Phoebe

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Phoebe has been discovered in 1899; this was the first discovery of an irregular satellite in the solar system and the first discovery of a moon through photography. With a size of 219 km × 217 km × 204 km and containing >98% of the mass of Saturn’s irregular-moon system, it is the giant of Saturn’s outer moons. Phoebe’s mean distance to Saturn is ∼13 million kilometers; this is among the smallest of all known Saturnian irregular moons, with one revolution around the planet on a retrograde orbit requiring 1 year and 6 months.
By synchronizing the shape model of Phoebe with ground-based stellar occultations and lightcurves, the rotation period was determined to 9 h 16 min 25.14 sec ± 0.07 sec. Different to the orbit direction, the spin direction is prograde. Since the north pole of Phoebe’s rotation axis points quite close to Saturn’s rotation north pole, the seasonal variations on Phoebe’s surface are very similar to the planet’s and the inner moons’s seasons; the sub-solar latitude varies by about ±25° over the course of one Saturn revolution about the Sun. On 11 June 2004, the Cassini spacecraft made a close flyby three weeks before arrival at Saturn.

Table of contents (1) Astronomical and physical properties (3) Images (5) Shape model (9) References for Phoebe

Fig. (left): Short animation of Cassini images of Phoebe while moving through constellation Virgo on 07 Jan 2015 (21 frames; time span: 2:04 h; exposure times: 1.5 sec; range: 12.7 million km; Cassini orbit: rev 211). In the first image, the object is marked by a black cross. The background stars move through the field of view because the camera of the fast moving Cassini spacecraft was tracking Phoebe. The slight horizontal jitter of Phoebe is due to improper tracking at the arcseconds level. Flickering bright spots stem from cosmic-ray hits on the camera’s CCD detector, fixed bright or dark pixels are incorrectly calibrated hot or cold pixels on the CCD, respectively. The space background, in reality pitch black, is displayed in dark gray because this makes visibility of the object easier. Image IDs: N1799321159 to N1799328599.

Fig. (right): Phoebe landscape — high-resolution view from Cassini’s targeted flyby on 11 Jun 2004. Image resolution is ∼65 meters per pixel, solar phase angle ∼83°.

This page is intended to compile (parts of) our knowledge of Phoebe in compact form. Its focus is on the documentation of my Cassini-ISS work (observation planning and data analysis), but it also provides general information obtained from other work, like discovery circumstances and orbital and physical parameters (including Phoebe’s rotation period). 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 Phoebe and on irregular moons of Saturn in general, see the reference list at my outer-Saturnian moons page or the reference list below.

This website is still under development, and more content will be added in the forseeable future. I will remove this note when the page will be close to completion. In the meantime, for example the Phoebe entry in Wikipedia is a rich first-order source of information and references.

Last update: 07 Jun 2024 — 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
Phoebe	million km	years	retrograde	km	h min	1899

Sheet (1) offers basic information about Phoebe in tabular form:
(1A) Designations and discovery circumstances
(1B) Orbit parameters
(1C) Physical parameters (body properties)
← Tables (1A) to (1C) in text format (last updated: 31 Jul 2019)
← Screenshot of Table 4 in Denk et al. (2018) (many values of the tables here in sheet (1) refer to it)

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)	Phoebe		IAU number(3)	Saturn IX		First observation date(7)	16 Aug 1898
Moon abbrev. (TD)(2)	Pho		Provisional desig.(4)	—		Announcement date(7)	17 Mar 1899
			SPICE ID(5)	609		Harvard Obs. announcement(7)	Bull. no. 49
			Also-used label(6)	S9		Discoverers(8)	W.H. Pickering, Stewart

Notes for Table 1A:

⁽¹⁾ Phoebe’s name was suggested by W.H. Pickering about a month after discovery in a note from E.C. Pickering published in the Harvard College Observatory Circular and in identical letters to the Journal of Popular Astronomy, the Astronomical Journal, and the Astronomische Nachrichten. It is named after Phoebe, a sister of Saturn and a Titaness in greek mythology, and follows the scheme of naming Saturnian moons by his siblings, introduced by John Herschel in his work Results of Astronomical Observations made at the Cape of Good Hope from 1847 (in paragraph (398) of chapter VI on page 415).

⁽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 and have not been invented at the time of Phoebe’s discovery.

⁽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⁾ Given are the date of the photography wherein Phoebe was spotted for the first time, and the discovery-announcement date. The Phoebe discovery was reported in a hand-written Bulletin of the Harvard College Observatory in Cambridge, MA. Phoebe was the first ever satellite in the solar system detected photographically.

