"Astronomers are still unsure of where Phobos came from. Did it form with Mars as a planet? Is it a captured asteroid, now trapped in orbit? Or is Phobos a chunk of Mars flung into orbit from an impact? (Or… maybe it’s an ancient alien spacecraft?? Just kidding.) Wherever it came from, as a moon Phobos is an oddity. In addition to its small size – only 16 miles across at its widest – low reflectivity (albedo) and irregular shape, it orbits its parent planet at a rather low altitude, only 5,840 miles (as compared to our own Moon’s 248,000 mile distance) and thus needs to travel at a very high speed in order to stay in orbit. It is actually orbiting Mars overthree times faster than Mars rotates, and rises in Mars’ western sky. Its orbit is so low, in fact, that it can’t even be seen from the polar regions on Mars !"
