Surtur (S/2006 S 7)

back to ‘Outer Moons of Saturn’

Surtur is ~4 kilometers in size and thus one of the smallest known irregular moons of Saturn. It has been discovered in 2006 joint with eight other outer moons. Surtur’s mean distance to Saturn is ~23 million kilometers; this is among the farthest of all known Saturnian irregular moons, with one revolution around the planet on a retrograde orbit requiring 3 years, 6 months and 2½ weeks.
Cassini tried to observe Surtur during two observation campaigns in 2012 and 2016, but pointing was apparently not precise enough and Surtur managed to escape us. Therefore, the rotational period is not known.

Table of contents

(1) Astronomical and physical properties
(2) Cassini observations: Overview
(8) Cassini ISS observations: Planning
(9) References for Surtur

Fig. (middle): From the 2-panel mosaic at apoapsis of orbit 233/234, all images of the 1st “footprint” were co-added, as were all of the 2nd. The resulting ‘image 1’ was then subtracted from ‘image 2’. If Surtur were in one or both frames, a fuzzy black or white spot would have shown up — but it did not.
Fig. (right): Planning plot of the 1×2 mosaic from orbit 233/234 in an inertial reference frame, showing the movement of the Cassini spacecraft (parallaxe). The pointing swiches between the two “footprints” were done approximately every hour. 

This page compiles (much of) our knowledge of Surtur in compact form. Its main focus lies on the documentation of my Cassini-ISS work (observation planning). In the 1st section, it also provides general information obtained from other work, like discovery circumstances and orbital and physical parameters. It does not include the raw data (images or spectra) taken by the Cassini spacecraft, these are available at NASA’s Planetary Data System (PDS). For further reading on Surtur and on irregular moons of Saturn in general, see the reference list at my ‘Outer Moons of Saturn’ page.

Last update: 30 Oct 2018 — 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
 million km

Basic information about Surtur is offered in tabular form:
(1A) Designations and discovery circumstances
(1B) Orbit parameters
(1C) Physical parameters (body properties)
← Tables (1A) to (1C) in ASCII format

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

(1A) Designations and discovery circumstances
Moon name(1) Surtur IAU number(3) Saturn XLVIII First observation date(7) 05 Jan 2006
Moon abbrev. (TD)(2) Sur Provisional desig.(4) S/2006 S 7 Announcement date(7) 30 Jun 2006
SPICE ID(5) 648 IAU circ. announcement(7) no. 8727
Also-used label(6) S48 Discoverers(8) S. Sheppard et al.

Notes for Table 1A:

(1) Surtur’s name was announced on 05 Apr 2007 in IAU circ. 8826. It is taken from the Norse mythology where Surtr is a Jötunn (frost giant) who plays a major role during the events of Ragnarök (a series of future events).

(2) I use this 3-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.

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

(8) The discoverer team included: Scott Sheppard, David Jewitt, and Jan Kleyna.

(1B) Orbit parameters
Orbit direction(1) retrograde Group member(2) Norse Dynamical family(3) Ymir
Periapsis range(4) 12.71 ⋅ 106 km Semi-major axis(5) 22.94 ⋅ 106 km Apoapsis range(6) 33.17 ⋅ 106 km
Semi-major axis(7) 380 R Semi-major axis(8) 0.153 au Semi-major axis(9) 0.35 RHill
Orbit eccentricity(10) 0.45 Orbit inclination(11) 169.7° Inclination supplemental angle(12) 10.3°
Orbital period(13) 1296 d Orbital period(14) 3 y 6 m 2½ w Mean orbit velocity(15) 1.29 km/s

Notes for Table 1B:

(1) Prograde (counterclockwise as seen from north) or retrograde (clockwise as seen from north)

(2) Norse, Inuit, or Gallic

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

(4) $r_{Peri}=a\cdot(1-e)$

(5) Orbit semi-major axis a, from Table 2 in Denk et al. (2018)

(6) $r_{Apo}=a\cdot(1+e)$

(7) Saturn radius R = 60330 km (100 mbar level)

(8) Astronomical Unit 1 au = 149 597 870.7 km

(9) Saturn’s Hill sphere radius $R_{Hill}=\sqrt[3]{m_♄/3m_☉}\cdot r_{♄↔☉}$= ∼65 ⋅ 106 km = ∼1085 R♄ = ∼3° as seen from Earth at opposition (with mass of Saturn m = 5.6836 ⋅ 1026 kg and perihel range Saturn↔Sun r♄↔ = 1.353 ⋅ 109 km)

(10) Orbit eccentricity e, from Table 2 in Denk et al. (2018)

(11) Orbit inclination i, from Table 2 in Denk et al. (2018)

(12) Orbit “tilt” or inclination supplemental angle i’ = i for prograde moons; i’ = 180°−i for retrograde moons