⁽⁸⁾ The discoverer team included: William H. Pickering, DeLisle Stewart. The discovery announcement was made by Edward C. Pickering, older brother of W.H.P. and director of Harvard College Observatory at that time.

(1B) Orbit parameters

Orbit direction(1)	retrograde		Group member(2)	Norse		Dynamical family(3)	?
Periapsis range(4)	10.81 ⋅ 106 km		Semi-major axis(5)	12.929 ⋅ 106 km		Apoapsis range(6)	15.05 ⋅ 106 km
Semi-major axis(7)	214 R♄		Semi-major axis(8)	0.086 au		Semi-major axis(9)	0.20 RHill
Orbit eccentricity(10)	0.164		Orbit inclination(11)	175.24°		Inclination supplemental angle(12)	4.76°
Orbital period(13)	550.3 d		Orbital period(14)	1 y 6 m		Mean orbit velocity(15)	1.71 km/s

Notes for Table 1B:

⁽¹⁾ Prograde (counterclockwise as seen from north) or retrograde (clockwise as seen from north) — this difficult problem was solved by Pickering (1905).

⁽²⁾ Norse, Inuit, or Gallic

⁽³⁾ Phoebe might be the parent body of several small irregular moons (esp. the Mundilfari group), but this is completely unclear so far; see also Fig. 1 and Table 2 in Denk et al. (2018)

⁽⁴⁾ $r_{Peri}=a\cdot(1-e)$

⁽⁵⁾ Orbit semi-major axis a, from JPL’s Solar System Dynamics Planetary Satellite Mean Elements website

⁽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 JPL’s Solar System Dynamics Planetary Satellite Mean Elements website

⁽¹¹⁾ Orbit inclination i, from JPL’s Solar System Dynamics Planetary Satellite Mean Elements website

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

⁽¹³⁾ From JPL’s Solar System Dynamics Planetary Satellite Mean Elements website

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

⁽¹⁵⁾ From Table 4 in Denk et al. (2018)

(1C) Physical parameters

Mean size(1)	213 $^{+1}_{−1}$ km		Min. equatorial axes ratio(4)	1.01		Mass(6)	8.29 ⋅ 1018 kg
Mean radius(2)	106.4 km		Axes radii (a × b × c)(5)	∼ 109 × 108 × 102 km		Mean density(7)	1.64 g/cm3
Equatorial circumference(3)	∼ 685 km					Surface escape velocity(8)	∼ 320 km/h
Rotation period (sidereal)(9)	9.27365 h		+/- (9)	0.07 s		Spin rate(9)	2.588 d−1
Spin direction(10)	prograde		Pole dir. (ecliptic longitude λ)(12)	61.2° ± 0.3°		Pole direction (geocentric, RA)(13)	356.6° ± 0.3°
Seasons(11)	none		Pole dir. (ecliptic latitude β)(12)	+64.5° ± 0.1°		Pole direction (geocentric, Dec)(13)	+78.0° ± 0.1°
Absolute visual magnitude(14)	6.6 mag		Apparent vis. mag. from Earth(15)	16 mag		Best apparent mag. for Cassini(16)	5.1 mag
Spectral slope(17)	∼ −2.5 %/100nm		B−R color index(17)	∼ 0.98 / ∼ 0.91		Albedo (V-band)(18)	0.086
Hill sphere radius(19)	∼ 18000 km		Hill sphere radius(20)	∼ 170 rPho

Notes for Table 1C:

⁽¹⁾ From Thomas et al. (2018); see also Table 4 in Denk et al. (2018) for a tabular compilation of many Phoebe properties.

⁽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.

⁽³⁾ Estimated from the equatorial axes radii, see note (5).

⁽⁴⁾ From Castillo-Rogez et al. (2012) (see also Table 3 in Denk et al. (2018)).

⁽⁵⁾ Here, a is the long equatorial, b the short equatorial, and c the polar axis dimension of the reference ellipsoid (see Table 4 in Denk et al. (2018)).

⁽⁶⁾ From Jacobson et al. (2006); see also Table 4 in Denk et al. (2018).

⁽⁷⁾ From Thomas et al. (2018); see also Table 4 in Denk et al. (2018).

⁽⁸⁾ From Porco et al. (2005); see also Table 4 in Denk et al. (2018).

⁽⁹⁾ From Gomes-Júnior et al. (2020), determined through synchronization of the Gaskell et al. (2013) shape model with six stellar occultations. The “older”, ∼30x less accurate value from Bauer et al. (2004) (9.2735 ± 0.0006 h) was used in Tables 3 and 4 in Denk et al. (2018). The spin rate is 24/P, measured in units of one per day.

