In a little over a day and a half, a spacecraft will be flying by Mercury for the first time in thirty years, providing images from the planet that have never been seen before. The spacecraft will flyover at a height of about 125 miles (202km) with eventual orbital insertion occurring in March 2011. The last time a spacecraft did a flyby of Mercury was March 29, 1974 when Mariner 10 took 140 frames of photographs which were assembled into a mosaic of Mercury’s surface. Messenger was launched August 3, 2004.

The spacecraft is named MESSENGER, which you might guess is an acronym. It stands for MEcury Surface, Space ENvironment, GEochemistry and Ranging. The acronym sort of explains the mission but basically, these six questions compose the goals of Messenger:

  1. Why is Mercury so dense?
    • Mercury’s core comprises 60% of the mass of the planet, twice that of Earth.
  2. What is the geological history of Mercury?
    • The other 55% of the planet not yet viewed will be surveyed.
  3. What is the nature of its magnetic field?
    • Mercury, like Earth, has an internal magnetic field. Venus & Mars do not.
  4. What is the structure of its core?
    • Mercury’s gravity field will be measured, inner core size calculated & outer core verified as molten.
  5. What is the unusual substance at its poles?
    • Ice. Duh! But wait, if it’s ice how can it exist on a planet that is so close to the sun?
  6. What volatiles are important in the formation of the planet?
    • Mercury’s thin exosphere will be evaluated to help determine how Mercury formed.

NASA’s Applied Physics Lab has an excellent web site concerning all aspects of the mission here. Also, NASA’s main web site has information on the mission here.

Why seven years to get into orbit around Mercury? To help keep the cost of the mission low, fuel use was minimized. The spacecraft used Earth and Venus for gravity assist by circling each a couple of times and Mercury three times in order to get into position without using fuel.