| Between 2031 and 2034, the outer or Irregular moons of Jupiter are planned to be a research topic of the European 🛰️Juice spacecraft mission at Jupiter. This website provides some information and links to these numerous objects, most of which are only a few kilometers in size. They orbit Jupiter at mean distances ranging from 7½ to 24 million kilometers, and it takes between 4¼ months and two years to complete one orbit. | Zwischen 2031 und 2034 sollen die als äußere oder Irreguläre Monde bezeichneten Satelliten des Jupiter ein Forschungsthema der 🛰️Juice-Raumsonde am Jupiter werden. Diese Website bietet einige Informationen und Links zu diesen zahlreichen, aber zum größten Teil nur wenige Kilometer großen Objekten. Sie umkreisen Jupiter in mittleren Entfernungen zwischen 7½ und 24 Millionen Kilometern und benötigen dafür zwischen 4¼ Monaten und zwei Jahren. |
As of 10 Apr 2026, 115 moons of Jupiter is the official count: 4 large Galilean moons, 4 small inner moons, and 107 outer or Irregular moons. There exist certainly a lot more; numerous objects have been sighted but not yet announced because no reliable orbits could be determined.
The figure on the right (Fig. 2 of my 2026-paper on Jovian Irregulars) shows the orbits of Jupiter’s moons, the planned trajectory of the 🛰️Juice spacecraft from 2031 to 2034 (black line), and as a scale bar the distance between Earth and Mars in July 2033 (~63 million kilometers).
The figure below (an updated version of Fig. 1 in the 2025 Juice JANUS-team paper) shows the orbit inclinations (tilts) over the semi-major axes (mean distances) of Jupiter’s moons. The quite clear orbit-dynamical grouping of the objects is obvious, with different groups shown by different symbols and colors. For the Irregular moons on prograde orbits (open symbols), the inclination values are shown at left; for retrograde moons (filled symbols) on the right. The single black dots indicate each group’s eponym. The large Galilean moons (“GalSats”) which contain almost all mass of the Jovian moon system fall at lower left in the diagram. The small inner moons are not shown, they are even closer to Jupiter. The small spacecraft symbols indicate the apoapses (furthest distances) of the Juice spacecraft at its first and second orbit in October 2031 and March 2032.
← List of all Jovian moons: Designations (names), absolute magnitudes, orbit grouping (.txt file)
← List of all giant planet moons: Numbers, names, TD abbreviations, SPICE/JPL IDs (.txt file)
Links to individual Jovian Irregular-moon pages
• (544) Kallichore
Images of Himalia and Elara (very low resolution)
• from Cassini (Dec 2000): Himalia
Notes: Label at left: date and time (UTC); on top: filter names; at bottom: effective filter wavelengths [nm].
• from New Horizons (Feb 2007): Himalia; Elara
Notes: Top row: 1x zoom; middle row: 5x zoom; bottom row: 5x zoom interpolated.
Paper on a Kallichore stellar occultation and HST observations to support the 🛰️Juice mission
• Rizos, J.L., et al. (2026): Kilometre-scale Jovian moon characterized for a potential JUICE flyby. Nature Astronomy. doi:10.1038/s41550-026-02929-z.
Paper on 🛰️Juice prospects for Jovian Irregular-moons observations (and small inner moons) (and Io)
Denk, T., Williams, D.A., et al. (2026): Io and the Minor Jovian Moons – Prospects for JUICE. Space Science Reviews 222:27 (93 pp.). doi:10.1007/s11214-025-01263-6.
Paper about the JANUS camera onboard the 🛰️Juice spacecraft (including sections on Jupiter’s Irregular moons)
• Palumbo, P., 133 colleagues (2025): The JANUS (Jovis Amorum ac Natorum Undique Scrutator) VIS-NIR Multi-Band Imager for the JUICE Mission. Space Science Reviews 221:32. doi:10.1007/s11214-025-01158-6.
Paper on Himalia’s rotation period
• Pilcher, F., Mottola, S., Denk, T. (2012): Photometric Lightcurve and Rotation Period of Himalia (Jupiter VI). Icarus 219, 741-742. doi:10.1016/j.icarus.2012.03.021.
Paper on Cassini’s Jupiter flyby in Dec 2000, including observations of Himalia
• Porco, C.C., 23 colleagues (2003): Cassini Imaging of Jupiter’s Atmosphere, Satellites, and Rings. Science 299, no. 5612, 1541-1547. doi:10.1126/science.1079462.
• Another source for this paper is here (near bottom of this page).