⁽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⁾ From Giese et al. 2006; see also the pole-direction sheet below for details.

⁽¹³⁾ As (12), but in geocentric coordinates (Right Ascension and Declination).

⁽¹⁴⁾ From Miller et al. 2011; see also Tables 2 and 4 in Denk et al. (2018). 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⁾ From Table 3 in Denk et al. (2018). Given is the best apparent magnitude as seen from Cassini at a time when a low-resolution lightcurve observation took place (i.e., not during the targeted flyby in June 2004 where Phoebe became much brighter and bigger).

⁽¹⁷⁾ Color information: Spectral slope S’₂  and B−R color index; from Table 3 in Grav and Bauer (2007). The higher the value, the “redder” the color of the object. Mean wavelengths: 445 nm for B (“blue”), 658 nm for R (“red”) filters. B−R of the Sun is 1.01 (Ramírez et al. 2012).

⁽¹⁸⁾ From Miller et al. 2011; see also Table 4 in Denk et al. (2018).

⁽¹⁹⁾ Hill radius at periapsis under the assumption of the given density (see note (7)).

⁽²⁰⁾ Hill radius at periapsis in Phoebe-radius units. With $R_{Hill}=\sqrt[3]{4\pi\rho_{Pho}/9m_♄}\cdot r_{Pho↔♄}$, 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)).

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(3) Images

Phoebe as seen with Cassini’s narrow-angle camera within ±1 hour of closest approach in June 2004.

Left: Before; right: after closest approach. The spatial resolution is ~80 m/pixel (left) and ~65 m/pixel (right). See NASA’s planetary photojournal (PIA18411) for more details on these two images.

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(5) Shape model

General:
– Available
– References: Porco et al. (2005); Giese et al. (2006); Gaskell (2013)

Shape model of Phoebe from PDS Small Bodies Node (Gaskell 2013). Link to original pdf file →here

Shape model of Phoebe with relative color-coded heights relative to an equipotential surface; adapted from Fig. S1 from Porco et al. (2005).

The calculation assumed homogeneous mass distribution, and accounts for rotational accelerations. Equatorial views with north at the top. Viewpoint longitudes are noted.

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(9) References for Phoebe

Discovery: Harvard College Observatory Bulletin no. 49 (17 Mar 1899)
Wikipedia:  Phoebe (moon)  Phoebe (Mond)
Planetary Photojournal (NASA/JPL): Phoebe images and maps
Cassini Imaging Team (CICLOPS): Approach to Saturn (June 2004)
My ‘Outer Moons of Saturn’ website: Sheet ‘links and references’
JPL’s Icy Moon Treks page: Phoebe Trek

References (that include my work) (time-ordered; newest first)

	Denk, T., Mottola, S. (2019): Studies of Irregular Satellites: I. Lightcurves and Rotation Periods of 25 Saturnian Moons from Cassini Observations. Icarus 322, 80-103. doi:10.1016/j.icarus.2018.12.040
	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 Series, The University of Arizona Press, pp. 409-434.
	Jaumann, R., Clark, R.N., Nimmo, F., Hendrix, A.R., Buratti, B.J., Denk, T., Moore, J.M., Schenk, P.M., Ostro, S.J., Srama, R. (2009): Icy Satellites: Geological Evolution and Surface Processes. In: Saturn after Cassini-Huygens, Dougherty, M., Esposito, L., Krimigis, S. (eds.), Springer-Verlag, chapter 20, 637-681.
	Roatsch, Th., M. Wählisch, F. Scholten, A. Hoffmeister, K.-D. Matz, T. Denk, G. Neukum, P.C. Thomas, P. Helfenstein, C.C. Porco (2006): Mapping of the icy Saturnian satellites: First results from Cassini-ISS. Planetary and Space Science 54, 1137-1145.
	Giese, B., G. Neukum, Th. Roatsch, T. Denk, C.C. Porco (2006): Topographic modeling of Phoebe using Cassini images. Planetary and Space Science 54, 1156-1166.
	Porco, C.C., Baker, E., Barbara, J., Beurle, K., Brahic, A., Burns, J.A., Charnoz, S., Cooper, N., Dawson, D.D., DelGenio, A.D., Denk, T., Dones, L., Dyudina, U., Evans, M.W., Giese, B., Grazier, K., Helfenstein, P., Ingersoll, A.P., Jacobson, R.A., Johnson, T.V., McEwen, A.S., Murray, C.D., Neukum, G., Owen, W.M., Perry, J., Roatsch, T., Spitale, J., Squyres, S.W., Thomas, P.C., Tiscareno, M., Turtle, E.P., Vasavada, A.R., Veverka, J., Wagner, R., West, R. (2005): Cassini Imaging Science: Initial Results on Phoebe and Iapetus. Science 307, 1237-1242.

