MESSENGER Returns to Mercury
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Oct. 1, 2008: NASA's MESSENGER spacecraft is returning to Mercury. On Monday, Oct. 6, 2008, the probe will conduct the second of three planned flybys and photograph most of Mercury's remaining unseen surface.
At closest approach MESSENGER will pass just 125 miles above Mercury's cratered surface, taking more than 1200 pictures. The flyby also will provide a critical gravity assist needed for MESSENGER to become, in March 2011, the first spacecraft to actually orbit the innermost planet.
Above: A color image of Mercury's giant Caloris Basin recorded during MESSENGER's first flyby on Jan. 14, 2008. [more]
During MESSENGER's first flyby on Jan. 14, 2008, its cameras photographed approximately 20 percent of Mercury's surface never before seen by space probes. The spacecraft spotted ancient volcanoes ringing Mercury's Caloris Basin, found that Mercury's magnetic field is "alive" (generated by an active dynamo in Mercury's core) and discovered a surprisingly rich plasma nebula trapped in Mercury's magnetic field. And those were just a few of the surprises; see Science@NASA's New Discoveries at Mercury for details.
The second flyby is expected to yield even more surprises. A laser altimeter on the spacecraft will measure the planet's topography, allowing scientists, for the first time, to correlate high-resolution topography measurements with high-resolution images. At the same time, MESSENGER's sensors will analyze the chemical and mineralogical composition of Mercury's surface.
Below: Much of Mercury's surface is still unknown. This map shows areas that will be covered by the second flyby of MESSENGER on Oct. 6, 2008. Solid purple denotes places that have never been photographed by a spacecraft before. 
"We will be able to do the first test of differences in the chemical
compositions between the two hemispheres viewed in the two flybys," says Ralph McNutt, the mission's project scientist at APL.
"The results from MESSENGER's first flyby of Mercury settled debates that were more than 30 years old," notes Sean C. Solomon, the mission's principal investigator from the Carnegie Institution of Washington. "This second encounter should uncover even more information about the planet."
Stay tuned to Science@NASA for results from the flyby.
Editor: Dr. Tony Phillips | Credit: Science@NASA
The MESSENGER project is the seventh in NASA's Discovery Program of low-cost, scientifically focused space missions. The Applied Physics Laboratory designed, built and operates the spacecraft and manages the mission for NASA. MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging. The Discovery and New Frontiers Program Office at the NASA Marshall Space Flight Center manages the Discovery Program for NASA Headquarters.
New Discoveries at Mercury (Science@NASA)
Dark Halos Discovered on Mercury (Science@NASA)
Surprises from Mercury (Science@NASA)
MESSENGER is more than halfway through a 4.9-billion-mile journey to enter orbit around Mercury that includes more than 15 trips around the sun. In addition to flying by Mercury, the spacecraft flew past Earth in August 2005 and past Venus in October 2006 and June 2007.
NASA's Future: US Space Exploration Policy