• An excerpt of the part describing the Himalia findings can be found here.
Outside links
• Wikipedia: Moons of Jupiter
• IAU Minor Planet Center: Natural Satellite Ephemeris Service; Recent MPECs
• JPL SSD: Planetary Satellite Mean Elements; Planetary Satellite Discovery Circumstances
• Scott Sheppard’s Moons of Jupiter page
• Johnston’s Archive on solar-system planets and satellites: Orbital and dynamical data; physical data
• 🛰️Juice at ESA: mission homepage • science homepage • Images/ESA Multimedia
• JPL SSD: HORIZONS Web-Interface (selected spacecraft ID codes: →list)
Jovian Irregular satellite discoveries
• Himalia: Harvard College Observatory Astronomical Bulletin no. 173 (06 Jan 1905)
• Elara: Harvard College Observatory Astronomical Bulletin no. 178 (27 Feb 1905)
• Pasiphae: MNRAS 68 (5), 373 (13 Mar 1908)
Jovian satellite naming
• J1 (Io) to J4 (Callisto): Marius, S. (1614): Mundus Iovialis.
Part relevant to modern naming in Latin and German →here
English translation of modern naming part: Barton (1946) (herein → p. 125)
• J5 (Amalthea) to J13 (Leda): IAU circ. no. 2846 (07 Oct 1975); Owen (1976)
• J14 (Thebe) to J16 (Metis): IAU WGPSN (1983); IAU circ. no. 3872 (30 Sep 1983)
• J17 (Callirrhoe) to J27 (Praxidike): IAU circ. no. 7998 (22 Oct 2002)
• J28 (Autonoe) to J38 (Pasithee): IAU circ. no. 8177 (08 Aug 2003)
• J39 (Hegemone) to J48 (Cyllene): IAU circ. no. 8502 (30 Mar 2005)
• J49 (Kore): IAU circ. no. 8826 (05 Apr 2007)
• J50 (Herse): IAU planetary naming news (09 Nov 2009)
• J53 (Dia): IAU planetary naming news (10 Mar 2015)
• J62 (Valetudo): IAU planetary naming news (03 Oct 2018)
• J57 (Eirene), J58 (Philophrosyne), J60 (Eupheme), J65 (Pandia), J71 (Ersa): IAU planetary naming news (19 Aug 2019)
Calar Alto related information and webpages
My DLR colleague Stefano Mottola and I have performed an observation campaign of the Irregular satellites of Jupiter to measure lightcurves for rotation period, spin-axis orientation, and shape determination. For this task, we used the 1.23-m telescope at Calar Alto in southern Spain which allowed us to investigate basic properties of the largest Jovian Irregular moons.
• 1.23-m telescope [HORIZONS ID code 493] — 357°27’15.1″ E, 37°13’24.7″ N, 2173.1 m altitude
• Weather forecast (continuous live data stream)
• Current Meteosat weather picture
• Calar-Alto webcams (continuous live data stream); the WEST, Southwest and NORTH views show the dome of the 1.23-m telescope
• Former DLR page about the 1.23-m telescope
• Observers timeline for fall 2026 and spring 2026
Table: Physical and astronomical properties of the 11 largest Irregular moons of Jupiter (H < 15 mag) and Kallichore
| Moon | JPL ID (1) | Orbit (2) | Apparent magnitude [mag] (3) |
Absolute magnitude H [mag] (4) |
Visible albedo range [%] (5) |
Mean diameter [km] (5) |
Rotation period [h] (6) |
Semi-major axis [million km] (7) |
Eccentricity [ ] (7) |
Inclination [deg] (7) |
Orbital period [d] (7) |
Moon (abbrev.) (8) |
| Themisto | 518 | prograde | 21.0 | 13.3 | 6 ? | ∼9 | ? | 7.40 | 0.34 | 44 | 130.0 | Thm |
| Himalia | 506 | prograde | 14.8 | 8.0 | 4.9–6.5 |
x
|
|
11.44 | 0.16 | 28 | 250.6 | Him |
| Elara | 507 | prograde | 16.6 | 9.5 | 3.9–5.3 | 80 | ? | 11.71 | 0.21 | 28 | 259.6 | Ela |
| Lysithea | 510 | prograde | 18.2 | 11.1 | 3.0–4.2 | 42 | (12.8) | 11.70 | 0.12 | 27 | 259.2 | Lys |
| Leda | 513 | prograde | 20.2 | 12.6 | 2.8–4.0 | 22 | ? | 11.15 | 0.16 | 29 | 240.9 | Led |
| Pasiphae | 508 | retrograde | 16.9 | 10.2 | 3.8–5.0 | 58 | ? | 23.47 | 0.41 | 148 | 743.6 | Pas |
| Carme | 511 | retrograde | 17.9 | 10.9 | 2.9–4.1 | 47 | (10.4) | 23.14 | 0.26 | 165 | 734.2 | Car |
| Sinope | 509 | retrograde | 18.3 | 11.3 | 3.6–4.8 | 35 | (13.2) | 23.68 | 0.26 | 157 | 758.8 | Sin |
| Ananke | 512 | retrograde | 18.9 | 11.8 | 3.2–4.4 | 29 | (8.3) | 21.03 | 0.24 | 148 | 629.8 | Ana |
| Callirrhoe | 517 | retrograde | 20.8 | 14.0 | 3.6–6.8 | 10 | ? | 23.80 | 0.30 | 145 | 758.9 | Clh |
| Praxidike | 527 | retrograde | 21.2 | 14.9 | 2.3–3.5 | 7 | ? | 20.94 | 0.25 | 148 | 625.4 | Pra |
| Kallichore | 544 | retrograde | 23.7 | 16.3 | 4 ? | ∼4½ ? | ? | 23.02 | 0.25 | 165 | 728.3 | Kch |
Table notes:
(1)…JPL’s 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.
