S/2019 S 1

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S/2019 S 1 is ∼5 kilometers in size and thus one of the small Irregular moons of Saturn. Its discovery has been announced in 2021. Its mean distance to Saturn is ∼11¼ million kilometers, with one synodic revolution around the planet (from “full moon” to “full moon”) on a prograde orbit requiring 1 year, 3 months and 1 week. This moon is a member of the Inuit group and might be a collisional remnant of Kiviuq and/or Ijiraq which share very similar orbital elements. Its mean distance to Saturn is the lowest and its orbit period the shortest of all Irregulars, barely besting the other members of the Kiviuq family.

With the orbit eccentricity tremendously oscillating between ∼0.07 and 0.67 just over a few centuries (see top Fig. below), the orbit of S/2019 S 1 is temporarily very elongated. This brings this object closer to Saturn than any other known Irregular moon, often even below 4 million kilometers (= 4 Gigameters = 4 Gm). For example, according to JPL’s Horizons orbit calculator, on 19 Jul 2003, the periapsis distance of S/2019 S 1 was just 3.678 Gm. Note that Iapetus, the outermost of Saturn’s large moons, orbits Saturn at a distance between 3.46 and 3.67 Gm; the theoretical missing distance was just ∼10000 kilometers! Thus, S/2019 S 1 and Iapetus are “hot” candidates for a future collision, which has so far been prevented by the orientations of the orbits as well as by “bad” timing of potential orbit crossing-point passages. Indeed, these two moons came closer than ∼2 Gm to each other several times in the past millenium, and will do so frequently in the future (see bottom Fig. below). Closest in our lifetimes was 1.37 Gm on 28 Mar 1985, and during the Cassini mission 1.50 Gm on 01 Apr 2013. A collision would vaporize S/2019 S 1 and produce a crater on Iapetus with a size of ∼50…100 kilometers — that’s a really big one!

We made no attempt to observe S/2019 S 1 with the Cassini spacecraft because it was unknown at the time Cassini was active. If known, Cassini could have used exceptionally good opportunities. In retrospective, I found no less than four passages below 3 million kilometers at very favorable solar phase angles. The closest approach (2.1 Gm; at 51° phase) occured on 23 Sep 2015 — a record proximity to an Irregular moon except for the targeted Phoebe flyby. In happy ignorance of this extraordinary chance, we observed Bestla instead the day before for almost 13 hours and received great data, too.

Table of contents

(1) Astronomical and physical properties
(9) References for S/2019 S 1

Fig. (top): Variations in orbit inclination, periapsis distance, and eccentricity of S/2019 S 1 over a time span of 400 years.
Fig. (bottom): Distance of S/2019 S 1 to Saturn and to Iapetus in the same time frame. Selected step size is 20000, this corresponds to one data point every ~7.3 days or ∼11 data points for each Iapetus orbit.
Both plots were created with JPL’s SPICE Geometry Calculator.

This page is intended to compile (much of) our knowledge of unnamed moon S/2019 S 1 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: 24 Jun 2023 — 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/2019 S 1
million km

Basic information about S/2019 S 1 is offered in tabular form:
(1A) Basic properties
← Table (basic properties) in text format [ not available yet; overview table of all Saturnian Irregulars is linked in the meantime ]

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) ∼ 5 km
Moon abbrev.(2) 19S1 Semi-major axis(8) 11.245 ⋅ 10km Absolute visual magnitude(12) ∼ 15.3 mag
IAU number(3) Orbit eccentricity(8) 0.384 Apparent R mag. from Earth(13) 25.3 mag
Provisional desig.(4) S/2019 S 1 Orbit inclination(8) 49.5° First observation date(14) 01 Jun 2019
SPICE ID(5) 65093 Orbital period(8) 445.5 d Announcement date(14) 16 Nov 2021
Also-used label(6) Group member(9) Inuit MPEC announcement(14) 2021-W14
Dynamical family(10) Kiviuq/ Ijiraq Discoverers(15) E. Ashton 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 MPC’s Natural Satellites Ephemeris Service; 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 R magnitude; 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: Edward J. Ashton, Brett J. Gladman, Matthew Beaudoin, Jean-Marc Petit, Mike Alexandersen.

(9) References for S/2019 S 1

Discovery announcement: MPEC 2021-W14
Wikipedia:  S/2019 S 1 S/2019 S 1 S/2019 S 1
My ‘Outer Moons of Saturn’ site: Sheet ‘links and references’

Ashton, E., Gladman, B., Beaudoin, M., Alexandersen, M., Petit, J.-M. (2022): Discovery of the Closest Saturnian Irregular Moon, S/2019 S 1, and Implications for the Direct/Retrograde Satellite Ratio. Planetary Science Journal 3:107 (5pp).

© Tilmann Denk (2023)