Should Phobos Serve as the Entry Port of SpaceX to Mars?

Phobos is the innermost, larger moon of the two satellites of Mars. Phobos orbits Mars every 7 hours, 39 minutes and 12 seconds. With a mass of ten trillion tons and a mean diameter of 22 kilometers — the length of Manhattan Island, the escape speed from Phobos is only 11.4 meters per second. This is equivalent to 41 kilometers per hour, a fifth of the speed achievable by the Tesla Cybertruck. In other words, a fast car could lift off from a runway on the surface of Phobos. This makes Phobos an excellent port for shipping cargos between Earth and the vicinity of Mars. Hence my question to Elon Musk: “Should Phobos be used as the entry port and service station of SpaceX near Mars?”

In full disclosure, this port will not last forever. Tides decrease the orbital radius of Phobos by 2 meters per century. Within 50 million years, Phobos will come too close to Mars and break up due to the Martian tidal force on it. If SpaceX will build a port on Phobos, the industrial base on it will need to be relocated within 50 million years. But we should also keep in mind that the debris from the disruption of Phobos could damage any technological infrastructure on the surface of Mars. Any dust and rocks that will remain in orbit around Mars might offer a spectacular view to human residents there, akin to the rings around Saturn.

Currently, the day and night sides of Phobos exhibit extreme temperature differences. The side illuminated by the Sun shows a temperature of -4 degrees Celsius, similar to a winter day in Boston, while the temperature on the night side — located merely a few kilometers away, dips to -112 degrees Celsius, cooler than Antarctica. Heat is lost rapidly as a result of the large surface area of the finely-grained regolith dust resting on the surface of Phobos. Based on data from the Mars Global Surveyor, this dust is estimated to be at least 100 meters thick, likely produced by impacts as the surface is heavily cratered. Phobos reflects a small fraction, about 7%, of the sunlight impinging on its surface. Data-based models indicate that Phobos may be a rubble pile held together by a thin crust.

Phobos is not a spherical body and along three axes it has different diameters of 26 by 22.8 by 18.1 kilometers. Its surface gravity is too small to make it round or to retain an atmosphere around it. Infrared spectra show that the surface material is carbon-rich resembling the primitive surface composition of Mars.

Phobos orbits 6,000 kilometers from the Martian surface, at an altitude comparable to the radius of Earth and closer than any known natural satellite of a planet. Mars rotates around its axis over a period of 24.6 hours, which is 3.2 times longer than the orbital period of Phobos. An astronaut on the surface of Mars would see Phobos twice every day, rising in the west and setting in the east.

Since Phobos occupies an equatorial orbit, it cannot be seen above the horizon from latitudes greater than 70.4 degrees. Its angular diameter, as seen by an astronaut on Mars, varies with position in the Martian sky from 0.14 degrees wide at the horizon to 0.2 degrees at the zenith, about a third of the angular diameter of the full Moon viewed from Earth.

The angular diameter of Phobos constitutes about half the angular diameter of the Sun as viewed from Mars. In contrast to the Earth’s Moon which changes phases over the course of a month, Phobos changes phases over a third of a day. A Martian astronaut monitoring Phobos could see regular partial eclipses of the Sun, as photographed by the Mars Rovers Opportunity and Perseverance.

Many space probes photographed Phobos in addition to the Mars Rovers. They include Mariner 7 in 1969, Mariner 9 in 1971, Viking 1 in 1977, Phobos 2 in 1989, Mars Global Surveyor in 1998 and 2003, Mars Express in 2004–2019, Spirit rover in 2005, and Mars Reconnaissance Orbiter in 2007–2008, and Mars orbiter in 2020.

The origin of Phobos remains unclear. The possibility that Phobos is a captured asteroid is not supported by analysis of radio data from the Mars Express mission. Based on the gravitational attraction that Phobos exerted on the spacecraft, it was concluded that the interior of this moon likely contains large voids, making its composition and structural strength less likely to be associated with a captured asteroid.

Alternatively, Phobos might have formed near Mars out of ejecta from impacts on the Martian surface, or from remnants of a previous moon which subsequently collided with a member of the asteroid belt. The primitive composition and equatorial, nearly circular orbit of Phobos suggests that it formed from materials in orbit around Mars.

There had been no successful sample-return mission that brought materials from Phobos to Earth. It was suggested that the Kaidun meteorite from 1980 was a piece of Phobos, but without a reference-sample from Phobos it is impossible to validate this conjecture.

Raw materials from Phobos could be used to develop a Martian space industry. This may include a space elevator, extending 6,000 kilometers from the Mars-facing side of Phobos to the edge of the Martian atmosphere. The space elevator could serve as an entry point for cargo after they arrive to the Martian system on a trip from Earth. The small escape speed from Phobos would make return trips much cheaper than from the surface of Mars.