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.

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

No that’s not a typo; it's a message Galileo sent to the brilliant German astronomer Johannes Kepler. And it led to the enduring belief that Mars possessed two moons, centuries before they were actually discovered.

Galileo was an observer, using his newly built telescope to sweep the skies and look at things no human had seen before. He saw the mountains on the Moon, the phases of Venus and the stars of the Milky Way. Kepler was a theoretician, gifted in mathematics. He had already performed a feat nobody had ever done by describing planetary motion in mathematical terms. At a stroke he had proved that the heavens were not the mysterious, unknowable realm of God that many believed.

When Kepler received the string of letters from Galileo, he knew at once that it was a coded discovery and he set about unscrambling the anagram.  Galileo had already announced the discovery of four moons at Jupiter, and Kepler’s analytical mind had formed a hypothesis. If Earth had one moon and Jupiter had four, then Mars should have two moons based upon the geometrical progress where you double the previous number to gain a series (1, 2, 4, 8 etc.).

Johannes Kepler

Kepler wrestled with the anagram until he squeezed out "Salve umbistineum geminatum Martia proles." He called it a barbaric verse, and it was a letter different from the original but it served his conviction that Mars had two moons. Kepler's translation reads: "Be greeted, double knob, children of Mars." Imagine reading that in a scientific journal today!

In fact, Kepler was wrong. Galileo had caught a blurry glimpse of the rings of Saturn and had written: "Altissimum planetam tergeminum observavi" meaning "I have observed the highest of the planets three-formed."

Nevertheless, the conviction that Mars had two moons persisted for centuries. In 1726, English satirist Jonathan Swift wrote about them in Gulliver's Travels. But it wasn’t until 1877 that Asaph Hall discovered Phobos and Deimos, the two moons of Mars, from the United States Naval Observatory in Washington DC. -- Stuart