giving a discovery date of sometime before July 28. The name was given on 16 September 1991.^[7]

A simulated view of Despina orbiting Neptune

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 smashed up by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit.^[8]

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

Notes

1. ^ Surface gravity derived from the mass m, the gravitational constant G and the radius r: Gm/r².
2. ^ Escape velocity derived from the mass m, the gravitational constant G and the radius r: √2Gm/r.

References

1. ^ Planet Neptune Data http://www.princeton.edu/~willman/planetary_systems/Sol/Neptune/
2. ^ Jacobson, R. A.; Owen, W. M., Jr. (2004). "The orbits of the inner Neptunian satellites from Voyager, Earthbased, and Hubble Space Telescope observations". Astronomical Journal 128 (3): 1412–1417. Bibcode 2004AJ....128.1412J. doi:10.1086/423037.  edit
3. ^ ^{a b} Karkoschka, Erich (2003). "Sizes, shapes, and albedos of the inner satellites of Neptune". Icarus 162 (2): 400–407. Bibcode 2003Icar..162..400K. doi:10.1016/S0019-1035(03)00002-2.  edit
4. ^ Williams, Dr. David R. (2008-01-22). "Neptunian Satellite Fact Sheet". NASA (National Space Science Data Center). http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptuniansatfact.html. Retrieved 2008-12-13. 
5. ^ ^{a b c d} "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2008-10-24. http://ssd.jpl.nasa.gov/?sat_phys_par. Retrieved 2008-12-13. 
6. ^ Marsden, Brian G. (August 2, 1989). "Satellites of Neptune". IAU Circular 4824. http://www.cbat.eps.harvard.edu/iauc/04800/04824.html. Retrieved 2011-10-26. 
7. ^ Marsden, Brian G. (September 16, 1991). "Satellites of Saturn and Neptune". IAU Circular 5347. http://www.cbat.eps.harvard.edu/iauc/05300/05347.html. Retrieved 2011-10-26. 
8. ^ Banfield, Don; Murray, Norm (October 1992). "A dynamical history of the inner Neptunian satellites". Icarus 99 (2): 390–401. Bibcode 1992Icar...99..390B. doi:10.1016/0019-1035(92)90155-Z.  edit

External links

• Despina Profile by NASA's Solar System Exploration
• Neptune's Known Satellites (by Scott S. Sheppard)
• Astronomy Picture of the Day featuring Despina and its shadow transiting Neptune