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Phobos takes a bite out of the Sun

Dr Amanda Doyle, Feature writer
Sep 19, 2012, 7:00 UTC

Sen—- People often go to exotic locations to try and get the perfect view of a solar eclipse, but the Curiosity rover on Mars only had to look up to see an eclipse of a different kind. Careful planning by the mission engineers ensured that the NASA rover had its cameras ready to capture the transit of Mars’s moon Phobos across the face of the Sun.

The partial eclipse occurred on September 13 (Sol (Martian day) 37 of Curiosity's time on the planet) and it took around 15 minutes for Phobos to graze the edge of the Sun. The Mastcam camera is capable of filtering out some sunlight, meaning that it can safely look at the Sun.

Curiosity took more photos of the Mars' two moons crossing the face of the Sun on September 17.

Unlike a total solar eclipse on Earth, the moon Phobos is not large enough to completely block out the Sun’s disc. Phobos is an irregular shaped moon measuring 27 by 22 by 18 kilometres, so it will only disrupt a portion of the Sun’s light. In contrast, our Moon is 3,480 kilometres across, is 400 times smaller than the Sun and 400 times closer to Earth than the Sun, meaning that on Earth we can occasionally see a total solar eclipse. Earth is the only planet in the solar system where a total solar eclipse can occur.

Mars’ moon Deimos is even smaller than Phobos, measuring 15 by 12 by 11 kilometres, and has a higher altitude, meaning that it will blot out far less of the Sun than Phobos when it transits the face of the disc.

Phobos has previously been caught in the act of eclipsing the Sun by the Opportunity rover in December 2010. Opportunity’s twin, Spirit, also witnessed Phobos fade from the night sky as it passed within Mars’ shadow, in the equivalent of a lunar eclipse.

Phobos orbits Mars at a very low altitude of 9,400 kilometres, so it needs to travel fast in order to stop it from spiralling down towards the red planet. This high speed means that it orbits Mars three times for every one rotation of the planet.

It is likely that these small moons are captured asteroids, rather than moons that formed around the red planet. Imaging the transits of the moons can reveal more information about their orbital dynamics. Similar eclipses of Phobos will be visible to Curiosity in the coming months, allowing for further study of this event.

Mars' moons were discovered in 1877 by Asaph Hall. Hall named Mars' moons after the mythological sons of Ares, the Greek counterpart of the Roman god, Mars. Deimos is the brother of Phobos.

The Curiosity rover landed on Mars on August 6, with a mission to search its surroundings to see if they were ever suitable for microbial life, as well as to study the geological features of the area. It is equipped with ten scientific instruments, including Mastcam. 

Since landing in Gale Crater the rover has driven about 850 feet (259 metres). On Sol 42 (September 17, 2012) Curiosity drove about 105 feet (32 meters).  Curiosity is making its way across Gale Crater to its first destination, Glenelg, which is about 400 metres from the rover's landing site. When Curiosity reaches Glenelg it will be the first rover to drill for a rock sample.

NASA's Jet Propulsion Laboratory (JPL), based in Pasadena, California, a division of the California Institute of Technology, manages the Mars Science Laboratory mission for NASA.