Coronagraphic detection of Earth-like planets with large, actively controlled space telescopes

We explore the capabilities of large segmented telescopes with active and adaptive optics, with a particular focus on a system view, which includes use of approaches that are routine for current large ground-based telescopes. Using a physically motivated order-of-magnitude model, we show that continuous control of telescope misalignments using adjustable optics in an exoplanet imaging instrument significantly relaxes stability requirements for the entire observatory. We start with the recent analysis by Nemati et al., (2020, JATIS 6, id. 039002), which asserts that small monolithic mirrors have an engineering advantage over larger segmented mirrors when it comes to obtaining images stable enough for direct exoplanet imaging and characterization, i.e., picometer stability. When we fold these results into our model of closed-loop operations and properly partition engineering challenges by optimizing error budget allocations, we find that even for the most sensitive modes, allowable drifts are actually of the order of nanometer over an hour, well within easily engineered tolerances. While this order-of-magnitude analysis does not include full end-to-end modeling or proper engineering margins, it showcases the importance of considering continuous wavefront sensing and control when discussing the feasibility of future exoplanet missions. We also quantify how large segmented architectures, in spite of appearing more complex at the observatory level, facilitate closed-loop operations due to their large photon collection abilities. We place our work in the context of larger discussions on aperture size that highlight a more fundamental challenge: the deeper uncertainties in performance of an exo-earth characterizing telescope primarily reside in our knowledge of the frequency of exo-earths; the effects of geological age on the resulting atmospheres; and, most importantly, on the likelihood of detectable life arising on such planets. A mission that sets out to establish whether we are alone among the nearby stars must adopt a mission architecture that is resilient against such intrinsic uncertainties: uncertainties that only direct observations can resolve. Large apertures enabled by segmented telescope designs historically have demonstrated such resilience.