On July 14, 2015, the New Horizons mission accomplished the first flyby of Pluto-Charon, achieving full mission success during its primary mission. Less than 4 years later, during its first extended mission, New Horizons flew by Arrokoth, a 36-km contact binary trans-Neptunian object in the Kuiper Belt, on January 1, 2019. Along the way, New Horizons imaged numerous distant Kuiper Belt objects, performed important heliophysics science including complex Lyman-alpha radiation scans, and measured the dust and zodiacal light from regions never before explored. This article provides an overview of the New Horizons spacecraft and its engineering performance, as well as potential strategies for extending the mission far beyond its original design lifetime. Details on the mass and power budgets, as well as descriptions of key innovations to meet the challenges posed by the mission, offer insight into the engineering accomplishments that led to mission success. Trended data on the power, thermal, and propulsion systems substantiate projections of the mission's potential to continue its exploration beyond the heliopause until similar to 2050.

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.

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission was designed to unlock the secrets of our solar system’s innermost planet, revealing clues to the planet’s enigmatic geological history, unusually high density, and radar-reflective materials at the poles, among many other decades-old unanswered questions. MESSENGER began its journey on 3 August 2004, when it was launched from the Cape Canaveral Air Force Station in Florida, and the spacecraft was successfully inserted into its destination orbit about Mercury on 18 March 2011. After expending the vast majority of its propellant to reach the planet, the MESSENGER dual-mode propulsion system required a new set of operating procedures to extract the remaining fuel and oxidizer. Those updated guidelines were used to successfully execute five orbit-correction maneuvers during the one-year primary orbital mission. After the completion of the primary mission objectives, an additional one-year extended mission began that required the propulsion system to exhaust all of its remaining usable oxidizer and empty the usable fuel from one of its main tanks. To accomplish this goal, the recently updated propellant extraction techniques had to be further modified to safely deplete the remaining propellant from the near-empty tanks.

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission was designed to unlock the secrets of our solar system’s innermost planet, revealing clues to the planet’s enigmatic geological history, unusually high density, and radar-reflective materials at the poles, among many other decades-old unanswered questions. MESSENGER began its journey on 3 August 2004, when it was launched from the Cape Canaveral Air Force Station in Florida, and the spacecraft was successfully inserted into its destination orbit about Mercury on 18 March 2011. On its way to Mercury, the MESSENGER spacecraft completed 12 trajectory-correction maneuvers, five deep-space maneuvers, and one critical Mercury orbit-insertion maneuver. The MESSENGER dualmode propulsion system is composed of 12 monopropellant Aerojet 4.4-N MR-111C thrusters, four monopropellant Aerojet 22-N MR-106E thrusters, and one large bipropellant AMPAC In-Space Propulsion (ISP) Leros 1b 660-N engine. This paper describes the operation and performance of the propulsion system during the MESSENGER spacecraft’s interplanetary cruise phase and through its insertion into orbit about Mercury.

Study of the planet Mercury is fundamental to acquiring insight into the evolution of the inner solar system. NASA’s seventh Discovery mission, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, will orbit a spacecraft around Mercury for 1 Earth year to gather scientific data. To make this mission possible, a lightweight dual-mode propulsion system capable of delivering 2,300 m/s was designed, developed, and qualified over a period of 30 months. Following integration with the spacecraft and an extensive checkout and test period, the spacecraft was launched by a Delta-II launch vehicle on 3 August 2004. The MESSENGER propulsion system includes three pressurized propellant tanks, an auxiliary fuel tank, propellant and pressurant control components, 16 monopropellant thrusters, and a single 667-N bipropellant large-velocity adjustment (LVA) thruster. The MESSENGER propulsion system has four operational modes: a passive thermal management or cruise phase mode and three active thruster operational modes. A mode-1 maneuver fires monopropellant thrusters with fuel fed in a blow-down mode from the auxiliary tank. A mode-2 maneuver fires monopropellant thrusters with fuel from pressurized main fuel tanks. A mode-3 maneuver fires the bipropellant LVA using propellants from pressurized fuel and oxidizer tanks. In both the mode-2 and -3 maneuvers, the propulsion system’s small auxiliary tank is refilled with fuel from one of the two main fuel tanks. In the passive thermal management mode, spacecraft and thermostatically controlled heaters maintain the propellant and propulsion components within their operational pressure and temperature ranges. This paper presents the performance of the MESSENGER propulsion system since launch. To date, all elements of the propulsion system have been exercised with the successful execution of nine propulsion maneuvers including five mode-1 maneuvers, three mode-2 maneuvers, and one mode-3 maneuver. Of the five mode-1 maneuvers, two were attitude control maneuvers only: one was used to detumble the spacecraft following launch while the second was a commanded momentum dump performed in early 2006. The single mode-3 maneuver, completed in December 2005, imparted the largest spacecraft velocity change to date. The 474-s burn of the propulsion system’s bipropellant thruster adjusted the spacecraft velocity by 315.72 m/s. * Aerojet Propulsion System Chief Engineer and MPS Integrated Product Team Lead and AIAA Member. † Aerojet MPS Systems Engineering Lead and AIAA Member. ‡ JHU/APL MPS Manager and Senior AIAA Member. § JHU/APL MESSENGER Spacecraft GN&C Manager and Senior AIAA Member. American Institute of Aeronautics and Astronautics 1 Nomenclature ACS = attitude control system BTI = burn time integration CMD = commanded momentum dump !V = delta-velocity DSM = deep space maneuver GN&C = guidance, navigation, and control JHU/APL = The Johns Hopkins University Applied Physics Laboratory LVA = large velocity adjustment MESSENGER = MErcury Surface, Space ENvironment, GEochemistry, and Ranging MPS = MESSENGER propulsion system MR = mixture ratio N2H4 = hydrazine N2O4 = nitrogen tetroxide PAUX-A = pressure, auxiliary tank side a PAUX-B = pressure, auxiliary tank side b PFF = pressure, fuel feed PVT = pressure, volume, temperature TVC = thrust vector control

