12 August: Deimos, Moon of Mars

On this date in 1877 American astronomer Asaph Hall discovered a moon of Mars, which he named Deimos. Here are ten facts about Deimos.

1. Mars has two moons, of which Deimos is the smaller and outermost. The other moon is Phobos.
2. Long before it was actually discovered that Mars had two moons, Jonathan Swift wrote about Mars having two moons in Gulliver's Travels, which are discovered by people aboard the flying island Laputa. Voltaire also mention s the two moons of Mars in a short story, Micromégas. It's thought Voltaire had probably read Gulliver's Travels so possibly got the idea from Swift. But where did Swift get it from? Lucky guess? Or something else?
3. Deimos has a mean radius of 6.2 km (3.9 miles). That's 0.56 times the size of Phobos. It isn't a sphere like our own Moon.
4. The names of Mars's moons were suggested by Henry Madan, a Science Master at Eton, who got the names from Book XV of the Iliad. Deimos is the Twin brother of Phobos. Deimos is the personification of dread and in the Iliad, is summoned by Mars along with Phobos, the personification of fear.
5. Deimos is 23,460 km (14,580 miles) from Mars.
6. If people ever land on Mars, Deimos will appear more like a star than a moon to the naked eye. Its brightness would range from as bright as Venus at its "full moon" phase to as bright as the star Vega during its quarter phases. Deimos's phases take 1.2648 days to run their course.
7. Deimos takes about 30.4 hours to orbit Mars. A person on the equator of Mars would see it rise in the east and set in the west, every couple of Martian days. Sometimes it transits the Sun, and would be observed as a small black dot moving across the sun. From Martian latitudes greater than 82.7°, it cannot be seen at all.
8. The studies people have made of Deimos so far show it to be similar to a C or D type Asteroid, leading to the theory that it was an asteroid which was thrown off course by Jupiter's gravity and captured by Mars.
9. Although several passing probes have taken pictures en route to Mars, and the Mars Rovers Spirit and Opportunity have photographed it from the planet's surface, nothing has ever landed there. Scientists have been talking about embarking upon a sample return mission, Gulliver, to collects about a kilogram of material from Deimos and return it to Earth.
10. Deimos has a smoother surface than its twin, Phobos. Possibly because its regolith (soil) has filled in its craters somewhat. There are only two surface features on Deimos which have been given names - two craters, which have been named Swift and Voltaire after the two writers.

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Phobos

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