Mars and Phobos imaged by Hubble on may 12, 2016. Credits: NASA, ESA, and Z. Levay (STScI), Acknowledgment: J. Bell (ASU) and M. Wolff (Space Science Institute).
On May 12, 2016, the Hubble Space Telescope captured a series of images of Mars and in them the planet’s moon Phobos can be seen appearing from behind the western limb. This was just 10 days before opposition which, in 2016, was the closest Mars had been to Earth since 2005, lending particularly good opportunity for picking out its largest—yet still quite small—moon.
Phobos orbits Mars at an altitude of about 6,000 km. Image: ESA
Phobos is a wee world, only about 16 miles (26 km) across. It orbits Mars at a very low 3,721 miles (5,989 km) altitude, and travels around it rapidly—7 hours and 39 minutes to complete a single orbit, which is considerably faster than the planet spins.
Unlike our Moon which is slowly but steadily moving away from us (about 4 cm every year) Phobos’ orbit is degrading. Eventually (some estimates say about 10-11 million years) Phobos will either crash into Mars or be ripped apart by tidal forces, forming a ring of material around the planet. (Yes, for ...