S/2004 S 24

S/2004 S 24 is ∼3½ kilometers in size and thus among the smallest known irregular moons of Saturn. Its discovery has been announced in 2019 joint with nineteen other outer Saturnian moons. Its mean distance to Saturn is ∼23 million kilometers, with one revolution around the planet on a prograde orbit requiring 3 years, 8 months and ½ week.
The orbit of this object is very unusual: Among the prograde moons, it has the by far largest distance to Saturn on an almost circular path. Usually, only the retrograde moons make it that far out. However, since the orbit tilts of almost all retrogrades are much lower — S/2004 S 24 hovers, so to speak, above or below the vast majority of the retrograde moons for most of the time —, the chance of a collision with another moon is quite limited. The orbit eccentricity of 0.07 is the lowest of all irregular moons of Saturn.
This moon has not been named yet. We made no attempt to observe it with Cassini because it was unknown at the time Cassini was active. Even if it would, Cassini and S/2004 S 24 never came closer than 20 million kilometers to each other, too far for useful observations. (For a few weeks during Cassini’s initial orbit in August 2004, the distance shrank to 16 million kilometers — not really better.)

This page is intended to compile (much of) our knowledge of unnamed moon S/2004 S 24 in compact form, including general information like discovery circumstances and orbital and physical parameters. For further reading on irregular moons of Saturn in general, see the reference list at my outer-Saturnian moons page.

Last update: 27 Apr 2022 — page content is best displayed on a screen at least 1024 pixels wide

(1) Astronomical and physical properties

 Moon name Saturn range Orbit period Orbit direction Size Rotation period Discovery year S/2004 S 24 million km years prograde ∼ km unknown 2019

Basic information about S/2004 S 24 is offered in tabular form:
(1A) Basic properties
← Table (Basic properties) in text format [ not available yet ]

Most fundamental values are highlighted in red. The notes offer explanations, calculations, accuracies, references, etc. The data were obtained from ground-based observations.

(1A) Basic properties
 Moon name(1) — Orbit direction(7) prograde Mean size(11) ∼ 3½ km Moon abbrev.(2) 4S24 Semi-major axis(8) 23.346 ⋅ 106 km Absolute visual magnitude(12) ∼ 16.0 mag IAU number(3) — Orbit eccentricity(8) 0.072 Apparent vis. mag. from Earth(13) 25.2 mag Provisional desig.(4) S/2004 S 24 Orbit inclination(8) 36.5° First observation date(14) 12 Dec 2004 SPICE ID(5) 65070 Orbital period(8) 1343.9 d Announcement date(14) 07 Oct 2019 Also-used label(6) — Group member(9) — MPEC announcement(14) 2019-T131 Dynamical family(10) — Discoverers(15) S. Sheppard et al.

Table notes:

(1) The object has no proper name yet.

(2) I use this 4-letter abbreviation in the diagrams of my publications simply for practicability reasons. These have no offcial character.

(3) Moon numbers are assigned by the International Astronomical Union (IAU)’s Committee for Planetary System Nomenclature. For satellites, Roman numeral designations are used.

(4) Designation given to the object in the first announcement; the guidelines are explained here.

(5) SPICE is a commonly-used information system of NASA’s Navigation and Ancillary Information Facility (NAIF). It assists engineers in modeling, planning, and executing planetary-exploration missions, and supports observation interpretation for scientists. Each planet and moon obtained a unique SPICE number. In case of this object, the number is still provisional.

(6) ‘S’ for ‘Saturnian moon’ plus the roman numeral designation in arabic numbers are often-used labels for satellites. Not sure how official that is.

(7) Prograde (counterclockwise as seen from north) or retrograde (clockwise as seen from north).

(8) Orbit semi-major axis a, eccentricity e, inclination i, orbit period P; from JPL’s Solar System Dynamics Planetary Satellite Mean Elements website.

(9) Norse, Inuit, or Gallic.

(10) Classification based on the a,e,i space in Fig. 1 and Table 2 in Denk et al. (2018).

(11) Determined from absolute visual magnitude H (see note (12)). The conversion from H to size (diameter of a reference sphere) was calculated through $D=1 \text{ au}\cdot \frac{2}{\sqrt{A}}\cdot 10^{−0.2·(H−M_☉)}$; with solar apparent V magnitude M = −26.71 ± 0.02 mag and Astronomical Unit 1 au = 149 597 870.7 km. For the object’s albedo A, a value of 0.06 is assumed (see discussions in Grav et al. (2015) and Denk et al. (2018) on albedo uncertainties). Due to the uncertain input values, a size determined this way may be uncertain to ∼ −15/+30% (for A ± 0.02 and H ± 0.1).

(12) From MPEC; the number may be uncertain by several tenths of magnitude. The absolute visual magnitude HV is the magnitude (brightness) of an object (in the visible wavelength range) if located 1 au away from the sun and observed at 0° phase angle (i.e., in this definition, the observer virtually sits at the center of the sun). The magnitude scale is logarithmic, with an object of 6th mag being 100x darker than a 1st mag object.

(13) Apparent visual magnitude V; from S. Sheppard’s website.

(14) The date of the photography wherein the object was spotted for the first time is given in the MPEC released on the announcement date.

(15) The discoverer team included: Scott Sheppard, David Jewitt, Jan Kleyna, Robert Jacobson.

© Tilmann Denk (2022)