Imaging extrasolar planets via formation flying with an external occulter

Currently, over 270 exosolar planets have been detected, but all have been significantly more massive than Earth and none have been imaged directly. A great deal of research is being done on various techniques for achieving the high contrast and exquisite control needed to image a terrestrial planet. NASA is currently sponsoring at least four concept studies for missions that might be flown in the next decade to detect and characterize such exosolar earthlike planets. In this paper we present an overview of one such mission that utilizes a large external occulter (on the order of 50 m) flying over 70,000 km from a conventional space telescope (with aperture on the order of 4 m). Innovative optical designs incorporating petal shaped occulters, leveraging off of the results for shaped pupil coronagraphs, have resulted in optimal systems with the smallest and closest possible occulters that still achieve the needed contrast over the full visible band. New hybrid approaches that combine external occulters with internal corona- graphs promise to reduce the size and distance of the occulter even further while still maintaining the wavefront control advantage. We will review the optical solution for the occulter/telescope and discuss the orbital trajectory design. We will discuss mission planning scenarios and optimal approaches to retargeting. We will also present some preliminary error analysis that drives the pointing and positioning requirements of both the occulter and telescope. Finally, we will show laboratory results of scaled occulters.