ESA GNC Conference Papers Repository

The Origins Spectral Interpretation Resource Identification Security Regolith Explorer (OSIRIS-REx) asteroid sample return mission will study, observe, and collect and return a sample from asteroid (101955) Bennu (previously known as 1999 RQ36) to Earth for further detailed analysis. This asteroid is both the most accessible carbonaceous asteroid and one of the most potentially hazardous asteroids known. Knowledge of its nature is fundamental to understanding planet formation and the origin of life, and knowledge of its future will allow us to better protect ourselves against potential impacts. In order to gain this knowledge, the detailed chemical composition of an asteroid sample must be understood. Thus, the primary goal of the mission is to successfully collect a sample from the surface of Bennu and return it to Earth such that scientists can perform the detailed analysis necessary in order to answer these questions. Set to launch in September 2016, the OSIRIS-REx spacecraft will travel in deep space for over two years on its journey to rendezvous with Bennu in late-2018. After reaching the asteroid, OSIRIS-REx will spend well over a year performing proximity operations near Bennu in order to analyze and characterize the asteroid, culminating in the collection of a sample from the asteroid's surface. Bennu poses several unique challenges for the navigation team early in the Proximity Operations phase. In particular, the first three phases of the proximity operations portion of the mission will require precise maneuver planning and execution in order to achieve stringent science requirements that provide key early observations, while little will be known regarding the dynamical environment around Bennu at that time. The first phase, known as Approach, begins with the first attempts of optical acquisition of Bennu from over one million kilometers range and features four asteroid approach maneuvers to slow the velocity of the spacecraft relative to the asteroid. The Approach phase has key observation opportunities necessary in order to begin generating a detailed shape model of the asteroid. In addition, this phase will also include a survey of the area surrounding Bennu to search for any natural satellites that may be present down to objects with diameters of 10cm. Each of these observation sets has particular viewing geometries required in order to achieve science requirements that place constraints on maneuver execution and orbit determination performance. The next phase, Preliminary Survey, features three separate flybys of the asteroid and continued imaging in order to increase the resolution of the generated shape model. In order to perform these flybys, five maneuvers have been scheduled to be performed over the span of ten days requiring a challenging operational schedule for the team. The third phase, known as Orbit A, begins following the last flyover of the Preliminary Survey phase and ends with the spacecraft in a closed orbit around Bennu. Importantly, the flybys performed during the previous Preliminary Survey phase will provide key information on the gravity field surrounding Bennu that will help the team design the maneuvers necessary to insert the spacecraft into orbit about the small 512-m diameter asteroid Bennu. In addition, once the spacecraft has successfully inserted into orbit, the navigation team will use the shape model derived from earlier imaging during the Approach and Preliminary Survey phases in order to transition from star-based to landmark-based optical navigation. Due to the estimated small size and low gravity of Bennu, the maneuvers needed to insert OSIRIS-REx into orbit are expected to be on the order of millimeters per second, and require great precision in order to achieve a stable orbit about such a small body. In total, the time from the first asteroid approach maneuver to orbit insertion covers less than three months, providing a very challenging beginning to the mission around Bennu. In order to prepare for these challenges, extensive analyses have been performed in order to provide an indication of the design and an assessment of the performance for each of the maneuvers in these phases. Monte Carlo analyses have been run in order to demonstrate that the science requirements can be met, and a closed orbit can be achieved given expected state uncertainties based on covariance analysis and maneuver execution errors based on pre-launch testing of the spacraft's propulsion system. In addition, several operational tests are planned for early 2017 using realistic simulations of these phases in order to test and train the team to be as prepared as possible during the actual encounter. These tests will include both nominal and off-nominal scenarios in order to help prepare the team. This paper will present analysis results and share key observations from the operational tests for the three early phases of Bennu proximity operations for the OSIRIS-REx mission. Details on the strategy of the orbit insertion maneuvers and satisfaction of science constraints via analysis will be discussed. Finally details of the completed operational readiness tests will be included with specific emphasis on the results obtained and the simulation environment and conditions used. Acknowledgments This material is based upon work supported by the National Aeronautics and Space Administration under Contract NNG13FC02C issued through the New Frontiers Program.