The new horizons pluto fly-by dust hazard assessment process

After the launch of the New Horizons spacecraft in 2006, the discovery of four new moons in the Pluto-Charon system led to the concern that there could be a significant dust environment that would endanger the spacecraft during the flyby. The best observations then available could not provide an acceptable upper limit of the dust environment, so a process was put into place to develop multiple contingency trajectories and attitude profiles that could be implemented within the last few weeks before closest approach. One part of that process was to quickly associate the range of potential dust environments with the probabilities that the spacecraft would survive the encounter. This paper provides an overview of the New Horizons Hazard Process and describes the details of the spacecraft hazard assessment. The New Horizons hazard assessment process began with images taken of the Pluto system using the high-resolution imager. Those images were passed to a group looking for new objects. The locations of any new moons were analyzed to develop the associated stable orbits. Using those orbits, a team of dust modelers developed a range of dust environments for the system using models of how the dust particles were generated by the moons and transported throughout the system. Other key assumptions included the optical properties that defined how the dust environments would be imaged by the spacecraft sensor. These environments were then coupled to the spacecraft trajectory and attitude profiles to predict how many hits the different spacecraft surfaces would see. The number of hits and the damage criteria for those surfaces were combined to develop the overall probability of survival of the spacecraft. There were two parts to the New Horizons hazard process that were unique and provide key features for how these types of studies could be improved in the future. The first part was the timing. All of the spacecraft operations were controlled by preloaded commanded sequences that needed to be developed, reviewed, and tested well beforehand. A set of alternate command sequences and trajectories were developed in advance of the encounter to respond to possible dust hazards in the event that significant hazards were found. The limits of the available imaging from the spacecraft meant that the process of final image acquisition, downlink, analysis, and hazard assessment had to be completed within a 3-day time period so that the appropriate trajectory and sequence could be selected in time. The unknowns associated with the dust environment led to the development of over 2000 dust model/trajectory/attitude combinations that had to be evaluated and assessed. As the number of cases to be evaluated increased, the processing time became the schedule driver. The second part was the parametric dust models used. In an encounter with an unknown dust environment, the calculation of the dust spatial distribution involves a range of parameters that need to be defined by the dust modelling experts. The relative likelihood of the different dust parameter sets used in the dust model allow the generation of confidence levels to be included with the overall spacecraft damage assessment.