Portal:Solar System/Selected article/3
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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 −2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun (greatest elongation) is only 28.3°. 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%–45% of the planet’s surface.
Physically, Mercury is similar in appearance to the Moon as it is heavily cratered. It has no natural satellites and no substantial atmosphere. The planet has a large iron core which generates a magnetic field about 0.1% as strong as that of the Earth. Surface temperatures on Mercury range from about 90 to 700 K (−180 to 430°C, −292 to 806°F), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.
