OVERVIEW
] MOONPhobos is a moon of Mars. It orbits Mars at a mean distance of 9375 . It has a mass of 1.06×10¹⁶ and a mean radius of 11.08 . It was discovered in 1877.
The larger of Mars's two moons, orbiting so low and so fast that it rises in the west, and spiralling in towards eventual destruction.
- WHAT IT IS
- Moon of Mars
- HOW FAR
- 9375
- HOW BIG
- 11.08
- ONE ORBIT TAKES
- 7.6
Its average distance from Mars, not from Earth.
Its mean radius: half the width across.
One trip round Mars.
THE MODEL
] CATALOGUED RADIUSCALCULATED · NOT AN OBSERVATION · CATALOGUED RADIUS · DENSITY 1.87 G/CM³ · RADIUS 11.1 KM · LIT FROM WHERE THE SUN IS
Drawn from the catalogued radius and from the same properties the picture at the top of this page uses, at the distance a flight to it would stop. Drag it, or use the arrow keys, to look from another direction. The stars behind are the 498,631 this catalogue holds real distances for, at their real positions and drawn from where the camera actually is — so the constellations are wrong, and that is the point. This is the sky from there, not the sky from here.
IMAGES
] 2 FRAMES- NASA / JPL-Caltech / University of Arizona
Public domain - Andrea Luck
CC BY 2.0
OBSERVING
] FROM WHERE YOU AREWhat it takes to see Phobos for yourself. Everything in this part of the page is worked out for a real horizon, a real night and the equipment you say you have.
CATALOG DATA
] 25 FIELDSThe raw catalogue values, as the source published them, with the error bars where one was published — sorted by what each set of figures is about rather than by the order they happened to be stored in.
WHAT KIND OF THING IT IS
What the object is in itself, rather than how it happens to look from here.
- Mean density
- 1.872 ±0.076
THE ORBIT
The shape of the path, where the object sits on it, and the instant those figures were true of. An orbit is a fit to observations and drifts away from the real one either side of its epoch.
- Orbits
- Mars
- Orbital period
- 0.3187
- Eccentricity
- Inclination
- 1.1
- Longitude of ascending node
- 169.2
- Argument of periapsis
- 216.3
- Mean anomaly
- 189.7
- Element epoch
- 2451545.0000
- Elements referred to
- the local Laplace plane — where the orbit's own node precesses
- Reference plane pole (right ascension)
- 317.7
- Reference plane pole (declination)
- 52.9
- Apsidal precession period
- 1.1
- Nodal precession period
- 2.3
- Mean anomaly fitted to the ephemeris
- 193.98
- Mean anomaly rate
- 1128.38
- Apsidal precession rate
- 0.896024
- Nodal precession rate
- −0.428533
- Agreement with the ephemeris
- 1.28
WHEN IT WAS FOUND
Who first saw it, from where, and by what method.
- Discovered by
- A. Hall
OTHER NAMES AND NUMBERS
The identifiers other catalogues file this object under, for anyone joining this data to theirs.
- Also known as
- Mars I, Mars I Phobos, Phobos (moon)
- JPL satellite code
- 401
- Wikidata item
- Q7547
- Wikipedia article
- Phobos (moon)
WHERE THESE FIGURES CAME FROM
Which catalogue, paper or model each of the fields above was read from, where the source named one.
- Ephemeris
- MAR099
ALSO KNOWN AS
] 1 DESIGNATIONS- Common name Phobos
CITE THIS PAGE
] DR1This page is live and its numbers move when an upstream refreshes, so the citation names the release the rows belong to rather than the day you read them. The release itself is frozen, republished as a bundle, and is what a data-availability statement should point at.
SpaceCatalog. Phobos [catalogue entry]. Data release DR1 (2026-08-21). https://spacecatalog.org/object/phobos
@misc{spacecatalog_phobos,
author = {{SpaceCatalog}},
title = {{Phobos}},
howpublished = {SpaceCatalog, data release DR1 (2026-08-21)},
year = {2026},
url = {https://spacecatalog.org/object/phobos},
note = {Catalogue entry, data release DR1 (2026-08-21). Values from JPL Planetary Satellite Data.}
}The values above were taken from JPL Planetary Satellite Data — No licence stated; citation requested. Citing this page does not replace citing that catalogue.
That catalogue asks for this acknowledgement: Solar System Dynamics. (Downloaded 2026, September 13). Planetary Satellite Mean Elements; Planetary Satellite Physical Parameters; Planetary Satellite Discovery Circumstances. https://ssd.jpl.nasa.gov
RECENT ACTIVITY
] 2 ITEMS- Perseverance Captures Another Phobos Transit2026-08-24 · NASA · MISSION UPDATE
- Perseverance Captures Another Phobos Transit2026-08-24 · NASA · RELEASE
NASA · UPDATED DAILY