New images and video captured by ESA’s Mars Express orbiter show two small Martian moons Phobos and Deimos drifting in front of the giant planet Saturn and background stars.
Both moons are named after the sons of Ares, the Greek god of war, who was known as Mars in Roman mythology. Phobos (panic or fear) and Deimos (terror or dread) accompanied their father into battle.
Phobos, Mars’ innermost and largest moon, has an oblong shape with an average diameter of about 14 miles (22 km). It is an unusual satellite, orbiting closer to its planet than any other moon in the Solar System.
The little moon orbits Mars about 3,721 miles (5,989 km) from the surface and completes an orbit in just 7 hours and 39 minutes.
It orbits so close to the Martian surface that the curvature of the planet would obscure its view from an observer standing in Mars’ polar regions. Its orbital period is about 3 times faster than the rotation period of the Red Planet, with the unusual result among natural satellites that Phobos rises in the west and sets in the east as seen from the planet.
Deimos has a mean radius of about 3.7 miles (6 km) and is less irregular in shape.
It orbits around Mars with a period of 30 hours at a distance of approximately 14,580 miles (23,460 km).
There is much we still wish to know about the Martian moons.
Phobos and Deimos remain particularly mysterious, with open questions about their origins, formation and composition. As a result, combined with their proximity to Mars, the moons have generated a lot of interest as a target for future missions.
Phobos in particular has been considered for a possible landing and sample-return mission.
Owing to its nearness to Mars and one side always facing its parent, the moon could also be a possible location for a more permanent observation post.
This would enable long-term monitoring and study of the Martian surface and atmosphere, and communications relay for other spacecraft.
ESA’s Mars Express spacecraft has been studying the Red Planet and its two moons for more than 14 years.
While several other orbiters are currently at Mars, Mars Express’ near-polar elliptical orbit gives it some advantages for certain observations.
In particular, its path takes it closer to Phobos than any other spacecraft, and allows it to periodically observe the moon close up from within 93 miles (150 km).
The images of Phobos and Saturn comprising the new video were taken on November 26, 2016, by Mars Express’ High Resolution Stereo Camera.
The orbiter was traveling at about 6,710 mph (3 km/s) when it obtained these views, highlighting the importance of knowing Phobos’ exact position: the spacecraft had just seconds to image the rocky body as it passed by.
Astronomers repeatedly refine our knowledge of the moons’ positioning in the sky and ensure it is up-to-date by observing each moon against background reference stars and other Solar System bodies. These calculated positions are incredibly precise, and can be accurate to just a couple of miles.
Alongside the view of Phobos set against Saturn, Mars Express also obtained images of the small moon against a reference star on January 8, 2018, close-up images of Phobos’ pockmarked surface on September 12, 2017, and images of Deimos with Saturn on January 15, 2018.
The frames of Phobos’ surface were taken during close flybys, and show the bumpy, irregular and dimpled surface in detail.
Phobos has one of the largest impact craters relative to body size in the Solar System: Stickney crater’s 5.6 mile (9 km) diameter is around a third of the moon’s diameter. It is visible as the largest crater in these frames.
The same side of the moon always faces the planet, which means multiple flybys are needed to build up a full map of its surface.
Deimos is visible as an irregular and partially shadowed body in the foreground of one of the new Mars Express images, with the delicate rings of Saturn just about visible encircling the small dot in the background.