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Phobos is the largest out of Mars' 2 moons. It is 27 by 22 by 18 km in diameter. It orbits Mars 3 times a day!

Phobos is getting closer to Mars at a rate of 1.8 meters every 100 years. This means in about 50 million years it will crash into Mars or break up into a ring.

There is a lot of variation in temperature between day and night. In the day, temperatures reach as high as minus 4°C. The lowest temperatures at night go as low as minus 112°C. This variation is due to the fine dust on the surface not being able to hold in the heat.