On October 6, for the second time in less than a year, NASA’s MESSENGER spacecraft will swoop just 200 kilometers (125 miles) above the cratered surface of Mercury, snapping hundreds of pictures and collecting a variety of other data from the planet as it gains a critical gravity assist that keeps the probe on track to become the first spacecraft ever to orbit the innermost planet beginning in March 2011.

“The results from MESSENGER’s first flyby of Mercury in January resolved debates that are more than 30 years old,” says Sean C. Solomon, MESSENGER principal investigator, from the Carnegie Institution of Washington. “Volcanic eruptions produced many of Mercury’s plains, its magnetic field appears to be actively generated in a molten iron core, and the planet has contracted more than we thought.” This second encounter, says Solomon, will uncover even more information about the planet.

Scoping New Territory

During MESSENGER’s first flyby, its cameras returned images of about 20% of Mercury’s surface not previously seen by spacecraft, revealing new and unexpected features. “During this second flyby, the cameras will take more than 1,200 high-resolution and color images of the planet, including 30% of Mercury’s surface that has never been seen by spacecraft,” says Louise M. Prockter, instrument scientist for MESSENGER’s Mercury Dual Imaging System (MDIS) at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. “MESSENGER’s second flyby will show us a completely new area of Mercury’s surface, opposite from the side of the planet we saw during the first flyby.”

The probe’s Mercury Laser Altimeter (MLA) will measure the planet’s topography, allowing scientists, for the first time, to correlate high-resolution topography measurements with high-resolution images. “Unlike the topographic data obtained during our first flyby – which covered terrain we hadn’t photographed from space – these MLA range measurements will cover areas MDIS imaged during that first pass,” explains APL’s Brian J. Anderson, MESSENGER’s deputy project scientist. “Moreover, terrain sampled by MLA in the first flyby will in turn be imaged by MDIS on the second encounter.” The second flyby is also expected to yield more surprises about the unique physical processes governing Mercury’s magnetosphere-exosphere system, as well as additional information about the charged particles located in and around Mercury’s dynamic magnetosphere.

Surface Composition

A major goal of the orbital phase of MESSENGER’s mission is to determine the composition of Mercury’s surface, APL’s Ralph L. McNutt, MESSENGER project scientist, points out. The instruments designed to make those compositional measurements – the Visible-Infrared Spectrograph (VIRS) on the Mercury Atmospheric and Surface Composition Spectrometer (MASCS), the X-Ray Spectrometer, and the Gamma-Ray and Neutron Spectrometer – will get another peek at Mercury during this flyby.

“The VIRS high-spatial-resolution spectral measurements of Mercury’s surface will overlap with high-resolution color images taken by MDIS, providing complementary spectral information about portions of Mercury’s surface in unprecedented detail,” says McNutt. “The second flyby will also provide a first test of the spectral uniformity or variability between the two hemispheres viewed by MASCS in the two flybys.”

MESSENGER is more than halfway through a 7.9-billion-kilometer (4.9-billion-mile) journey to Mercury orbit that includes more than 15 trips around the Sun. It has already flown past Earth once (August 2, 2005), Venus twice (October 24, 2006, and June 5, 2007), and Mercury once (January 14, 2008). The upcoming flyby and an additional pass of Mercury, in September 2009, will use the pull of the planet’s gravity to guide MESSENGER progressively closer to Mercury, so that orbit insertion can be accomplished at the fourth Mercury encounter in March 2011.

“In addition to providing data that are already being used to start answering the guiding science questions of the mission, the observations made during the Mercury flybys are critical to the science planning effort,” says MESSENGER Project Manager Peter D. Bedini, of APL. “The performance of the spacecraft and instruments during the flybys helps us prioritize and organize the observations to be made during the orbital phase.”

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.

The MESSENGER Science Team, augmented by an impressive array of experts participating in NASA’s MESSENGER Participating Scientist Program, includes 46 scientists from 25 institutions. A complete list can be found at http://messenger.jhuapl.edu/who_we_are/science_team.html. Additional information about MESSENGER is available on the Web at: http://messenger.jhuapl.edu.