(13) From the Planetary Satellite Mean Orbital Parameters page of JPL’s solar system dynamics website

(14) Value from (13) in units of years, months, weeks

(15) $v=\sqrt{Gm_♄/a}$ (Gravitational constant G = 6.6741 ⋅ 10−20 kmkg−1 s−2 )

(1C) Physical parameters
Mean size(1) 4 $^{+1}_{−1}$ km Min. equatorial axes ratio(4) unknown Mass(6) ∼ 1.5 ⋅ 1013 kg
Mean radius(2) ∼ 1.9 km Axes radii (a × b × c)(5) unknown Mean density(7) 0.5 g/cm3 (?)
 Equatorial circumference(3)  ∼ 12 km Surface escape velocity(8) ∼ 3 km/h
Rotation period(9) unknown +/- (9) Spin rate(9) unknown
Spin direction(10) unknown Pole dir. (ecliptic longitude λ)(12) unknown Pole direction (geocentric, RA)(13) unknown
Seasons(11) unknown Pole dir. (ecliptic latitude β)(12) unknown Pole direction (geocentric, Dec)(13) unknown
Absolute visual magnitude(14) ∼ 15.8 mag Apparent vis. mag. from Earth(15) 24.8 mag Best apparent mag. for Cassini(16) 16.0 mag
Color(17) unknown Albedo(18) 0.06 (?)
Hill sphere radius(19) ∼ 260 km Hill sphere radius(20) ∼ 135 rSur

Notes for Table 1C:

(1) Determined from absolute visual magnitude H (see note (14)). 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 Surtur’s albedo, see note (18). 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).

(2) Half the diameter value. While the diameter is the intuitive size number, the radius r is mainly used in formulas to calculate other quantities. Important: While the given number is the formal result from the equation of note (1), the true precision is much lower (also see note (1)).

(3) Estimated under assumption of a spherical equatorial circumference.

(4) Ratio between long equatorial reference axis a and short equatorial reference axis b; unknown because no lightcurve is available.

(5) Unknown because no shape model is available.

(6) The mass is a very rough guess, estimated through density ρ and volume $\frac{4\pi}{3}r^3$; see notes (7) and (2).

(7) The density of Surtur is not known, the given number is speculative. There are indications from other Saturnian irregular moons that these objects have quite low densities (well below 1 g/cm3), similar to comets or some of the inner small moons of Saturn. However, a higher density, maybe up to 2.5 g/cm3, cannot be ruled out.

(8) $v_{esc}=\sqrt{\frac{2GM}{R}}$; very rough guess as well since it depends on Surtur’s mass (note (6)) and radius (notes (1) and (2)) which are not well known. = 6.674 · 1011 mkg−1 s−2 (Gravitational constant).

(9) Unknown because Cassini could not observe the object.

(10) Valid entries: Prograde (counterclockwise as seen from north), retrograde (clockwise as seen from north), ‘lying on the side’ (pole direction almost perpenticular to ecliptic pole), or ‘unknown’.

(11) Valid entries: “None” (rotation axis points close to one of the ecliptic poles), “moderate” (rotation axis is moderately tilted), or “extreme” (rotation axis is highly tilted, points somewhere close to the ecliptic equator), or ‘unknown’.

(12) —


(14) From Table 2 in Denk et al. (2018); the number may be uncertain by several tenths of magnitude. The absolute visual magnitude H 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.

(15) Apparent visual magnitude V; from Table 2 in Denk et al. (2018).

(16) Given is the best apparent magnitude as seen from Cassini at a time when an observation took place.


(18) Might vary by ±0.03; see discussion in Denk et al. (2018).

(19) Hill radius at periapsis under the assumption of the given density (see note (7)). The number would be larger for a higher density, or lower for a lower density.

(20) Hill radius at periapsis in Surtur-radius units. With $R_{Hill}=\sqrt[3]{4\pi\rho_{Sur}/9m_♄}\cdot r_{Sur↔♄}$, this number only depends on the object’s distance to the central body (Saturn; linear dependency) and on the object’s density (proportional to the cubic root; see also note (7)).

(2) Cassini observations: Overview

Table of contents:
(2A) Availability of Cassini observations and results
(2B) Imaging observations (ISS)

Note: ISS is the abbreviation for the Cassini cameras (Imaging Science Subsystem) and has nothing to do with the International Space Station which was named ISS many years later. A high-level instrument’s description is given in Porco et al. (2004).

(2A) Availability of Cassini observations and results

General overview on product and data availability. For imaging (ISS) high-level planning details → section (8).

