 Halimede imaged by the Very Large Telescope during follow-up observations on 3 September 2002 | |
| Discovery[1][2] | |
|---|---|
| Discovered by | |
| Discovery date | August 14, 2002 |
| Designations | |
Designation | Neptune IX |
| Pronunciation | /hæləˈmiːdiː/[3] |
Named after | Ἁλιμήδη Halimēdē |
| S/2002 N 1 | |
| Adjectives | Halimedean /ˌhæləməˈdiːən/ |
| Orbital characteristics[4] | |
| Epoch 2020 January 1 | |
| 16 590 500 km | |
| Eccentricity | 0.521 |
| 1,879 d (5.14 a) (retrograde) | |
| Inclination | 119.6° |
| Physical characteristics | |
| 62 km (for albedo 0.04)[5] | |
| Albedo | 0.04 (assumed)[5] |
Spectral type | neutral (grey) B-V=0.73 V-R=0.35[6] |
Halimede /hæləˈmiːdiː/, or Neptune IX, is a retrograde irregular satellite of Neptune. It was discovered by Matthew J. Holman, John J. Kavelaars, Tommy Grav, Wesley C. Fraser and Dan Milisavljevic on August 14, 2002.[7]
Name
Halimede, like many of the outer satellites of Neptune, is named after one of the Nereids, the fifty daughters of Nereus and Doris. Before the announcement of its name on February 3, 2007 (IAUC 8802), Halimede was known by the provisional designation S/2002 N 1.
Orbit
Halimede has the second most eccentric and third most inclined orbit around Neptune.[8] This is illustrated on the diagram in relation to other irregular satellites of Neptune. The satellites above the horizontal axis are prograde, the satellites beneath it are retrograde. The yellow segments extend from the pericentre to the apocentre, showing the eccentricity. It is also worth mentioning that Sao and Laomedeia are similar to Halimede but they both have prograde orbits unlike Halimede which has a retrograde orbit.[9]
Physical characteristics
Halimede is about 62 kilometers in diameter (assuming an albedo of 0.04)[5] and appears neutral (grey) in the visible light, a very similar measured colour to that of Nereid.
Origin
Given the very similar colour of the satellite to that of Nereid together with the high probability (41%[7]) of collision in the past lifespan of the Solar System, it has been suggested that the satellite could be a fragment of Nereid.[6]
Alternatively, Halimede could share an origin with Psamathe and Neso, which it has similar inclinations with but not semi-major axes or eccentricities, if it had a "chaos-assisted capture". Moons would first be trapped in the Hill sphere of giant planets as a temporary satellite, then later having their orbits stabilized through a process such as gas drag and becoming permanently captured. Objects captured in this way would have their inclinations unaltered but their other orbital elements scattered.[10]
References
- JPL (2011-07-21). "Planetary Satellite Discovery Circumstances". Jet Propulsion Laboratory. Retrieved 2011-10-24.
- Green, Daniel W. E. (January 13, 2003). "Satellites of Neptune". IAU Circular. 8047. Retrieved 2011-10-24.
- Noah Webster (1884) A Practical Dictionary of the English Language
- "Planetary Satellite Mean Elements". ssd.jpl.nasa.gov. Retrieved 2026-05-30.
- Sheppard, Scott S.; Jewitt, David C.; Kleyna, Jan (2006). "A Survey for "Normal" Irregular Satellites around Neptune: Limits to Completeness". The Astronomical Journal. 132 (1): 171–176. arXiv:astro-ph/0604552. Bibcode:2006AJ....132..171S. doi:10.1086/504799. S2CID 154011.
- Grav, Tommy; Holman, Matthew J.; Fraser, Wesley C. (2004-09-20). "Photometry of Irregular Satellites of Uranus and Neptune". The Astrophysical Journal. 613 (1): L77–L80. arXiv:astro-ph/0405605. Bibcode:2004ApJ...613L..77G. doi:10.1086/424997. S2CID 15706906.
- Holman, M. J.; Kavelaars, J. J.; Grav, T.; et al. (2004). "Discovery of five irregular moons of Neptune" (PDF). Nature. 430 (7002): 865–867. Bibcode:2004Natur.430..865H. doi:10.1038/nature02832. PMID 15318214. S2CID 4412380. Retrieved 24 October 2011.
- Williams, Dr. David R. (2008-01-22). "Neptunian Satellite Fact Sheet". NASA (National Space Science Data Center). Retrieved 2011-11-03.
- "In Depth | Halimede". NASA Solar System Exploration. Retrieved 2020-11-10.
- Brozović, Marina; Jacobson, Robert A.; Sheppard, Scott S. (2011-04-01). "THE ORBITS OF NEPTUNE'S OUTER SATELLITES". The Astronomical Journal. 141 (4): 135. doi:10.1088/0004-6256/141/4/135. ISSN 0004-6256.