Phobos is the innermost, larger moon of Mars’ two satellites. Phobos orbits Mars every 7 hours, 39 minutes and 12 seconds. With a mass of ten trillion tons and an average diameter of 22 kilometers (the length of Manhattan Island), the escape velocity from Phobos is only 11.4 meters per second. This corresponds to 41 kilometers per hour, one fifth of the speed of the Tesla Cybertruck. In other words, a fast car could take off from a runway on the surface of Phobos. This makes Phobos an excellent port for shipping cargo between Earth and the Martian environment. Hence my question to Elon Musk: “Should Phobos be used as SpaceX’s gateway and refueling station near Mars?”
In full disclosure, this haven won’t 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 break up due to the tidal force of Mars on it. If SpaceX wants to build a port on Phobos, the industrial base on it will have to be moved within 50 million years. But we should also keep in mind that the debris from Phobos’ disruption could damage any technological infrastructure on the surface of Mars. Any dust and rock left in orbit around Mars could provide its human inhabitants with spectacular views, similar to the rings around Saturn.
Currently, the day and night sides of Phobos exhibit extreme temperature differences. The sunlit side shows a temperature of -4 degrees Celsius, comparable to a winter day in Boston, while the temperature on the night side (just a few kilometers away) drops to -112 degrees Celsius, cooler than Antarctica. Heat is lost quickly due to the large surface area of the fine-grained regolith dust resting on Phobos’ surface. Based on data from the Mars global surveyorit is estimated that this dust is at least 100 meters thick, probably caused by impacts as the surface is heavily cratered. Phobos reflects a small portion, about 7%, of the sunlight that strikes its surface. Data-based models indicate that Phobos may be a pile of rubble held together by a thin crust.
Phobos is not a spherical body and has different diameters along three axes of 26 by 22.8 by 18.1 kilometers. The force of gravity at the surface is too small to make it round or to maintain an atmosphere around it. Infrared spectra show that the surface material is carbon-rich, which resembles the primitive surface composition of Mars.
Phobos orbits at a distance of 6,000 kilometers from the surface of Mars, at an altitude comparable to the radius of Earth and closer than any known natural satellite of any planet. Mars rotates on its axis in 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 a day, rising in the west and setting in the east.
Because Phobos occupies an equatorial orbit, it cannot be seen above the horizon from latitudes greater than 70.4 degrees. The angular diameter, as seen by an astronaut on Mars, varies depending on position in the Martian sky from 0.14 degrees wide at the horizon to 0.2 degrees at the zenith, about one-third the angular diameter of the full moon when viewed from Earth.
The angular diameter of Phobos is about half the angular diameter of the Sun as seen from Mars. Unlike Earth’s moon, which changes phase over the course of a month, Phobos changes phase over a third of a day. A Mars astronaut monitoring Phobos could regularly see partial solar eclipses, as photographed by the Mars Rovers Possibility And Perseverance.
In addition to the Mars Rovers, many space probes have also photographed Phobos. 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 pull that Phobos exerted on the spacecraft, it was concluded that this moon’s interior likely contains large voids, making its composition and structural strength less likely to be related to a captured asteroid.
Alternatively, Phobos could have formed near Mars from ejecta from impacts on the Martian surface, or from remnants of a previous moon that subsequently collided with a member of the asteroid belt. Phobos’s primitive composition and equatorial, nearly circular orbit suggest that Phobos 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 1980 Kaidun meteorite was a piece of Phobos, but without a reference sample of Phobos it is impossible to validate this suspicion.
Raw materials from Phobos could be used to develop a space industry on Mars. This could 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 it arrives in the Martian system during a journey from Earth. The small escape velocity from Phobos would make the return flights much cheaper than from the surface of Mars.
