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TOPIC: Neptune


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The Planet Neptune

The Planet Neptune
10 seconds exposure on Friday 9 June 2006 (04:37 UTC)



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RE: Neptune Satellites

Title: A Survey for "Normal" Irregular Satellites Around Neptune: Limits to Completeness
Authors: Scott S. Sheppard (Carnegie Inst. of Wash.), David C. Jewitt, Jan Kleyna

Researchers have surveyed 1.75 square degrees of sky near Neptune to an R-band 50% detection efficiency of 25.8 mags (corresponding to radii of about 17 km for an assumed albedo of 0.04). They discovered one new outer satellite, Psamathe (S/2003 N1), about 20 km in radius and having a distant retrograde orbit and moderate eccentricity.
Until 2003 Neptune was only known to have two satellites which exhibited orbital signatures indicative of capture. Both of these, Triton and Nereid, are unusual when compared to the irregular satellites of other giant planets. With recent discoveries of four additional satellites by Holman et al. (2004) it is now apparent that Neptune has a distant ''normal'' irregular satellite system in which the satellites have radii and orbital properties similar to those of the satellites of other giant planets.

The researchers find that the satellite size distribution at Neptune is not well determined given the few objects known to date, being especially sensitive to the inclusion of Triton and Nereid in the sample. Finally, they note that Psamathe and S/2002 N4 have similar semi-major axes, inclinations and eccentricities. They may be fragments of a once larger satellite.

*Orbital data are from Robert Jacobson at JPL. Fits are over a 1000 year time span. Epochs are June 10, 2003 TT for the Neptune satellites discovered in 2002 and 2003 and Aug 25, 1989 TT for Triton and Nereid.
aMean semi-major axis with respect to Neptune.
bMean inclination of orbit with respect to the ecliptic.
cMean eccentricity.
dThe argument of Pariaphis.
eThe longitude of the ascending node.
fThe mean anomaly.
gOrbital period of satellite around Neptune.
hApparent red (0.65 µm wavelength) magnitude. Uncertainties are around 0.2 mags.
iRadius of satellite assuming a geometric albedo of 0.04.
jTriton is an “unusual” retrograde satellite and not classified as an irregular satellite under the definition of Burns (1986). Triton likely has had significant modification of its orbit from tidal interactions with Neptune (Goldreich et al. 1989; Chyba et al. 1989). Triton’s mean inclination as shown here is with respect to Neptune’s equator.
kNereid is an “unusual” irregular satellite because of its relatively low inclination, small semi-major axis and large eccentricity. It may be a perturbed regular satellite.

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The Hubble Space Telescope captures the distant blue-green world, Neptune, and its satellites in this portrait. Astronomers used Hubble’s assortment of filters to pinpoint high altitude clouds floating above the methane rich atmosphere. The images have been assembled into a time-lapse movie revealing the orbital motion of the satellites. The icy moons seen in this view are Proteus (the brightest), Larissa, Despina, and Galatea. Neptune had 13 moons at last count.
In Roman mythology, Larsissa and Despina were Neptuness daughters.

Methane Band

Enhanced Colour

Natural Colour

MOVIE (1.64Mb mpeg movie)

This time-lapse movie of Neptune was assembled from combining the Hubble Space Telescope images taken over a 15-hour period. The movie shows a dynamic atmosphere and captures the fleeting orbits of Neptunes satellites . The natural colour and enhanced colour views of Neptune were assembled from images in 14 different coloured filters. This allows atmospheric features to be seen above Neptune's methane haze, which gives the planet its blue-green colour.
The animation has four sections, each one corresponding to approximately one rotation of Neptune.

The first section shows Neptune and its largest satellite Triton, which is about as large as our moon. Probably a captured Kuiper Belt object, Triton orbits Neptune in a "backwards" or retrograde path relative to the other major satellites, and opposite to Neptune's rotation. The view is close to what a human eye would see looking through the Hubble Space Telescope.

The second scene zooms in on Neptune, and the colours have been enhanced to better show the subtle detail of clouds in Neptune's atmosphere.
The third scene has the spectral region of light changed from the visible to special methane bands in the near infrared. Most of Neptune's atmosphere becomes very dark, except for high-altitude clouds.

The fourth scene traces the satellite orbits. Four of the small, inner satellites are visible orbiting Neptune. Dutifully obeying Kepler's laws of planetary motion, the outer moons take longer to revolve around Neptune than the inner ones. From outside to inside, these satellites are Proteus, Larissa, Galatea, and Despina. These moons are so faint that only the longest Hubble exposures can capture them.
The outermost layers of Neptune's thick atmosphere do not rotate like a solid body. The polar atmospheric regions finish a full rotation in a speedy 15 hours, while the equatorial regions lag behind, taking more than 18 hours for a complete circuit of the planet.

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