On May 1, 1959, it was reported that Soviet scientist Iosif Shklovsky had found evidence that the two moons of Mars, Phobos and Deimos, may be artificial. In other words, they may have been placed in orbit by Martians.
Shklovsky had studied data that had been collected by others and concluded that Phobos, in particular, was most likely hollow inside with what could be something like a thin sheet metal exterior. Its behavior could not be explained by comparing it to any known natural satellite in our solar system. Instead, it behaved much like the artificial satellites that man had placed in orbit around Earth. The logical conclusion was that Martians had placed the two moons into orbit some two or three million years prior.
Further study later determined that the data that Shklovsky used to make these predictions, which he did not collect himself, had systematic errors. It’s not that Shklovsky did bad science – the whole Martian idea excluded – it’s just that he had really bad data to work with.
A number of space probes have since been sent to study these two moons. Today we are certain that they are solid, naturally made, and very similar to many of the asteroids out there.