Scientists used to think that the same side of Mercury always faces the Sun, but in 1965 astronomers discovered that the planet rotates three times during every two orbits. Mercury speeds around the Sun every 88 days, traveling through space at nearly 50 kilometers (31 miles) per second faster than any other planet.
Rather than an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off its surface by the solar wind and striking meteoroids. Because of the planet's extreme surface temperature, the atoms quickly escape into space.
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