View Video Presentation: https://doi.org/10.2514/6.2022-2470.vid The NASA Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) spacecraft began close proximity operations at the small (500-m diameter) B-type asteroid (101955) Bennu in December 2018 and concluded in May 2021. Hundreds of grams of pristine surface regolith were collected on October 20, 2020, when the OSIRIS-REx spacecraft successfully executed the Touch and Go (TAG) sample collection sequence. The spacecraft touched down within 1 meter of the targeted site on the surface of Bennu. TAG was the culmination of over 2 years of navigation performance refinement as a result of extraordinary interagency teamwork between the Flight Dynamics System (FDS), science and spacecraft teams while in close proximity to Bennu. This paper will discuss the navigation processes, planning and performance during the proximity operations at Bennu.

One of the more challenging aspects of the trajectory design for the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) mission at asteroid Bennu was maneuvering while in orbit. The orbital dynamics were highly perturbed by various sources, most notably solar radiation pressure, which degraded accuracy of long term predictions of the spacecraft’s location in orbit. Generally, the Navigation team had to solve three separate issues: correcting a perturbed orbit, changing to a different orbit, or phasing the orbit to place the spacecraft at a specific location at a specific time. The team composed a common framework using up to two maneuvers that could solve all of these problems using an identical schedule that allowed for consistent planning long before the final trajectory could be designed. This orbit trim strategy was successfully used for the first time in the Orbital B phase of the mission to maximize the duration of usable observing geometry in a time-variable orbit with strict operational limits. It was used an additional 3 times throughout the mission to adjust and/or change the orbit, most notably altering the orbit in the weeks prior to the successful Touch-And-Go (TAG) sample collection attempt. This same strategy was used to phase the orbit a total 10 times in preparation for each of the science sorties over potential sample sites, the TAG Rehearsals, and TAG. The trim strategy was demonstrated to be robust and performed exceptionally well in all aspects, which proved critical to a successful sample collection.

This proposed interstellar precursor mission will penetrate into the nearby interstellar medium by achieving 1000 Astronomical Units from the Sun within the working lifetime (about 50 years) of those who started the mission. A variety of science goals and orbital mechanics constraints contribute to the formulation of a list of candidate stars for targeting the solar system escape trajectory direction. An approach is developed that will minimize both launch energy to Jupiter and a subsequent near-Sun, high-ISP, high-thrust maneuver in order to attain a high-speed escape from the solar system towards a target star. Detailed performance characteristics are given for selected reference trajectories.

Lucy is NASA’s next Discovery-class mission and will explore the Trojan asteroids in the Sun-Jupiter L4 and L5 regions. This paper details the design of Lucy’s interplanetary trajectory using a two-point direct shooting transcription, nonlinear programming, and monotonic basin hopping. These techniques are implemented in the Evolutionary Mission Trajectory Generator (EMTG), a trajectory optimization tool developed at NASA Goddard Space Flight Center. We present applications to the baseline trajectory design, Monte Carlo analysis, and operations.

Recent advances linking medium-fidelity trajectory optimization and high-fidelity trajectory propagation/maneuver design software with Monte Carlo maneuver analysis and parallel processing enabled realistic statistical delta-V estimation well before launch. Completing this high-confidence, refined statistical maneuver analysis early enabled release of excess delta-V margin for increased dry mass margin for the Lucy Jupiter Trojan flyby mission. By 3.3 years before launch, 16 of 34 TCMs had 1000 re-optimized trajectory design samples, yielding tens of m/s lower 99%-probability delta-V versus targeting maneuvers to one optimal trajectory. One year later, 1000 re-optimized samples of all deterministic maneuvers and subsequent flybys further lowered estimated delta-V.

Lucy, NASA’s next Discovery-class mission, will explore the diversity of the Jupiter Trojan asteroids. The Jupiter Trojans are thought to be remnants of the early solar system that were scattered inward when the gas giants migrated to their current positions as described in the Nice model. There are two stable subpopulations, or “swarms,” captured at the Sun-Jupiter L4 and L5 regions. These objects are the most accessible samples of what the outer solar system may have originally looked like. Lucy will launch in 2021 and will visit five Trojans, including one binary system. This paper discusses the target selection process, including a description of “alternate Lucys” that were ultimately passed over in favor of the final design. The mathematics of the trajectory optimization are also discussed.

Lucy is NASA’s next Discovery-class mission and will explore the Trojan asteroids in the Sun-Jupiter L4 and L5 regions. This paper details the design of Lucy’s interplanetary trajectory using a two-point direct shooting transcription, nonlinear programming, and monotonic basin hopping. These techniques are implemented in the Evolutionary Mission Trajectory Generator (EMTG), a trajectory optimization tool developed at NASA Goddard Space Flight Center. We present applications to the baseline trajectory design, Monte Carlo analysis, and operations.

