The paths of Deimos and Phobos are nearly circular, and they both lie on the plane of Mars’ equator. They also show the same face to the Red Planet as they revolve around it. But there ends the similarity!
The average orbital distance of Deimos, the smaller of the two moons, is around 23,500 kilometres! So from Mars on a full-moon night, Deimos looks like a bright star, just about one-twelfth the size of our Moon (as seen from the Earth)! It takes around 30.3 hours to complete an orbit (lesser than the rotational period of Mars, 24.7 hours) and so it rises in the east and sets in the west.
In comparison, Phobos, with an orbital distance of nearly 9,400 kilometres, travels quite close to Mars. This is why, despite its (relatively) bigger size, Phobos is not visible from the polar regions of Mars! It makes Phobos quite fast too – its orbital period is only about 7.7 hours – and so, if you are standing on Mars, you will see Phobos rise from the west and set in the east twice a day! But an unfortunate side-effect of this proximity to Mars is that Phobos is slowly getting pulled closer to the planet, and will be destroyed within a span of 50 million years or so. Deimos, on the other hand, is moving further away from Mars, and will one day be lost in space!
Picture Credit : Google
