Thalassa (moon)

---

title: "Thalassa (moon)" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["moons-of-neptune", "astronomical-objects-discovered-in-1989", "moons-with-a-prograde-orbit"] description: "Moon of Neptune" topic_path: "general/moons-of-neptune" source: "https://en.wikipedia.org/wiki/Thalassa_(moon)" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Moon of Neptune ::

::data[format=table title="Infobox planet"]

Field	Value
name	Thalassa
image	Neptune Trio.jpg
discoverer	Richard J. Terrile and Voyager Imaging Team
discovered	September 1989
caption	Thalassa (1989 N5) seen by Voyager 2 on 23 August 1989
mpc_name	Neptune IV
pronounced
adjective	Thalassian
named_after	Θάλασσα Thalassa
orbit_ref
epoch	18 August 1989
semimajor
eccentricity
period	d
inclination
satellite_of	Neptune
dimensions	() × () × () km
mean_radius
mass	~ (calculated)
density
rotation	synchronous
axial_tilt	zero
albedo	0.091
magnitude	23.32
single_temperature	~51 K mean (estimate)
note	no
::

| name = Thalassa | image = Neptune Trio.jpg | discoverer = Richard J. Terrile and Voyager Imaging Team | discovered = September 1989 | caption = Thalassa (1989 N5) seen by Voyager 2 on 23 August 1989 | mpc_name = Neptune IV | pronounced = | adjective = Thalassian | named_after = Θάλασσα Thalassa | orbit_ref = | epoch = 18 August 1989 | semimajor = | eccentricity = | period = d | avg_speed = | inclination = | satellite_of = Neptune | dimensions = () × () × () km | mean_radius = | mass = ~ (calculated) | density = | rotation = synchronous | axial_tilt = zero | albedo = 0.091 | magnitude = 23.32 | single_temperature = ~51 K mean (estimate) | note = no

Thalassa , also known as Neptune IV, is the second-innermost satellite of Neptune. Thalassa was named after sea goddess Thalassa, a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea".

Discovery

Thalassa was discovered sometime before mid-September 1989 from the images taken by the Voyager 2 probe. It was given the temporary designation S/1989 N 5. The discovery was announced (IAUC 4867) on 29 September 1989, and mentions "25 frames taken over 11 days", implying a discovery date of sometime before 18 September. The name was given on 16 September 1991.

Physical properties

Thalassa is irregularly shaped. 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. Unusually for irregular bodies, it appears to be roughly disk-shaped.

Orbit

::figure[src="https://upload.wikimedia.org/wikipedia/commons/1/16/Naiad-Thalassa_73-69_orbital_resonance.jpg" caption="Depiction of Naiad's orbital motion (red) in a view that co-rotates with Thalassa (central yellow dot)" alt=""] ::

Since the Thalassian orbit is below Neptune's synchronous orbit radius, it is slowly spiralling inward due to tidal deceleration and may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching. Relatively soon after, the spreading debris may impinge upon Despina's orbit.

Thalassa is currently in a 69:73 orbital resonance with the innermost moon, Naiad, in a "dance of avoidance". As it orbits Neptune, the more inclined Naiad successively passes Thalassa twice from above and then twice from below, in a cycle that repeats every ~21.5 Earth days. The two moons are about 3540 km apart when they pass each other. Although their orbital radii differ by only 1850 km, Naiad swings ~2800 km above or below Thalassa's orbital plane at closest approach. Thus this resonance, like many such orbital correlations, stabilizes the orbits by maximizing separation at conjunction. However, the role of Naiad's nearly 5° orbital inclination in this avoidance in a situation where eccentricities are minimal is unusual.

References

Planet Neptune Data http://www.princeton.edu/~willman/planetary_systems/Sol/Neptune/

| title = Planetary Satellite Physical Parameters | date = 2010-10-18 | publisher = JPL (Solar System Dynamics) | url = http://ssd.jpl.nasa.gov/?sat_phys_par |access-date = November 15, 2019

| last = Green | first = D. W. E. | title = Neptune | date = September 29, 1989 | journal = IAU Circular | volume = 4867 | url = http://www.cbat.eps.harvard.edu/iauc/04800/04867.html | access-date = 2011-10-26

| last = Marsden | first = B. G. | title = Satellites of Saturn and Neptune | date = September 16, 1991 | journal = IAU Circular | volume = 5347 | url = http://www.cbat.eps.harvard.edu/iauc/05300/05347.html | access-date = October 26, 2011

| doi = 10.1016/0019-1035(92)90155-Z | last1 = Banfield | first1 = D. | last2 = Murray | first2 = N. | date=October 1992 | title = A dynamical history of the inner Neptunian satellites | journal = Icarus| volume = 99| issue = 2| pages = 390–401| bibcode = 1992Icar...99..390B}}

|url = https://www.jpl.nasa.gov/news/news.php?feature=7540 |title = NASA Finds Neptune Moons Locked in 'Dance of Avoidance' |author = |date = November 14, 2019 |website = Jet Propulsion Laboratory |access-date = November 15, 2019

|last1 = Brozović |first1 = M. |last2 = Showalter |first2 = M. R. |last3 = Jacobson |first3 = R. A. |last4 = French |first4 = R. S. |last5 = Lissauer |first5 = J. J. |last6 = de Pater |first6 = I. |title = Orbits and resonances of the regular moons of Neptune |date = October 31, 2019 |journal = Icarus |volume = 338 |issue = 2 |article-number = 113462 |arxiv = 1910.13612 |doi = 10.1016/j.icarus.2019.113462 |bibcode = 2020Icar..33813462B |s2cid = 204960799

References

1. {{OED. thalassal
2. ''The complete poetical works of Robert Browning'' (1912)
3. (2019). "The seventh inner moon of Neptune". Nature.

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