Study of the planet Mercury is fundamental to acquiring insight into the evolution of the inner solar system. NASA’s seventh Discovery mission, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, will orbit a spacecraft around Mercury for 1 Earth year to gather scientific data. To make this mission possible, a lightweight dual-mode propulsion system capable of delivering 2,300 m/s was designed, developed, and qualified over a period of 30 months. Following integration with the spacecraft and an extensive checkout and test period, the spacecraft was launched by a Delta-II launch vehicle on 3 August 2004. The MESSENGER propulsion system includes three pressurized propellant tanks, an auxiliary fuel tank, propellant and pressurant control components, 16 monopropellant thrusters, and a single 667-N bipropellant large-velocity adjustment (LVA) thruster. The MESSENGER propulsion system has four operational modes: a passive thermal management or cruise phase mode and three active thruster operational modes. A mode-1 maneuver fires monopropellant thrusters with fuel fed in a blow-down mode from the auxiliary tank. A mode-2 maneuver fires monopropellant thrusters with fuel from pressurized main fuel tanks. A mode-3 maneuver fires the bipropellant LVA using propellants from pressurized fuel and oxidizer tanks. In both the mode-2 and -3 maneuvers, the propulsion system’s small auxiliary tank is refilled with fuel from one of the two main fuel tanks. In the passive thermal management mode, spacecraft and thermostatically controlled heaters maintain the propellant and propulsion components within their operational pressure and temperature ranges. This paper presents the performance of the MESSENGER propulsion system since launch. To date, all elements of the propulsion system have been exercised with the successful execution of nine propulsion maneuvers including five mode-1 maneuvers, three mode-2 maneuvers, and one mode-3 maneuver. Of the five mode-1 maneuvers, two were attitude control maneuvers only: one was used to detumble the spacecraft following launch while the second was a commanded momentum dump performed in early 2006. The single mode-3 maneuver, completed in December 2005, imparted the largest spacecraft velocity change to date. The 474-s burn of the propulsion system’s bipropellant thruster adjusted the spacecraft velocity by 315.72 m/s. * Aerojet Propulsion System Chief Engineer and MPS Integrated Product Team Lead and AIAA Member. † Aerojet MPS Systems Engineering Lead and AIAA Member. ‡ JHU/APL MPS Manager and Senior AIAA Member. § JHU/APL MESSENGER Spacecraft GN&C Manager and Senior AIAA Member. American Institute of Aeronautics and Astronautics 1 Nomenclature ACS = attitude control system BTI = burn time integration CMD = commanded momentum dump !V = delta-velocity DSM = deep space maneuver GN&C = guidance, navigation, and control JHU/APL = The Johns Hopkins University Applied Physics Laboratory LVA = large velocity adjustment MESSENGER = MErcury Surface, Space ENvironment, GEochemistry, and Ranging MPS = MESSENGER propulsion system MR = mixture ratio N2H4 = hydrazine N2O4 = nitrogen tetroxide PAUX-A = pressure, auxiliary tank side a PAUX-B = pressure, auxiliary tank side b PFF = pressure, fuel feed PVT = pressure, volume, temperature TVC = thrust vector control