Phobos is doomed. It is gradually spiralling towards Mars and eventually could slam into the planet’s surface, leaving a large crater as its parting gift. Believe it or not, this discovery led to speculation that Phobos could be a space station launched by an advanced Martian civilization.
At the time, calculations showed that the moon’s orbit was decaying at around 5 cm per year which was subsequently shown to be an overestimate. Phobos is in an unusually low orbit around Mars, and so it was thought that this drag could be caused by the upper atmosphere of the planet. Russian astrophysicist Iosif Samuilovich Shklovsky set about calculating whether the atmosphere could indeed be responsible. What he found surprised not only him, but many others too.
For the atmosphere to be responsible, Phobos would have to be hollow, like an Easter Egg. If the moon were solid rock, the atmosphere would have little effect. A hollow moon would be susceptible because it contained so much less mass. But if the moon were hollow, it could not be a natural object.
That's no moon, it's a... no wait, it is a moon.
Credits: ESA/ DLR/ FU Berlin (G. Neukum)
Writing in the Irish Astronomical Journal in 1964, Estonian astronomer Ernst Öpik noted that there were in fact three possible reasons for Phobos’s orbit. The first was that the observations were in error and Phobos was not spiralling inwards. The second was as Shklovsky suggested – and Öpik agreed that if it were hollow then Phobos was artificial. The third suggestion was that Mars’s gravity acted across the moon producing a so-called tidal force, which could rob the moon of energy.
Dr S. Fred Singer, an American physicist, joined forces with Öpik to investigate. Singer doubted the decay rate was as large as 5 cm per year. He was right. Sadly for the UFO enthusiasts, Phobos was found to be decaying at just 1.8 cm per year and this allowed Singer and Öpik to show that the third case is the correct one. Tidal forces are responsible for the moon slowly spiralling downwards. Star Wars fans will remember the classic line from the first movie, “That’s no moon, it’s a space station.” For a while in the 1960s, some astronomers actually thought this might be true about Phobos. -- Stuart