"The MESSENGER Science Team is extremely excited about this flyby," says Dr. Sean C. Solomon, MESSENGER principal investigator, from the Carnegie Institution of Washington. " We are about to enjoy our first close-up view of Mercury in more than three decades, and a successful gravity assist will ensure that MESSENGER remains on the trajectory needed to place it into orbit around the innermost planet for the first time."

During the flyby, the probe's instruments will make the first up-close measurements of the planet since Mariner 10's third and final flyby of Mercury on March 16, 1975, and will gather data essential to planning the MESSENGER mission's orbital phase. MESSENGER's seven scientific instruments will begin to address the mission goals of:

- mapping the elemental and mineralogical composition of Mercury's surface;

- imaging globally the surface at a resolution of hundreds of meters or better;

- determining the structure of the planet's magnetic field;

- measuring the planet's gravitational field structure; and

- characterizing exospheric neutral particles and magnetospheric ions and electrons.

A Close-up of Mercury
The cameras onboard MESSENGER will take more than 1,200 images of Mercury from approach through encounter and departure. "When the Mariner 10 spacecraft did its flybys in the mid-1970s, it saw only one hemisphere - a little less than half the planet," notes Dr. L. M. Prockter, instrument scientist for the Mercury Dual Imaging System, and a scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md.

"During this flyby we will begin to image the hemisphere that has never been seen by a spacecraft and at resolutions that are comparable to or better than those acquired by Mariner 10 and in a number of different color filters so that we can start to get an idea of the composition of the surface."

One site of great interest is the Caloris basin, an impact feature about 1,300 kilometers (808 miles) in diameter and one of the largest impact basins in the solar system. "Caloris is huge, about a quarter of the diameter of Mercury, with rings of mountains within it that are up to three kilometers high," says Prockter.

"Mariner 10 saw a little less than half of it. During this first flyby, we will image the other side of Caloris. These impact basins act like giant natural drills, pulling up material from underneath the surface and spreading it out around the crater. By looking through different color filters we can start to understand what the composition of the Caloris basin may be and learn something about the subsurface of Mercury."

MESSENGER instruments will provide the first spacecraft measurements of the mineralogical and chemical composition of Mercury's surface. The visible-near infrared and ultraviolet-visible spectrometers will measure surface reflectance spectra that will reveal important mineral species. Gamma-ray, X-ray, and neutron spectrometer measurements will provide insight into elemental composition.

During the flyby, Doppler measurements will provide the first glimpse of Mercury's gravity field structure since Mariner 10. The long-wavelength components of the gravity field will yield key information on the planet's internal structure, particularly the size of Mercury's core.

The encounter provides an opportunity to examine Mercury's environment in ways not possible from orbit because of operational constraints. The flyby will yield low-altitude measurements of Mercury's magnetic field near the planet's equator. These observations will complement measurements that will be obtained during the later orbital phase.

The flyby is an opportunity to get a jump start on mapping the exosphere with ultraviolet observations and documenting the energetic particle and plasma population of Mercury's magnetosphere. In addition, the flyby trajectory enables measurements of the particle and plasma characteristics of Mercury's magnetotail, which will not be possible from Mercury orbit.

MESSENGER is slightly more than halfway through a 4.9-billion mile (7.9-billion kilometer) journey to Mercury orbit that includes more than 15 trips around the Sun. It has already flown past Earth once (August 2, 2005) and Venus twice (October 24, 2006, and June 5, 2007). Three passes of Mercury, in January 2008, October 2008, and September 2009, will use the pull of the planet's gravity to guide MESSENGER progressively closer to Mercury's orbit, so that orbit insertion can be accomplished at the fourth Mercury encounter in March 2011.

"The complexity of this mission, with its numerous flybys and multitude of maneuvers, requires close and constant attention," says MESSENGER project manager P. D. Bedini, of APL. "MESSENGER is being driven by a team of extremely talented and dedicated engineers and scientists who are fully engaged and excited by the discoveries before them."

The MESSENGER project is the seventh in NASA's Discovery Program of low-cost, scientifically focused space missions. Solomon leads the mission as principal investigator; APL manages the mission for NASA's Science Mission Directorate and designed, built, and operates the MESSENGER spacecraft.

MESSENGER's science instruments were built by APL; NASA Goddard Space Flight Center, Greenbelt, Md.; University of Michigan, Ann Arbor; and the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder; with the support of subcontractors across the United States and Europe. GenCorp Aerojet, Sacramento, Calif., and Composite Optics Inc., San Diego, Calif., respectively, provided MESSENGER's propulsion system and composite structure.

earlier related report
Brown Planetary Geologists Lend Expertise to Mercury Mission
Providence RI (SPX) Jan 14, 2008 What lies on the uncharted side of mysterious Mercury, the smallest planet in the solar system? Brown University students, led by planetary geologist James Head, will study never-before-seen images of Mercury when a NASA spacecraft makes the first visit to Mercury in nearly 33 years.

On Monday, Jan. 14, 2008, the NASA spacecraft MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) will begin collecting more than 1,200 images of Mercury's ancient Moon-like surface, which is filled with wide craters, rocky plains and curved cliffs. The images will be unprecedented: When the Mariner 10 probe made its final Mercury flyby in 1975, it left with images from less than half the planet.

Determining the composition of rocks on Mercury's surface, as well as the age and origin of these rocks, will be the central work of MESSENGER's geology group. James Head, a planetary geologist and the Louis and Elizabeth Scherck Distinguished Professor at Brown, is leading this international team of scientists as well as a cadre of Brown students involved in the mission.

The MESSENGER mission, part of NASA's Discovery program, is led by Sean Solomon, a geophysicist at the Carnegie Institution of Washington.

"This is pioneering work," Head said. "When data from MESSENGER starts coming in, we'll be seeing terrain no human has ever seen before. And when you don't know what one half of a planet looks like, you're in for some surprises."

On Monday, Head and graduate students Laura Kerber and Debra Hurwitz, along with research analyst Jay Dickson, will be at the Johns Hopkins University Applied Physics Laboratory in Maryland to join in MESSENGER lead operations and data analysis. Graduate student Caleb Fassett will lead a "home team" of students from the Department of Geological Sciences, who will begin to study geological data down-linked from the MESSENGER probe.

The spacecraft is equipped with a camera and other instruments that will provide information on the mineralogical and chemical composition of Mercury's surface, as well as up-close images of Mercury's terrain, which includes the spectacular Caloris Basin, an 800-mile-wide impact crater that is one of the largest in the solar system.

Head and the Brown students, along with the rest of the MESSENGER science teams, will use the data from the mission to better understand Mercury's volcanic history, the origin of its craters, and the ages of different surface rock units.

This information will answer fundamental questions about the formation of Mercury, the planet closest to the Sun. Along with information on Mercury's geological composition and evolution, MESSENGER instruments will also give scientists a better understanding of the planet's magnetic field, its gravity field, its atmosphere and its iron core.

"In the 50 years since Sputnik, more than 30 spacecraft have been sent to the Moon, yet only now is a second spacecraft hurtling toward Mercury," Head said. "The MESSENGER mission is a ground-breaking effort, one that will address key questions about this enigmatic planet."

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