This animation is comprised of three images acquired by ESA’s Mars Express spacecraft on Sept. 12, 2017 with its High Resolution Stereo Camera (HRSC). It shows parts of the grooved and pitted surface of Phobos, the larger of Mars’ two natural satellites.
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.
It may be in its 14th year on Mars but Opportunity still has some surprises to show us—like this, a series of images captured on May 3, 2017 showing the Sun as seen from Mars. But that’s not the special part: see the change in brightness along the Sun’s edge near the end? That was a brief transit of Phobos, the largest (and nearest) of Mars’ two moons!
Can’t see it very well? It’s quick, I know—so check out a cropped and enlarged version below:
Mars isn’t a planet well-known for its natural satellites but it actually does have two small moons. The larger, Phobos, is an irregularly-shaped, heavily grooved and cratered world only about 17 miles (27 km) across at its widest. It orbits Mars so closely that it completes 3 orbits every day, and isn’t even visible from some parts of the planet. But Phobos has an even smaller companion in orbit: Deimos, which at the most 7.5 miles across is half Phobos’ size. Deimos orbits Mars much further away as well, taking about 30 hours to complete one orbit.
Do you love to look up at the Moon? Well so does NASA’s Curiosity rover! Feel free to correct me if I’m wrong (I have not confirmed this) but this appears to be an image of Phobos, the larger of Mars’ two small moons, imaged by Curiosity’s Mastcam on mission Sol 1002 (June 1, 2015). I spotted it while looking though some raw images on JPL’s MSL mission page.
Phobos is a very small world, only about 16 miles (26 km) across, and orbits Mars at about 3,700 miles (6,000 km) altitude. Curiosity has imaged it before, once actually crossing in front of the Sun during an eclipse event on Aug. 20, 2013.
Both Phobos and its smaller, more distant sibling Deimos have been imaged together by Curiosity as well, during an occultation on Aug. 1, 2013. See an animation of those observations here.
Planned observations of Phobos help scientists more precisely determine its orbit.
See a color image of Phobos acquired by the HiRISE camera aboard Mars Reconnaissance Orbiter here.