Today, NASA's spacecraft MESSENGER, or the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, is expected to begin its two day mission at about noon (eastern time), of data collecting and photographing of the planet Mercury. It is the first spacecraft to visit the planet in 34 years, since Mariner 10's visit to the planet in 1974.
"This is raw scientific exploration and the suspense is building by the day. What will MESSENGER see? Monday will tell the tale," said Alan Stern, associate administrator for NASA's Science Mission Directorate in Washington, D.C..
This encounter will provide a critical gravity assist needed to keep the spacecraft on track for its March 2011 orbit insertion, beginning an unprecedented yearlong study of Mercury. The flyby also will gather essential data for mission planning. It will flyby an impact crater called the Caloris basin which is almost 800 miles (1,287 kilometers) in diameter. The basin is one of the largest impact craters in our solar system.
"Caloris is huge, about a quarter of the diameter of Mercury, with rings of mountains within it that are up to two miles high. Mariner 10 saw a little less than half of the basin. During this first flyby, we will image the other side," said Louise Prockter, the instrument scientist for the Mercury Dual Imaging System at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.
It also will study the global magnetic field and improve our knowledge of the gravity field from the Mariner 10 flyby. The long-wavelength components of the gravity field provide key information about the planet's internal structure, particularly the size of Mercury's core. The flyby also will map Mercury's tenuous atmosphere with ultraviolet observations and document the energetic particle and plasma of Mercury's magnetosphere. In addition, the flyby trajectory will enable unique particle and plasma measurements of the magnetic tail that sweeps behind Mercury.
MESSENGER was launched on August 3, 2004 and will travel just under five billion miles in total. It already has flown past Earth once and Venus twice. The spacecraft will use the pull of Mercury's gravity during this month's pass and others in October 2008 and September 2009 to guide it progressively closer to the planet's orbit. Insertion will be accomplished with a fourth Mercury encounter in 2011.

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NASA spacecraft to begin collecting data on Mercury