♦ References (work from colleagues) (alphabet-ordered)

	Bauer, J.M., Buratti, B.J., Simonelli, D.P., Owen, W.M. (2004): Recovering the rotational light curve of Phoebe. Astrophys. J. 610, L57-L60.
	Bauer, J.M., Grav, T., Buratti, B.J., Hicks, M.D. (2006): The phase curve survey of the irregular saturnian satellites: A possible method of physical classification. Icarus 184, 181-197.
	Bottke W.F., Nesvorný D., Vokrouhlický D., Morbidelli A. (2010): The irregular satellites. The most collisionally evolved population in the solar system. Astron. J. 139, 994-1014.
	Buratti, B.J., Hicks, M.D., Davies, A. (2005): Spectrophotometry of the small satellites of Saturn and their relationship to Iapetus, Phoebe, and Hyperion. Icarus 175, 490-495. [ Word of caution: Nomenclature is erroneously mixed in this paper. S/2000 S3 is (S29) Siarnaq, not (S21) Tarvos. S/2000 S11 is (S26) Albiorix, not (S29) Siarnaq. The four irregulars observed for this work were Ymir, Paaliaq, Siarnaq, and Albiorix — but not Tarvos. ]
	Castillo-Rogez, J.C., Johnson, T.V., Thomas, P.C., Choukroun, M., Matson, D.L., Lunine, J.I. (2012): Geophysical evolution of Saturn’s satellite Phoebe, a large planetesimal in the outer Solar System. Icarus 219, 86-109.
	Gaskell, R.W. (2013): Gaskell Phoebe Shape Model V2.0. CO-SA-ISSNA-5-PHOEBESHAPE-V2.0. NASA Planetary Data System.
	Gomes-Júnior, A.R., Assafin, M., Braga-Ribas, F., Benedetti-Rossi, G., Morgado, E., Camargo, J.I.B., Vieira-Martins, R., Desmars, J., Sicardy, B., Barry, T., Campbell-White, J., Fernández-Lajús, E., Giles, D., Hanna, W., Hayamizu, T., Hirose, T., De Horta, A., Horvat, R., Hosoi, K., Jehin, E., Kerr, S., Machado, D.I., Mammana, L.A., Maybour, D., Owada, M., Rahvar, S., Snodgrass, C. (2020): The first observed stellar occultations by the irregular satellite Phoebe (Saturn IX) and improved rotational period. Month. Not. Roy. Astron. Soc. 492, 770-781.
	Grav, T., Bauer, J.M. (2007): A deeper look at the colors of the saturnian irregular satellites. Icarus 191, 267-285.
	Grav, T., Bauer, J.M., Mainzer, A.K., Masiero, J.R., Nugent, C.R., Cutri, R.M., Sonnett, S., Kramer, E. (2015): NEOWISE: Observations of the irregular satellites of Jupiter and Saturn. Astrophys. J. 809(3), 9 pp.
	Graykowski, A., Jewitt, D. (2018): Colors and Shapes of the Irregular Planetary Satellites. Astron. J. 155(184), 10 pp.
	Jacobson, R.A. (1998): The orbit of Phoebe from Earthbased and Voyager observations. Astronomy & Astrophysics Suppl. 128, 7-17.
	Jacobson, R.A., Antreasian, P.G., Bordi, J.J., Criddle, K.E., Ionasescu, R., Jones, J.B., Mackenzie, R.A., Meek, M.C., Parcher, D., Pelletier, F.J., Owen, W.M., Roth, D.C., Roundhill, I.M., Stauch, J.R. (2006): The Gravity Field of the Saturnian System from Satellite Observations and Spacecraft Tracking Data. Astron. J. 132, 2520-2526.
	Miller, C., Verbiscer, A.J., Chanover, N.J., Holtzman, J.A., Helfenstein, P. (2011): Comparing Phoebe’s 2005 opposition surge in four visible light filters. Icarus 212, 819-834.
	Pickering, W.H. (1905): The ninth satellite of Saturn. Annals of Harvard College Observatory 53, 45-74.
	Thomas, P.C., Tiscareno, M.S., Helfenstein, P. (2018): The Inner Small Satellites of Saturn and Hyperion. 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 Series, The University of Arizona Press, 387-408.

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© Tilmann Denk (2024)