While Mars may be one of the most interesting planets in our solar system, its moon Phobos is no doubt one of the most speculated on moons ever identified. Even now in the aftermath of the failed Russian mission to send a probe to the Martian moon, there is still much in the way of mystery about the obscure and possibly hollow satellite. The idea that Phobos might be hollow was first put forth by a Russian astrophysicist by the name of Iosif Samuilovich Shklovsky to explain the odd orbit it has around the red planet. Could Phobos be hollow or even, as some theorists suggest, artificial?
Early examinations of Phobos in the 1950's and 60's suggested that something odd was going on with the orbit of the small moon less than one fifth of one percent the size of Earth. As powerful telescopes focused in on its orbital trajectory, the object was difficult to examine at first, as it had such a low reflective capacity - roughly similar to eyes on Earth as many asteroids hurtling through the void of space. But even with its relative darkness, those observing it closely noted that it did not behave like a solid body. Soon after Iosif began formulating a hypothesis that the structure could be hollow.
This theory was examined extensively by fellow astronomers, including the science adviser to Dwight D. Eisenhower, Fred Singer. Singer suggested the theory that the Phobos moon might be hollow could be explained by miscalculations made by using several different bodies of data in conjunction from different sources. By the end, Iosif Samuilovich Shklovsky's theory was considered debunked. But this would not be the last time Russia considered the possibility that there was something strange about Phobos.
After the 1989 Phobos II probe disappearance, some of those affiliated with the program began sharing photographs suggesting that the probe may have been destroyed by an unexplained extraterrestrial craft. The shadowy elliptical shape shown in one of the photographs was one of the last the probe ever sent back to Earth before vanishing. Understandably interest in Phobos grew considerably.
But while Phobos may not have been as hollow as Samuilovich may have thought at first, subsequent calculations of the moon suggest that it may in fact have a substantial area within it that is completely hollow or at least porous. It's estimated that the planet has either voids within it or is of a porous nature with somewhere between 25% and 35% of the moon being hollow. Samuilovich's theory of a hollow Phobos was partially correct, so it would appear.
And while the most recent examinations suggest that the porous rock of this Martian moon may be on a very small scale much like the lava rock found in the volcanic regions of Hawaii's Mauna Loa or Sicily's Mount Etna, there may be some hollow areas within Phobos with far more room - and possibly more. Could Phobos host secrets somewhere within it just waiting to be discovered? That was one of the things the Phobos Grunt was supposed to uncover - before it mysteriously malfunctioned just like the other Russian probes sent to investigate it.