Starshade orbital maneuver study for WFIRST

The Wide Field Infrared Survey Telescope (WFIRST) mission, scheduled for launch in the mid-2020s will perform exoplanet science via both direct imaging and a microlensing survey. An internal coronagraph is planned to perform starlight suppression for exoplanet imaging, but an external starshade could be used to achieve the required high contrasts with potentially higher throughput. This approach would require a separately-launched occulter spacecraft to be positioned at exact distances from the telescope along the line of sight to a target star system. We present a detailed study to quantify the Δv requirements and feasibility of deploying this additional spacecraft as a means of exoplanet imaging. The primary focus of this study is the fuel use of the occulter while repositioning between targets. Based on its design, the occulter is given an offset distance from the nominal WFIRST halo orbit. Target star systems and look vectors are generated using Exoplanet Open-Source Imaging Simulator (EXOSIMS); a boundary value problem is then solved between successive targets. On average, 50 observations are achievable with randomly selected targets given a 30-day transfer time. Individual trajectories can be optimized for transfer time as well as fuel usage to be used in mission scheduling. Minimizing transfer time reduces the total mission time by up to 4.5 times in some simulations before expending the entire fuel budget. Minimizing Δv can generate starshade missions that achieve over 100 unique observations within the designated mission lifetime of WFIRST.