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) spacecraft achieved a number of technical milestones and successfully accomplished all of the planned science objectives for its nominal mission as well as the first two mission extensions, termed XM1 and XM2. The orbital phase of the mission through XM2 lasted 4 years, a duration four times as long as that of the baseline mission. Key to the success of the mission was a robust vehicle design that defied the designed mission lifetime and a creative team that developed operational concepts and science collection methods that allowed the continued collection of novel science data products. The mission culminated in one final mission extension, termed XM2., during which the spacecraft periapsis altitude ranged over unprecedentedly low values. This vantage point allowed novel studies of Mercury, but it forced an elevated cadence of propulsive maneuvers, the last few using helium pressurant to impart velocity corrections to the spacecraft. On 30 April 2015, with the vehicle nearly out of pressurant and the XM2. observation campaign complete, MESSENGER ended the flight phase of the mission by impacting the surface of Mercury.

The great success of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission was possible only through a series of technological advances and a number of innovative uses of existing technologies. A Mercury orbital mission had been studied for 30 years before MESSENGER and was believed to require a multi-billion-dollar effort. However, the innovations developed by the MESSENGER team enabled the mission to be accomplished within NASA's low-cost planetary program, known as Discovery. Not only were key enabling developments put into practice before launch, but also a number of innovations were implemented after launch, and in-depth planning for critical events greatly enhanced the scientific return from the mission. These postlaunch improvements also simplified mission operations and saved enough propellant to permit extending the mission beyond the originally planned 1 Earth year at Mercury. The communications practices were optimized to ensure that even more science data could be returned to Earth, and the orbital period was lowered to give 50% more low-altitude coverage. When part of the Gamma-Ray Spectrometer had exceeded its useful life, the rest of the instrument was repurposed to give new insight into the rapidly varying magnetosphere.

MESSENGER launched on 3 August 2004, entered orbit about Mercury on 18 March 2011 (UTC), and impacted Mercury's surface on 30 April 2015. After a 6.6-year cruise phase with one flyby of Earth, two of Venus, and three of Mercury, MESSENGER spent 4.1 years in orbit about the innermost planet. Initially in a 12-h orbit, MESSENGER maintained periapsis altitudes of 200-505 km before transferring to an 8-h orbit on 20 April 2012. MESSENGER's low-altitude campaign included periapsis altitudes between 15 and 200 km. In its final 44 days, MESSENGER maintained unprecedented minimum altitudes less than 38 km above Mercury's terrain before impact.

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 used six planetary gravity assists in order to enable capture into orbit about Mercury. A key element of MESSENGER's successful trajectory was achieving the proper gravity assist from each planetary flyby. The criticality of the MESSENGER gravity assists levied tight accuracy requirements on the planetary-flyby targeting. Major errors could have precluded Mercury orbit insertion or required modifications to the trajectory that increased mission complexity, cost, and risk by requiring additional Mercury flybys and extending mission duration. Throughout the mission, MESSENGER modified its strategy for achieving accurate planetary flybys. By using solar sailing, the MESSENGER team was able to eliminate all of the flyby approach maneuvers without sacrificing flyby accuracy, thereby saving mission ΔV margin. The elimination of these approach maneuvers also markedly reduced mission risk, as these approach maneuvers were nominally planned during a time of heightened sensitivity to errors and precluded unique flyby science opportunities. The paradigm shift used by MESSENGER may be useful for other interplanetary missions, particularly if their trajectories require gravity assists in the inner solar system.

After MESSENGER's 18 March 2011 Mercury orbit insertion (MOI), the spacecraft began its year-long primary science mission. Trajectory perturbations from solar gravity, Mercury's gravity field, and solar radiation pressure shift orbit periapsis higher in altitude and Mercury latitude during the primary mission. Five orbit-correction maneuvers (OCMs) will either lower periapsis altitude or increase orbit period. After the primary mission, MESSENGER will either drift until impacting Mercury or begin an extended mission. Extended mission options require OCMs to establish and maintain a new orbit. Final results for MOI and OCM-1 indicate a successful start to the primary mission.

On 18 March 2011, the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft became the first probe to orbit Mercury. The spacecraft's 6.6-year journey to Mercury orbit included six large trajectory-correction maneuvers and six planetary flybys. These planetary gravity assists imparted the vast majority of velocity change required to transform the spacecraft trajectory from Earth orbit departure to Mercury arrival. This paper summarizes the design and performance of all planetary flybys and course-correction maneuvers through orbit insertion, as well as the results of targeting the planetary-flyby aim points using the acceleration on the spacecraft imparted by solar radiation pressure.

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.