S/2004 S 21 – Tilmann Denk

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S/2004 S 21 is ∼3 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 retrograde orbit requiring 3 years, 7 months and 2½ weeks.

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.

(1) Astronomical and physical properties

This page is intended to compile (much of) our knowledge of unnamed moon S/2004 S 21 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: 19 May 2023 — page content is best displayed on a screen at least 1024 pixels wide

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(1) Astronomical and physical properties

Moon name	Saturn range	Orbit period	Orbit direction	Size	Rotation period	Discovery year
S/2004 S 21	million km	years	retrograde	∼ km	unknown	2019

Basic information about S/2004 S 21 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.

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(1A) Basic properties

Moon name⁽¹⁾	—	Orbit direction⁽⁷⁾	retrograde	Mean size⁽¹¹⁾	∼ 3 km
Moon abbrev.⁽²⁾	4S21	Semi-major axis⁽⁸⁾	23.131 ⋅ 10⁶km	Absolute visual magnitude⁽¹²⁾	∼ 16.3 mag
IAU number⁽³⁾	—	Orbit eccentricity⁽⁸⁾	0.409	Apparent vis. mag. from Earth⁽¹³⁾	25.4 mag
Provisional desig.⁽⁴⁾	S/2004 S 21	Orbit inclination⁽⁸⁾	155.0°	First observation date⁽¹⁴⁾	12 Dec 2004
SPICE ID⁽⁵⁾	65079	Orbital period⁽⁸⁾	1327.1 d	Announcement date⁽¹⁴⁾	07 Oct 2019
Also-used label⁽⁶⁾	—	Group member⁽⁹⁾	Norse	MPEC announcement⁽¹⁴⁾	2019-T127
		Dynamical family⁽¹⁰⁾	Kari	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 H_V 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.

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Table of contents

(1) Astronomical and physical properties

(1A) Basic properties