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The newspaper of The Johns Hopkins University August 8, 2005 | Vol. 34 No. 41
MESSENGER Spacecraft Completes Earth Swingby En Route to Mercury

By Michael Buckley
Applied Physics Laboratory

NASA's MESSENGER spacecraft, headed toward the first study of Mercury from orbit, swung by its home planet on Aug. 2 for a gravity assist that propelled it deeper into the inner solar system.

Mission operators at the Johns Hopkins Applied Physics Laboratory say MESSENGER's systems performed flawlessly as the spacecraft swooped around Earth, coming to a closest approach point of about 1,458 miles over central Mongolia at 3:13 p.m. EDT. The spacecraft used the tug of Earth's gravity to change its trajectory significantly, bringing its average orbit distance nearly 18 million miles closer to the sun and sending it toward Venus for another gravity-assist flyby next year.

"One flyby down, five more to go," said APL's Mark Holdridge, MESSENGER mission operations manager. "Now, the mission begins."

Launched Aug. 3, 2004, from Cape Canaveral Air Force Station, Fla., the solar-powered spacecraft is about 581 million miles into a 4.9-billion-mile voyage that includes 14 more loops around the sun. It will fly past Venus twice and Mercury three times before moving into orbit around its target planet. The Venus flybys in October 2006 and June 2007 will use the pull of the planet's gravity to guide MESSENGER toward Mercury's orbit. The Mercury flybys in January 2008, October 2008 and September 2009 help MESSENGER further match that planet's speed, setting up the maneuver in March 2011 that starts a yearlong science orbit around Mercury.

"This Earth flyby is the first of a number of critical mission milestones during MESSENGER's circuitous journey toward Mercury orbit insertion," said Sean C. Solomon, the mission's principal investigator, from the Carnegie Institution of Washington. "Not only did it help the spacecraft sharpen its aim toward our next maneuver, it presented a special opportunity to calibrate several of our science instruments."

MESSENGER's main camera had snapped several approach shots of Earth and the moon over the past week, then began taking a series of color images, beginning with South America and continuing for one full Earth rotation, that science team members will string into a "movie" documenting MESSENGER's departure. On approach, the atmospheric and surface composition spectrometer also made several scans of the moon in conjunction with the camera observations, and during the flyby, the particle and magnetic field instruments spent several hours measuring Earth's magnetosphere. The team will download the data and images through NASA's Deep Space Network over the next several weeks, continuing its assessment of the instruments' performance.

MESSENGER will conduct the first orbital study of Mercury, the least explored of the terrestrial ("rocky") planets that also include Venus, Earth and Mars. Over one Earth year — or four Mercury years — MESSENGER will provide the first images of the entire planet and collect detailed information on the composition and structure of Mercury's crust, its geologic history, the nature of its atmosphere and magnetosphere, and the makeup of its core and polar materials.

MESSENGER, short for MErcury Surface, Space ENvironment, GEochemistry and Ranging, is the seventh mission in NASA's Discovery Program of lower cost, scientifically focused exploration projects. APL designed, built and operates the MESSENGER spacecraft and manages the mission for NASA's Science Mission Directorate. For more information, go to


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