Mercury

Mercury (pronounced /'m?kj??ri/) is the deepest and smallest planet in the solar system, orbiting the Sun once every 88 days. Mercury is bright when viewed from Earth, range from -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 and evening twilight. Comparatively little is known about it; the first of two spacecraft to move toward Mercury was Mariner 10 from 1974 to 1975, which mapped only about 45% of the planet’s surface. The second was the MESSENGER spacecraft, which mapped another 30% of the planet throughout its flyby of January 14, 2008. MESSENGER will make two more passes by Mercury, follow by orbital insertion in 2011, and will survey and map the whole planet.