The closest planet to the Sun is mercury, and it is the second-smallest planet in the Solar System.The magnitude of the Mercury has its ranges from 0.4 to 5.5 and its greatest angular separation from the Sun (greatest elongation) is only 28.3°, signify that it is only ever seen in evening. The planet remains relatively little-known: the only spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, and only 40–45% of the planet has been mapped.Actually, Mercury is similar in appearance to the Moon, being heavily cratered. It has no natural satellites and no atmosphere, but has a large iron core which generates a magnetic field about 1% as burly as the Earth's. Surface temperatures on Mercury range from about 90-700 K.