Phobos & Deimos
Other Names:
The Brother Moons, Mars I/II
Astronomical Information
Rock asteroid-moon (colonized)
1st & 2nd
Orbital Information
Orbital Period: 7.66 hours 1.26 days
Semimajor Axis: 9,375 km 23,458 km
Periapsis: 9,234 km 23,453 km
Apoapsis: 9,517 km 23,463 km
Inclination: 26.04° 27.58°
Eccentricity: 0.01 0
Physical Information
Radius: 11.1 km 6.2 km
Gravity: 581.4 µg 306 µg
Rotational Period:
Axial Tilt: 1.07° 1.68°
4.503 billion years
-40.1 °C
Exploration Information
- 1969/2013 (Mariner 7/PADME, first fly-bys)
- 1988/2020 (Phobos 1/Aladdin, first orbiters)
- 1994/2024 (Phobos 2/Gulliver, first landers)
- 2028/2030 (Eris 1/2, first manned landings)
- 2042 (Laputa & Hall Bases)
- 2089 (UNAMM founded)
Population: 26,053 (2110) 5,949 (2110)

Phobos and Deimos are the only natural satellites of Mars. Compared to Luna (and indeed, most major moons), both moons are small and potato-shaped, likely being asteroids captured by Mars’ gravity. Phobos, the larger of the two, is only 22.2 km in diameter, while Deimos is only 12.6 km. They serve an important role in the colonization of Mars by functioning as orbital bases for resupply and refueling for travel between planets, as well as waystations for transitioning between Martian orbit and the surface.


The recorded history of both moons starts in 1726. The popular novel Gulliver's Travels, by Johnathan Swift accurately predicted the existence of the moons. In the book, this was said about Mars:

"They have likewise discovered two lesser stars, or satellites, which revolve around Mars, whereof the innermost is distant from the center of the primary exactly three of his diameters, and the outermost five: the former revolves in the space of ten hours, and the latter in twenty-one and a half."

The moons were first observed directly in 1877 by American astronomer Asaph Hall, and scientific interest in the moons began immediately thereafter. Exploration of the moons via probes began as early as 1988 through the Phobos program of the Soviet Union, but the first successful mission to carry out all of its intended functions were the PADME missions launched by NASA in 2020 along with Ares-1. In 2026, the Eris 1 mission (which was a sub-mission of Ares-4) carried out the first manned exploration of Phobos, with 2028’s Eris 2 (a sub-mission for Ares-5) doing the same for Deimos.

As the colonization of Mars began in the late 2030s, the Enyo program was enacted in 2042 to cut back on costs associated with Martian colonization by establishing an outpost on Phobos to become a waystation between newcomers arriving on Mars and those transferring between surface and orbit. With the success of Laputa Base and new missions carrying out explorations of the Asteroid Belt and outer planets, a similar station dubbed Hall Base was constructed on Deimos to serve as a waystation for missions that would simply stop and refuel in high Martian orbit rather than to the planet itself. Civilian colonization of Phobos and Deimos began in 2072 and 2078, respectively.

Both Phobos and Deimos serve an important function in the colonization and exploration of the Solar System. Being small and having very little gravity, mineral resources mined from the two moons can easily be transported out of the moons and either to the planet’s surface or even back to Earth or other planets where they are needed. Perhaps more critical, however, is the significant amount of water ice on the two moons, which not only allows for the Mars settlements to gather large amounts of water necessary to survive, but can be broken apart into both hydrogen and oxygen to resupply unmanned missions going further into the outer Solar System. Efforts have been taken to further developments on the moons for both civilian and exploratory purposes, with the long-term goal of making both Laputa Base and Hall Base self-sufficient and have the ability to expand using on-site materials by 2120. The United Nations Space Administration is also expected to open up some parts of Phobos to private corporations for commercial purposes soon, as many large conglomerates have openly expressed interesting in reaping the material rewards the moons hold.

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