The Role of Survey Telescopes for Planetary Defense

Ground-based telescopic surveys have been working for decades to search for objects that could pose a direct impact hazard to the Earth. These searches have nearly completed the inventory of large (D>1km) objects and none of those objects pose any significant risk in the foreseeable future. Ongoing efforts are working to push that inventory down to ever smaller and far more numerous sizes and the current focus is on reaching 90% completeness down to D>140m. The B612 Foundation has been studying the value of infrared space-based facilities, such as Sentinel, toward this goal. In the process of this work, we have also worked to understand what this technology can achieve for objects as small as D=15m, roughly the smallest size that could have measurable consequences on the ground. Sentinel combined with LSST has over a 40% chance of finding the next Tunguska-like object, which is due on the next several hundred years. We do that in 10 years and then we have decades to fill in the missing ones with more ground observations and future space missions. Two important results from our work show that the observational strategy can depend on object size as well as the orbital properties of the objects. In particular, Near-Earth asteroids (NEAs) have a more diverse range of orbital elements than those objects that will hit the Earth and take longer to reach the same level of completeness for the survey designs we tested. We suggest in this work that a survey dedicated to the goal of planetary defense should be optimized for smaller Earth-impacting objects - not finding 140m NEAs. We will review the results of our survey modeling efforts and discuss performance and completion metrics for the next generation of survey tools.