(2)…The Irregular satellites of Jupiter can be subdivided into objects with prograde and objects with retrograde orbits. Themisto is the Irregular moon closest to the planet. The moons in the prograde Himalia group share an inclination close to 28°. The retrograde Irregulars orbit Jupiter in three distinct groups at inclinations ranging from ∼145° to ∼165°. The naming convention is that all prograde Irregular moons of the Himalia group have names ending with an ‘a’, the other prograde Irregulars have names ending with ‘o’, and the retrograde moons’s names end with an ‘e’.
(3)…Apparent magnitude as seen from Earth (R-band); from S. Sheppard’s Jupiter satellites website. Smaller numbers indicate brighter objects. The magnitude scale is logarithmic, with an object of 6th mag being 100x darker than a 1st mag object. The human eye can spot celestial objects down to ∼6th magnitude.
(4)…The absolute magnitude is the magnitude (brightness) of an object if located 1 au away from the sun and observed at 0° phase angle (i.e., the observer virtually sits at the center of the sun in this definition). Smaller numbers again indicate brighter objects. The H values listed here were taken from Table 1 of the paper of Grav et al. (2015) who refers to Rettig et al. (2001). The Themisto, Callirrhoe, Praxidike and Kallichore values are from MPEC. Be aware that different sources list quite different values for H; e.g., compare with values from MPEC, a NASA Fact Sheet, JPL’s ssd page, S. Sheppard, or Table 12.1 in Jewitt et al. (2004) (which has the same values as the Sheppard site).
(5)…Albedos and diameters are from WISE and NEOWISE data published in Table 3 of Grav et al. (2015). The axes of Himalia are from our Cassini work, published in Porco et al. (2003). The Themisto diameter is again from S. Sheppard (and might well be underestimated); the Themisto albedo is just a guess. The Kallichore albedo is a guess based on Carme’s, with the corresponding diameter taken from this table.
(6)…All values are synodic rotation periods. The Himalia value is from our Pilcher et al. (2012) paper. The Lysithea, Sinope, Carme, and Ananke periods are from Luu (1991). Because of the short observation timelines, they are likely inaccurate and thus notated in brackets only. Luu (1991) also observed Elara, Pasiphae, and Leda, but could not deduce periods.
For Elara, JPL’s HORIZONS Web-Interface gives ∼0.5 d, but I could not find a proper source from where they got it. Maybe it was taken from Table 6.1 in Stanton Peale’s chapter in the 1977 Planetary Satellites book of the University of Arizona Press? If so, it would definitely be wrong because all unknown periods in this table are listed as “0.5 d”. Thus, no Elara rotation period is added to my table here.
Leda was observed by Rettig et al. (2001) over three consecutive nights, but they could not extract a rotation period. They thus note ∼24 h as a possible value for Leda, but I do not believe that because a non-detection of a period does not necessarily point to a period close to Earth’s. It may also mean that the equatorial cross-section of the object is rather spherical with no prominent albedo markings, that the object’s pole axis was pointing close to the observer, that the rotation itself is very slow, or that the observation’s signal-to-noise ratio was insufficient.
(7)…The data for the orbital parameters were taken from JPL SSD (May 2025).
(8)…These abbreviations are my own and not official.
© Tilmann Denk (2026)