A NASA Discovery mission to conduct the first orbital study of the innermost planet
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The Elusive Planet

The Planet Mercury

Mercury Symbol

It is fitting that we know Mercury by the name of the swift Roman messenger god, since it travels around the Sun faster than any other planet. During one year on Earth, Mercury makes over four orbits of the Sun. On the other hand, Mercury rotates slowly on its axis – almost 60 times more slowly than does our home planet. The amazing outcome is that a single Mercury day takes two Mercury years. The symbol for Mercury also represents the planet’s swift orbit around the sun. Click on Mercury’s symbol to the left to learn more about its origin.

Mercury is the planet closest to the Sun. Since it never strays far in the sky from the Sun’s glare, early astronomers had a difficult time viewing it, and considered it a "wandering star" appearing just before sunrise or just after sunset.

planet surface

Mercury is the smallest planet in our Solar System, and not much bigger than our own Moon. The surface of Mercury is in fact very Moon-like, covered with large and ancient craters, while its interior is like Earth’s, with a large core of iron. Mercury has a very thin atmosphere, and no moons of its own. It is a world of extreme temperatures in which the surface can heat to over 400°C and cool to almost –200 °C.

Spacecraft

Before MESSENGER, much of what we knew about Mercury was discovered through flybys of the planet during the Mariner 10 mission in the 1970’s. But Mariner 10 photographed only half the planet’s surface, and many questions remain.

In 2004, NASA launched the MESSENGER spacecraft to make more detailed observations. MESSENGER flew past the planet three times before going into orbit around Mercury in March, 2011. We have already advanced our understanding of Mercury dramatically. Principal Investigator, Sean Solomon, of the Carnegie Institution of Washington said on April 4, 2011: "With the beginning today of the primary science phase of the mission, we will be making nearly continuous observations that will allow us to gain the first global perspective on the innermost planet. Moreover, as solar activity steadily increases, we will have a front-row seat on the most dynamic magnetosphere–atmosphere system in the Solar System."