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Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 88 days. It ranges in brightness from about minus 2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun or greatest elongation is only 28.3 degrees. It can only be seen in morning or evening twilight. Comparatively little is known about the planet: the only spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only 40 to 45% of the planet's surface. The period of rotation is 58.6462 days.
