It is the largest of the two satellites on Mars, orbiting at 9,380 km. of the planet performing around you a complete turn in 7 hours and 39 minutes. Its period of rotation is identical to that of revolution, and therefore this body, like our Moon, always directs the same face to its planet.
Phobos is shaped like an elongated and pitted potato, with dimensions of just 27 x 21 x 19 km; He is therefore considered, as his youngest brother Deimos, an asteroid captured by the planet. Its mass is 9.6 x 10 raised to 15 kg. and its density of 1.9 g / cm3.
The physical properties of Phobos have been known for a relatively short time, when the Mariner 9 space probe photographed it from about 1971. Before that date the Mars satellite had made many astronomers fantasize. Speculating on the fact that its movement around the planet is in slight but constant acceleration, and attributing this phenomenon to a difference in orbit due to braking (and therefore the descent into a lower and faster orbit) by the rarified Martian atmosphere, Carl Sagan and others had advanced the hypothesis that only a hollow body inside and low density, such as an artificial satellite, had been able to experience this unique phenomenon.
A Russian astronomer, F. Zigel, had even come to claim that, having been discovered by Asaph Hall in 1877 and by J Herschel, who had observed him with a larger instrument and in much more favorable conditions during the Mars opposition in 1862, the artificial satellite Fobos would have been launched by the Martians precisely between 1862 and 1877.
To demonstrate how the myth that the planet Mars was inhabited, fed by Lowell in the early years of the twentieth century, hypothesized that a Martian civilization in danger of extinction, had been forced into orbit large space stations to be Safe from any kind of natural catastrophe.
The explorations from near Mars and its satellites, threw many assumptions to the ground and all the strange thing that can be said now about Phobos is that it looks like a potato eaten by a lobster, as defined by Carl Sagan after being disenchanted.
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