The spacecraft trajectory and the associated course-correction maneuvers provided a primary means for accomplishment of the scientific objectives of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. Whereas other articles in this issue offer a quantitative performance assessment of MESSENGER's course-correction maneuvers, this account identifies unique aspects and lessons learned from the examination of the processes and team interactions for maneuver design through maneuver reconstruction at the core of this successful NASA mission. Keys to mission success included forward thinking in the creative use of maneuvers as a means of preparing for future important maneuvers, as well as exercising flexibility to allow change from the nominal plan when this change would either increase scientific return or enable new scientific observations to answer questions that arose during the mission. Cautious use of propellant reserves and a willingness to accept higher risk near the end of flight operations enabled a 3.1-year extension of the yearlong orbital phase of the primary mission.

Having completed its primary and first extended missions by mid-March 2013, the MESSENGER spacecraft in orbit about Mercury began a 2.1-year final mission extension that brought substantial opportunity for low-altitude science, along with many technical challenges successfully overcome by the flight operations and science teams. After four orbit-correction maneuvers (OCMs) between June 2014 and January 2015 targeted minimum altitudes near 25 km and 15 km, seven OCMs in March and April 2015 maintained minimum altitude between 5 km and 37 km. Engineering challenges at mission end included the efficient utilization of accessible propellant and helium gas pressurant to delay Mercury impact.

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission is the seventh in NASA's Discovery Program. The spacecraft was launched from Cape Canaveral Air Force Station in August 2004 to begin an interplanetary cruise that culminated in orbit insertion about Mercury in March 2011 for a nominal one-year scientific investigation. An extension to the mission was initiated in March 2012, and in order to optimize the scope and return of the onboard scientific instruments and the stability of the spacecraft orbit about the planet, the orbital period was reduced from 12 to 8 hours in April 2012. This paper describes MESSENGER navigation operations and trajectory estimation performance for the orbital mission phase from Mercury orbit insertion through the end of the primary mission and into the first 9 months of the ongoing extended mission.

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission is the seventh in NASA's Discovery Program series. The spacecraft was launched in August 2004 and began an interplanetary cruise that culminated in insertion into orbit about Mercury in March 2011 for a nominal one-year scientific investigation. The cruise phase included six planetary gravity-assist flybys and eighteen propulsive events, which included five large deep-space maneuvers, one in two parts, and twelve smaller trajectory-correction burns. From the approach to the first Mercury flyby through orbital insertion about the innermost planet, an interval that spanned over three years, solar sailing was employed successfully for trajectory correction. This paper describes the navigation performance achieved for the three Mercury flybys and how experiences gained during the mission cruise phase have been applied to support Mercury orbit insertion and maintenance operations during the Mercury orbital phase of the MESSENGER mission.

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission is the seventh mission in NASA's Discovery Program. The spacecraft, launched from Cape Canaveral Air Force Station in August 2004, arrived in orbit about Mercury in March 2011 to begin a one-year scientific investigation. While in orbit, the spacecraft is subject to a variety of forces, including Mercury and solar gravity, solar and planetary radiation effects, and propulsive events associated with orbit correction and momentum desaturation. This paper describes the challenges for navigation in terms of achieving the highest accuracy possible for relevant force models to support orbit determination and reconstruction over the Mercury orbital phase of the MESSENGER mission.