* Astronomy

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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