The moon of Neptune – Psamathe (X) – was previously temporarily designated as S/2003 N1. It was discovered in late August 2003 by the following team of astronomers: D.C. Jewitt, J. Kleyna and S.S. Sheppard. This discovery was achieved by the 8.3 meter Subaru telescope na Mauna Kea (Hawaii). Almost simultaneously astronomers' team under management M. Holman, also discovered this moon by the 4 meter Blanco telescope in Cerro Tollolo (Chile). You can read additional details in a The International Astronomical Union Circular (→IAU).
    This is the first (and anyway only) moon of Neptune discovered in 2003, since the →Voyager-2 Neptune system fly-by which occured 14 years ago (1989).
    The name of that moon is of mythological origin. Psamathe was one of the nymphs, the daughter of Poseidon and Demeter...
    The moon is in elliptical retrograde orbit (→eccentricity e
 = 0.4499) with a →semimajor axis a = 46,695,000 km. At pericenter (closest to the planet) Psamathe is separated from the Neptune a distance of q = 25,686,920 km. At apocenter (furthest from the planet) this moon is separated from the Neptune a distance of Q = 67,703,081 km. In future these parameters may vary due to a large orbital distance from Neptune and the orbital motion being disturbed by the Sun and other factors. Perhaps it is a captured asteroid (with a diminutive mass: ~1.4987×1016 kg – it equals 2 mountains like Giewont!), and in future will possibly return to heliocentric orbit.

    An interesting feature of S/2002 N1 is its retrograde orbital motion (inclination of orbit –42.6°) just like Triton! It is also one of the most remote satellites in the Solar System! Its orbit is comparable to the orbit of Mercury! The existenc of such a remote satellite of Neptune is connected with the enormous radius of the planet's sphere of influence known as the →Hill sphere.

     But let's go back to that remote satellite: its orbital period equals almost 25 earthly years! Which means that the average speed of its celestial motion is only 0°02'22'' per day. When this motion was detected and compared to the changes in Neptune's position, this body was classified as a true planetary satellite (and not a distant celestial body with its position projected near the planet).
    Other obstacle in detecting this moon earlier was its brightness of only 24.6m (over 27.5 million times fainter than the faintest stars visible to naked eye). For comparison: Neptune's brightness is about 7.8m, so Psamathe is shining fainter by about 16.8m. The corresponding difference in brightness is almost 5.25 million times!

I have given some crucial data of Psamathe below. Table one contains the basic information. Whereas table two gives more detailed parametrs of its orbit (calculated using the following formulae).

Translated by Karol Pankowski


Mean distance from the planet  [×103 km] 46,695.0
Mean distance from the planet  [planetary R] 1,885.6
Orbital period  [days] 9,115.91
Orbital eccentricity  [e] 0.4499
Orbit inclination  [degrees] 137.4
Mean diameter  [km] 40.0
Main discoverer and year of the discovery D.C. Jewitt   2003
Visual magnitude  [mag] 24.6
Mass  [kg] ~1.4987 × 1016

Orbital Parameters

Distance from the planet  [×103 km] 25,686.9 67,703.1
Distance from the planet  [planetary R] 1,037.3 2,733.9
Angular size of the moon's orbit observed from the Earth*  [degrees] 0°20'18.76'' 0°53'32.41''
Angular diameter of the planet's disc as observed from the moon  [degrees] 0°06'37.71'' 0°02'30.89''
Brightness of the planet as observed from the moon**  [mag] –3.3 –1.2
Diameter of the moon's disc as observed from the planet's "surface"  [degrees] 0°00'00.32'' 0°00'00.12''
Brightness of the moon as observed from the planet's "surface"**  [mag] 13.5 15.6
Orbital velocity  [km/sec] 0.62 0.24
  * This value is calculated for Neptune at opposition (distance 4.4 billion km = 29.1 AU)
** The given value of magnitude is not corrected for some decreasing factors (e.g. the changing phase of illumination)

See other related links:
Monde des MonatsPsamathe
One Moon for Neptune, Another for Uranus

The Astronomy Workshop – Satellite Viewer
Planetary Satellite Mean Orbital Parameters
Natural Satellite Physical Parameters

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