Cassini observations yes (but object was not in ISS field-of-view)
Rotational period no
Object size approx. (but not from Cassini data)
Pole direction no
Shape model no
Phase curve no
Color (ISS, visible) no
Spectra (UV; vis., IR) no
(2B) Imaging observations (ISS)

General data-acquisition overview for Surtur:

No. of observation requests (“visits”)  — 3 (last as a 1×2 mosaic)
No. of successful requests of duration >6 h  — 3 (but no object found in data)
Observation dates  — 12/2012 + 03/2016
Apparent visual magnitudes  — 16.0 – 16.3 mag
Distances to Cassini  — 11.6 – 15.5 ⋅ 106 km
Phase angles  — 12° – 44°
Approx. number of images  — ≈ 480

Detailed ISS planning → section (8)

(8) Cassini ISS observations: Planning

Table of contents:
(8A) Table: Surtur observations overview
(8B) Map: Surtur in the sky of Cassini
(8C) Surtur geometry and visibility graph (range, phase, and magnitude vs. time)

General overview on ISS planning
sheet (2B)

High-level camera planning commands:
(includes all three IOI files plus post-downlink notes)

Fail notes (Cassini ISS)
– ISS_177OT_SURROT015 (22 Dec 2012): Surtur not seen, very likely mispointed due to insufficient ephemeris.
– ISS_177OT_SURROT012 (24 Dec 2012): As two days before. In addition, straylight from nearby Saturn saturated the images of the last third of the observation.
– ISS_233OT_SURROT044 (21 Mar 2016): Object again not found despite 1×2 mosaic alongtrack and rather good capture prognosis; likely mispointed.

(8A) Table: Surtur observations overview

Observational circumstances and geometry information for the three ISS observation requests of Surtur.

← Table (8A) in ASCII format


a The naming scheme used for Cassini Solstice Mission observations (“requests”) gives information on the object (first three letters), the request’s primary goal (Rot = rotation period), and the approximate observation phase angle (three digits). The three digits between the first and the second underline indicate Cassini’s orbit number. “OT” stands for “other target”.

b Rev. = revolution = Cassini orbit number; peri = periapsis segment. Note that (for some reasons) the true number of orbits since Saturn arrival in 2004 is off (ahead) by 1.5 compared to the official count which is noted here and used in all technical aspects.

c Time difference between shutter mid-time of first and of last image used for the lightcurve.

d A uniform Earth-to-object distance of 1.319·10km (range at opposition in April 2013) is used here for the distance between the irregular moons and Earth.

e For full-resolution images. The values must be doubled when the NAC was operated in 2×2 summation mode.

f Calculated from the absolute magnitude H, the observation phase angle α, and the distances of Surtur to the sun and to Cassini.

g Coordinates of the irregular moons (geocentric RA/Dec and ecliptic λ/β) as seen from Cassini during the observations, see also sheet 8B.

h Phase-angle bisector vector (longitude and latitude). For definition and use, see appendix in Harris et al. (1984).

(8B) Map: Surtur in the sky of Cassini

Surtur as seen from Cassini between 2010 and 2017. Orthographic projection of the sky; geocentric coordinate system (RA/Dec). As a retrograde object, Surtur moves from left to right.

Shown are Surtur’s locations (colored, wiggled lines), the Surtur locations during Cassini observations (light-yellow circles with annotations), the anti-directions of the Cassini observations (bluish diamonds), the location of the sun from 2004 to 2017 (smooth sinusoidal line from RA ∼ 290° to ∼ 85°; the sun moves from right to left), the location of the anti-sun (0° phase) from 2004 to 2017 (pale sinusoidal line from RA ∼ 110° to ∼ 265°), the Milky Way (yellow band) and the Magellanic clouds (yellow spots to the lower right), the ecliptic poles (gray diamonds), and Saturn’s poles (gray diamonds).

NP = north pole; SP = south pole.

(8C) Surtur geometry and visibility graph

This graph has been used for Surtur observation scheduling.

Notes: Left axis: Range of Surtur to Cassini (red) and to Saturn (orange); right axes: phase angle (green) and apparent magnitude of Surtur (blue) as seen from Cassini for the time range 01 Jan 2009 to 15 Sep 2017 (end-of-mission).

(9) References for Surtur

IAU circular, discovery: no. 8727
IAU circular, naming: no. 8826
Wikipedia:  Surtur (moon) Surtur (Mond)
My ‘Outer Moons of Saturn’ site: Sheet ‘links and references’

Denk, T., Mottola, S., Tosi, F., Bottke, W.F., Hamilton, D.P. (2018): The Irregular Satellites of Saturn. In: Enceladus and the Icy Moons of Saturn (Schenk, P.M., Clark, R.N., Howett, C.J.A., Verbiscer, A.J., Waite, J.H., editors), Space Science SeriesThe University of Arizona Press, pp. 409-434.

© Tilmann Denk (2018)