[meteorite-list] MESSENGER Spacecraft Prepares for Final Pass by Mercury

[Ron Baalke]

Sept. 23, 2009

Dwayne Brown 
Headquarters, Washington 
202-358-1726 
dwayne.c.brown at nasa.gov 

Paulette Campbell 
Johns Hopkins University Applied Physics Laboratory, Laurel, Md. 
240-228-6792 
paulette.campbell at jhuapl.edu 

RELEASE: 09-221

MESSENGER SPACECRAFT PREPARES FOR FINAL PASS BY MERCURY

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 
	
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