Phobos
Phobos is the larger and innermost moon of Mars, notable for its irregular shape and heavily cratered surface.
Also known as: Mars I
How big is it?
22.53 km · representative mean diameter
Representative mean diameter · approximate
Typical recorded range: 18 km to 27 km.
A little perspective
Phobos vs Neutron star: diameter comparison
The mean diameter of Phobos is about 1.13 times the diameter of a neutron star.
Phobos is the larger and innermost moon of Mars, notable for its irregular shape and heavily cratered surface.
A neutron star is the dense remnant core left after a massive star explodes as a supernova, composed mostly of neutrons.
Both rulers start at the same zero and share one linear scale. Artwork keeps its original proportions.
Measurements, assumptions & sources +
Phobos
Representative mean diameter: 22.53 km; approximate; range 18 km–27 km.
Mars’s larger moon is irregular, about 27 × 22 × 18 km. The 22.533 km comparison diameter is representative rather than a spherical edge.
The marked representative mean diameter is calibrated. Unmeasured illustration details are representative.
Neutron star
Diameter: 20 km; representative.
A 20 km, 1.4 solar-mass teaching model. The precise radius depends on stellar mass and the uncertain behaviour of dense matter.
The marked diameter is calibrated. Unmeasured illustration details are representative.
Use the visual comparison to explore its scale, then check the measurements and sources.
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Why this measurement?
Why we use 22.53 km
We compare the mean diameter of Phobos. This is an estimate; check the notes for its rounding and uncertainty. The recorded range is 18 km to 27 km.
Mars’s larger moon is irregular, about 27 × 22 × 18 km. The 22.533 km comparison diameter is representative rather than a spherical edge.
The marked horizontal span follows the measured feature, not the edges of the image. How calibrated comparisons work →
Recorded properties
Dimensions and measurements
| Property | Selected value | Recorded range | Basis |
|---|---|---|---|
| 22.53 km | 18 km to 27 km | approximate | |
| 1.066 × 10¹³ t | Not recorded | derived |
Around this scale
Explore nearby values.
Compare examples with lower, similar and higher values. Each card names the measurement being compared.
Nearest lower valuePlanets, moons and spaceNeutron star20 kmDiameterA neutron star is the dense remnant core left after a massive star explodes as a supernova, composed mostly of neutrons.
Around the same scalePeople and human anatomyOne-hour cycle20 kmDistanceA one-hour cycle is the distance covered by cycling at a constant speed for one hour without stopping.Nearest higher valuePlaces and natural featuresOnyx River32 kmApproximate main-channel lengthThe Onyx River is a seasonal meltwater stream in Antarctica’s McMurdo Dry Valleys, regarded as the continent’s longest river.Sources and method
Want to check the number?
The source records and measurement method remain available for inspection.
- NASA — PhobosIrregular body approximately 27 × 22 × 18 km.
- NASA JPL — Planetary satellite physical parametersPublished mean radii and gravitational parameters GM. Spherical areas, volumes, masses and local gravity are explicitly derived.
There’s always another scale
Follow your curiosity.
SpaceMars6,779 kmMars is a rocky planet orbiting the Sun, with a reddish surface due to iron oxide.
SpaceDeimos12.4 kmDeimos is the smaller and outermost natural satellite of Mars, with an irregular, rocky shape.
SpaceMoon3,475 kmThe Moon is Earth’s natural satellite and the brightest large object in Earth’s night sky.SpaceNeutron star20 kmA neutron star is the dense remnant core left after a massive star explodes as a supernova, composed mostly of neutrons.PeopleOne-hour cycle20 kmA one-hour cycle is the distance covered by cycling at a constant speed for one hour without stopping.PlacesChallenger Deep10.94 kmChallenger Deep is the deepest known point in the Earth's oceans, located in the Mariana Trench.
Places100 km² island100 km²An island is a landmass entirely surrounded by water.TransportOne-hour drive at 100 km/h100 kmA one-hour drive at a set speed is the distance a vehicle travels in one hour of uninterrupted motion.