The United States has launched a spacecraft to explore the mysteries of the planet Mercury. Mercury is the least understood of the nine planets in our solar system. As the smallest and closest planet to the Sun, Mercury is difficult to see with telescopes.

The U.S Mariner 10 spacecraft flew by three times in the mid 1970s, but mapped only about half the planet's surface.

The Messenger probe will enter orbit for at least one year and complete the job with much higher photographic sharpness, enduring searing daytime temperatures of more than 400 degrees Celsius. In addition, it will study the planet's thin methane atmosphere, magnetic field, and composition, and look for water ice in the dark recesses of polar craters.

A key question scientists hope to answer is why Mercury is much denser with a higher proportion of iron than the other terrestrial inner planets, Earth, Venus, and Mars, even though they formed from the same cloud of gas and dust billions of years ago.

Messenger's journey will take it on a circuitous six-and-a-half year, eight billion kilometer path around Earth, Venus, and Mercury before easing into orbit.