Callisto
Reference data for Jupiter's moon Callisto: 4,821 km diameter, 16.69-day non-resonant orbit, Valhalla multi-ring basin, and internal ocean.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Opposition Magnitude (V) | 5.65 mag | ± 0.15 mag | JPL Horizons DE440/441 planetary and satellite ephemeris solution | definitive | NASA/JPL Solar System Dynamics & Planetary Fact Sheets |
| Sidereal Orbital Period | 16.689018 d | ± 0.000002 d | JPL Horizons DE440/441 planetary and satellite ephemeris solution | definitive | NASA/JPL Solar System Dynamics & Planetary Fact Sheets |
| Jovicentric Semi-Major Axis | 0.012585 AU | ± 0.000001 AU | Mean orbital distance of 1,882,700 km (26.33 Jupiter radii), outside the Laplace resonance. | definitive | NASA/JPL Solar System Dynamics & Planetary Fact Sheets |
| Mean Volumetric Radius | 0.3783 R⊕ | ± 0.0001 R⊕ | Mean radius of 2,410.3 km (diameter 4,820.6 km—99% the size of Mercury). | definitive | NASA/JPL Solar System Dynamics & Planetary Fact Sheets |
| Mass | 0.01803 M⊕ | ± 0.00001 M⊕ | 1.0759 × 10²³ kg. | definitive | NASA/JPL Solar System Dynamics & Planetary Fact Sheets |
| Opposition Angular Diameter | 0.0268 arcmin | ± 0.003 arcmin | JPL Horizons DE440/441 planetary and satellite ephemeris solution | definitive | NASA/JPL Solar System Dynamics & Planetary Fact Sheets |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Callisto (Jupiter IV) — Outer Galilean Moon at RA 04h 55m 20.00s, Dec +22° 05′ 20.0″ within Taurus, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
Orbiting 1,882,700 kilometers from Jupiter with a period of 16.689 days, Callisto (Jupiter IV) is the outermost of the four Galilean moons and the third-largest satellite in the Solar System after Ganymede and Titan. At 4,820.6 kilometers in diameter, it is practically identical in size to the planet Mercury yet carries only one-third of Mercury's mass (bulk density 1.834 grams per cubic centimeter). Because Callisto does not participate in the inner 4:2:1 Laplace mean-motion resonance that heats Io, Europa, and Ganymede, it has never experienced strong tidal heating or tectonic resurfacing.
Gravity data from the Galileo spacecraft yielded a moment-of-inertia factor near 0.355, indicating that Callisto is only partially differentiated: below an outer icy lithosphere roughly 100 to 150 kilometers thick, rock and water ice remain mixed in a slowly convecting interior without a large segregated iron core. Even so, induced magnetic field signatures detected as Callisto sweeps through Jupiter's rotating magnetosphere point to a salty liquid-water ocean buried 150 kilometers deep. Its dark, four-billion-year-old surface holds the highest impact-crater density in the Solar System, crowned by the Valhalla multi-ring impact basin whose concentric troughs span 3,800 kilometers.
Because it orbits well outside Jupiter's lethal inner Van Allen radiation belts, Callisto receives hundreds of times less ionizing radiation than Europa or Io. Its wide 26-Jupiter-radius orbit also means that for roughly three years around Jupiter's solstices, Callisto passes north or south of Jupiter's disk without transiting or being eclipsed.
Practical Visual Observing Guide
- Reaches elongations up to 10.3 arcminutes from Jupiter—nearly twice as far out as Ganymede—and shines at V = 5.65.
- Despite being almost as large as Ganymede, its dark carbonaceous-coated surface (albedo 0.22) makes it the faintest of the four Galilean moons, though it shows the strongest opposition surge at zero phase angle.
- Only transits and undergoes eclipses during multi-year 'mutual event seasons' when Jupiter'sequatorial plane sweeps across Earth and the Sun.
Catalog Cross-Identifications & Aliases
Common Aliases: Jupiter IV · Outermost Galilean Moon
| Catalog Authority | Identifier |
|---|---|
| SIMBAD | Callisto |
Related Objects, Comparisons & Guides
Ganymede (Jupiter III) — Largest Moon Profile
Reference data for Jupiter's moon Ganymede: 5,268 km diameter, 7.15-day orbit, intrinsic magnetosphere, subsurface ocean, and JUICE target.
Europa (Jupiter II Subsurface Ocean World)
Orbital parameters (3.551 d around Jupiter in 1:2:4 Laplace resonance), magnitude 5.29, icy shell & subsurface ocean, and observing guide for Europa.
Jupiter (Gas Giant & Galilean Moons)
JPL orbital parameters (5.2038 AU, 11.862 yr), opposition magnitude −2.94, 50.1″ angular disk, Galilean moons, and observing guide for Jupiter.
Angular Size, Separation & Field of View
How angular diameter (degrees, arcminutes, arcseconds), spherical angular separation, and small-angle physical diameter relate to telescope field of view.
Apparent vs Absolute Magnitude Explained
Understand the logarithmic Pogson astronomical magnitude scale, apparent visual magnitude (m), absolute magnitude (M at 10 parsecs), and bolometric corrections.
Scientific Sources & Dataset Provenance
- NASA/JPL Solar System Dynamics & Planetary Fact Sheets (DE441 (2021/2026)): Park, R. S., et al. 2021, AJ, 161, 105 (The JPL Planetary and Lunar Ephemerides DE440 and DE441) — NASA Jet Propulsion Laboratory / GSFC