McAdams of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., "optimized the trajectory and maneuver schedule, designing one of the most challenging planetary missions in history," said APL's Tom Strikwerda, who on May 28 presented the award: a plaque and a 24-inch-high trophy that McAdams will keep until passing it on to the next winner a year from now.

Because Mercury lies deep within the Sun's gravity well, travel to the planet requires an extremely large velocity change. A spacecraft travelling to Mercury speeds up as it falls toward the Sun; so MESSENGER's trajectory had to be designed to most effectively utilize the gravitational pull of Venus and Mercury to achieve most of the required velocity change.

To make the trip possible, the trajectory uses six gravity-assist flybys: one by Earth, two by Venus, and three by Mercury. These gravity-assists, along with five large course-correction maneuvers, reduce the energy (and thus fuel) requirements but greatly prolong the trip. These maneuvers will also slow the spacecraft's speed just enough relative to Mercury to enable its thruster to place the probe into orbit around Mercury.

Upon arrival at Mercury in March of 2011 the spacecraft will enter an elliptical orbit that passes as close as 200 kilometers to Mercury's far northern surface every 12 hours. Such an orbit will allow MESSENGER to measure solar wind and magnetic fields at a variety of distances from the planet yet still obtain close-up measurements and images of the surface.

"The implementation of this complex mission plan has been a significant challenge," says McAdams, who also worked on the Near Earth Asteroid Rendezvous mission. "It's a privilege to join two other MESSENGER team engineers as recipients of this award," he adds, referring to Robin Vaughan and Adrian Hill, two other engineers from the team who received the award in 2004 and 2006, respectively.

McAdams, who holds an M.S. in Aeronautical and Astronautical Engineering from Purdue University, also created and led the development of MESSENGER education and outreach products, as well as trajectory data distribution to the science community. Last summer, he played a critical role in the creation of the Mercury Flyby Visualization Tool, which provides simulated views of Mercury from MESSENGER's perspective, during approach, flyby, and departure, or in real time (as the observations actually occur).

Members of MESSENGER's Geology Discipline Group used the tool both before and after the probe's first flyby of Mercury in January to gain information about imaging sites on Mercury. The tool will be updated for upcoming Mercury encounters.

"Jim McAdams has been a critical member of the MESSENGER team," offers MESSENGER Principal Investigator Sean Solomon. "Jim is usually several steps ahead of the rest of us with respect to planning for mission-critical events, and we can always count on his results to high precision. The journey to orbiting Mercury is long and complex, but we have a terrific guide."

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