page_title: Phobos (moon)
section_title: Orbital characteristics
hierarchical_section_title: Phobos (moon) / Orbital characteristics
caption_attribution_description: English: PIA17351: Illustration Comparing Apparent Sizes of Moons http://photojournal.jpl.nasa.gov/catalog/PIA17351 This image from NASA's Mars rover Curiosity provides a comparison for how big the moons of Mars appear to be, as seen from the surface of Mars, in relation to the size that Earth's moon appears to be when seen from the surface of Earth. This illustration provides a comparison for how big the moons of Mars appear to be, as seen from the surface of Mars, in relation to the size that Earth's moon appears to be when seen from the surface of Earth. Earth's moon actually has a diameter more than 100 times greater than the larger Martian moon, Phobos. However, the Martian moons orbit much closer to their planet than the distance between Earth and Earth's moon. Deimos, at far left, and Phobos, beside it, are shown together as they actually were photographed by the Mast Camera (Mastcam) NASA's Mars rover Curiosity on Aug. 1, 2013. The images are oriented so that north is up. The size-comparison image of Earth's moon, on the right, is also oriented with north up. Deimos has a diameter of 7.5 miles (12 kilometers) and was 12,800 miles (20,500 kilometers) from the rover at the time of the image. Phobos has a diameter 14 miles (22 kilometers) and was 3,900 miles (6,240 kilometers) from the rover at the time of the image. Earth's moon has a diameter of 2,159 miles (3,474 kilometers) and is typically about 238,000 miles (380,000 kilometers) from an observer on Earth. Malin Space Science Systems, San Diego, built and operates Mastcam. NASA's Jet Propulsion Laboratory manages the Mars Science Laboratory mission and the mission's Curiosity rover for NASA's Science Mission Directorate in Washington. The rover was designed, developed and assembled at JPL, a division of the California Institute of Technology in Pasadena. For more about NASA's Curiosity mission, visit http://www.jpl.nasa.gov/msl, http://www.nasa.gov/mars, and http://marsprogram.jpl.nasa.gov/msl.
context_page_description: Phobos is the innermost and larger of the two natural satellites of Mars, the other being Deimos. Both moons were discovered in 1877 by American astronomer Asaph Hall.
  Phobos is a small, irregularly shaped object with a mean radius of 11 km and is seven times as massive as the outer moon, Deimos. Phobos is named after the Greek god Phobos, a son of Ares and Aphrodite and the personification of fear.
  Phobos orbits 6,000 km from the Martian surface, closer to its primary body than any other known planetary moon. It is so close that it orbits Mars much faster than Mars rotates, and completes an orbit in just 7 hours and 39 minutes. As a result, from the surface of Mars it appears to rise in the west, move across the sky in 4 hours and 15 minutes or less, and set in the east, twice each Martian day.
  Phobos is one of the least reflective bodies in the Solar System, with an albedo of just 0.071. Surface temperatures range from about −4 °C on the sunlit side to −112 °C on the shadowed side. The defining surface feature is the large impact crater, Stickney, which takes up a substantial proportion of the moon's surface.
context_section_description: The orbital motion of Phobos has been intensively studied, making it "the best studied natural satellite in the Solar System" in terms of orbits completed. Its close orbit around Mars produces some unusual effects. With an altitude of 5,989 km (3,721 mi), Phobos orbits Mars below the synchronous orbit radius, meaning that it moves around Mars faster than Mars itself rotates. Therefore, from the point of view of an observer on the surface of Mars, it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice each Martian day (every 11 h 6 min). Because it is close to the surface and in an equatorial orbit, it cannot be seen above the horizon from latitudes greater than 70.4°. Its orbit is so low that its angular diameter, as seen by an observer on Mars, varies visibly with its position in the sky. Seen at the horizon, Phobos is about 0.14° wide; at zenith it is 0.20°, one-third as wide as the full Moon as seen from Earth. By comparison, the Sun has an apparent size of about 0.35° in the Martian sky. Phobos's phases, inasmuch as they can be observed from Mars, take 0.3191 days (Phobos's synodic period) to run their course, a mere 13 seconds longer than Phobos's sidereal period. As seen from Phobos, Mars would appear 6,400 times larger and 2,500 times brighter than the full Moon appears from Earth, taking up a quarter of the width of a celestial hemisphere.
caption: The relative sizes of Deimos and Phobos as seen from the surface of Mars, compared to the relative size in the sky of the Moon as seen from Earth
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