A Tiny Moon Meets a Giant Planet
18 May 2014

Fig. 1

This is a series of images from the Mars-orbiting Mars Express spacecraft, of the European Space Agency (ESA), showing Phobos, a small moon of Mars, passing in front of Jupiter, the fifth and largest planet, visible in the background, as a pale point-like object. Phobos moves from right to left.

Credit: ESA/DLR/FU Berlin (G. Neukum)

The European Space Agency (ESA) recently published an intriguing series of images (Fig. 1), showing an alignment of a tiny moon, the Martian satellite Phobos, and mighty Jupiter, the largest planet. The images of Fig. 1 were acquired from Mars orbit, by ESA’s Mars Express spacecraft. This sequence shows Phobos, the biggest of Mars’ two satellites, crossing, from right to left, in front of Jupiter, visible in the distance as a pale dot.

Phobos (22 km across) is one of the smallest known moons in the solar system. Markedly, its shape is irregular, resembling rocks or potatoes, rather than round or spherical. This is due to the small mass of Phobos. Only objects with large masses possess sufficient gravity, for attaining round forms.

Phobos orbits Mars every 7h 39m, at a distance of only 9,400 km. For comparison, the Moon orbits Earth every 27.3 days, at an average distance of 384,000 km.

Imaged on 1 June 2011, the unusual planetary alignment shown in Fig. 1 is known as a conjunction. A conjunction occurs when two or more celestial bodies appear close together in the sky, near the line of sight. At the time of obtaining the photos of Fig. 1, Mars Express was nearly 11,400 km from Phobos, while Jupiter was 529 million km away from the Mars-orbiting spacecraft.


Aymen Mohamed Ibrahem
Senior Astronomy Specialist
News Center

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