The orbits of Mars and Earth approach each other about every 2 years. During the most recent near passage, on Oct. 28, the planets came within 69 million kilometers of each other, the closest they'll come until 2018. A Hubble Space Telescope image taken a day later shows equatorial dust clouds covering an area up to 1,500 km across. NASA scientists say that the likely cause of the clouds was a large dust storm.
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