Mars hosts a pair of small, irregularly shaped moons named Phobos and Deimos. Unlike Earth’s single, large, and spherical moon, these diminutive Martian satellites orbit closely to the planet’s surface. They present a dramatically different celestial viewing experience for any observer on Mars. Both Phobos and Deimos are readily observable from the Martian surface, but they look nothing like the familiar full moon that dominates Earth’s night sky.
The Visual Reality: Seeing Both Moons
Phobos, the larger and closer of the two moons, appears noticeably disk-shaped and is bright enough to be visible even during the Martian day. At its peak, Phobos can reach an apparent magnitude of around –9, making it significantly brighter than any star or planet visible from Earth. Deimos, conversely, is much smaller and orbits farther out, appearing more like an exceptionally bright star in the night sky. It is still brighter than any natural star in the Martian sky, peaking at a magnitude of about –5.
The apparent size difference is considerable, reflecting their distance from the observer. Phobos can appear up to one-third to one-half the angular size of Earth’s full moon, particularly when it passes overhead near the equator. Deimos, however, only appears as a point of light or a small, round disk, roughly twice the maximum apparent size of Venus as seen from Earth. This visual disparity means the two moons never present a traditional “moonlight” experience, but rather a dynamic, two-part show.
Observing Phobos: The Rapid Transit
Phobos is irregularly shaped, with dimensions of about 26 by 22 by 18 kilometers. Due to its close orbit, an observer can sometimes see the jaggedness of its terminator, the dividing line between light and shadow. The moon’s speed is its most remarkable visual feature, as it completes an orbit in only 7 hours and 39 minutes.
This rapid orbital period means Phobos streaks from the horizon to the opposite horizon in approximately four hours. It can complete this journey up to three times during a single Martian day. Furthermore, the phases of Phobos change noticeably during a single transit, unlike Earth’s moon which maintains its phase throughout the night. This swift motion and quickly changing phase make it a unique and dynamic object to track.
Observing Deimos: The Star-Like Companion
Deimos is the smaller of the pair, with a mean diameter of about 12.5 kilometers. It orbits far enough away that it appears almost star-like, best described as an intensely bright point of light or a tiny disk. For an observer on Mars, Deimos would shine brightly at night, easily outshining all background stars.
Deimos has a much longer orbital period of just over 30 hours, only slightly longer than the Martian day. Because of this near-synchronous orbit, it moves very slowly across the sky. Once Deimos rises, it can remain visible above the horizon for an extended period, taking about 2.7 days to finally set. This slow pace contrasts sharply with the frantic speed of its inner companion.
Unique Orbital Mechanics
Phobos orbits Mars at an exceptionally close mean distance of just 9,377 kilometers, making it the closest natural satellite to its planet in the solar system. This low altitude places Phobos well below Mars’s synchronous orbit, the altitude where an object’s orbital period matches the planet’s rotation. Because Phobos is faster than the planet’s rotation, it exhibits apparent retrograde motion.
This mechanical reality causes Phobos to rise in the west and set in the east, which is highly unusual for a natural satellite. Deimos, conversely, orbits at a greater distance of approximately 23,460 kilometers. This places Deimos just outside the synchronous orbit radius. Its slightly slower orbital speed relative to the planet’s rotation allows it to follow the expected eastward movement, rising in the east and setting in the west, but at a very slow pace.