MESSENGER Spacecraft Prepares for Final Pass by Mercury

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WASHINGTON -- NASA's Mercury Surface, Space Environment, Geochemistry,
and Ranging spacecraft known as MESSENGER will fly by Mercury for the
third and final time on Sept. 29. The spacecraft will pass less than
142 miles above the planet's rocky surface for a final gravity assist
that will enable it to enter Mercury's orbit in 2011.

Determining the composition of Mercury's surface is a major goal of
the orbital phase of the mission. The spacecraft already has imaged
more than 90 percent of the planet's surface. The spacecraft's team
will activate instruments during this flyby to view specific features
to uncover more information about the planet.

"This flyby will be our last close look at the equatorial regions of
Mercury, and it is our final planetary gravity assist, so it is
important for the entire encounter to be executed as planned," said
Sean Solomon, principal investigator at the Carnegie Institution in
Washington. "As enticing as these flybys have been for discovering
some of Mercury's secrets, they are the hors d'oeuvres to the
mission's main course -- observing Mercury from orbit for an entire
year."

The spacecraft may observe how the planet interacts with conditions in
interplanetary space as a result of activity on the sun. During this
encounter, high spectral- and high spatial-resolution measurements
will be taken again of Mercury's tenuous atmosphere and tail.

"Scans of the planet's comet-like tail will provide important clues
regarding the processes that maintain the atmosphere and tail," said
Noam Izenberg, the instrument's scientist at the Johns Hopkins
University Applied Physics Laboratory, or APL, in Laurel, Md. "The
Mercury Atmospheric and Surface Composition Spectrometer will give us
a snapshot of how the distribution of sodium and calcium vary with
solar and planetary conditions. In addition, we will target the north
and south polar regions for detailed observations and look for
several new atmospheric constituents."

As the spacecraft approaches Mercury, cameras will photograph
previously unseen terrain. As the spacecraft departs, it will take
high-resolution images of the southern hemisphere. Scientists expect
the spacecraft's imaging system to take more than 1,500 pictures.
Those images will be used to create a mosaic to complement the high
resolution, northern-hemisphere mosaic obtained during the second
Mercury flyby. The first flyby took the spacecraft over the eastern
hemisphere in January 2008, and the second flyby took it over western
side in October 2008.

"We are going to collect high resolution, color images of
scientifically interesting targets that we identified from the second
flyby," said Ralph McNutt, a project scientist at APL. "The
spectrometer also will make measurements of those targets at the same
time."

Two spacecraft maneuvers will improve the ability of the spacecraft's
Neutron Spectrometer to detect low-energy neutrons sensitive to the
abundances of iron and titanium on Mercury's surface. These two
elements absorb neutrons and are critical to an understanding of how
the planet and its crust formed. A combination of day and night
measurements will enable scientists to test the influence that
planetary surface temperature has on the neutron population. The data
are important for interpreting measurements that will be made after
the probe is in orbit around Mercury.

An altimeter will make a topographic profile along the instrument
ground track of Mercury's surface. The data gathered will provide
additional topography of Mercury's surface features for ongoing
studies of the form and structure of its craters and large faults.
The information also will extend scientists' equatorial view of
Mercury's global shape and allow them to confirm the discovery made
during the first and second flyby that Mercury's equatorial region is
slightly elliptical.

The spacecraft has completed nearly three-quarters of its
4.9-billion-mile journey to enter orbit around Mercury. The trip
includes more than 15 trips around the sun. In addition to flying by
Mercury, the spacecraft flew past Earth in August 2005 and Venus in
October 2006 and June 2007.

The project is the seventh in NASA's Discovery Program of low-cost,
scientifically focused space missions. The spacecraft was designed
and built by APL. The mission also is managed and operated by APL for
NASA's Science Mission Directorate in Washington.

For more information about the mission, visit:

http://www.nasa.gov/messenger