Double Asteroid Redirection Test: The Earth Strikes Back

The NASA Double Asteroid Redirection Test (DART) is a technology demonstration mission that will test the kinetic impactor technique on a binary near-Earth asteroid system, Didymos. Didymos is an ideal target, since the 780 m primary, Didymos A, is well characterized, and the 163 m secondary, Didymos B, is sufficiently small to allow measurement of the kinetic deflection. Didymos also represents the population of Near Earth Objects that are the asteroids most likely to pose a near-term threat to Earth. Scheduled to launch in June 2021, the DART spacecraft will autonomously intercept Didymos B in October 2022, altering the orbit period of Didymos B with respect to Didymos A. The impact will occur when the Earth-Didymos range is close enough to allow observation by Earth-based optical and radio telescopes. The spacecraft will be guided to the impact by its on-board autonomous real-time system Small-body Maneuvering Autonomous Real-Time Navigation (SMART Nav). In addition, DART is carrying a 6U CubeSat provided by Agenzia Spaziale Italiana (ASI). The CubeSat will provide imagery documentation of the impact, as well as in situ observation of the impact site and resultant ejecta plume. The development is currently in Phase C, with mission Critical Design Review (CDR) planned for summer 2019. It is a substantial challenge to navigate the DART spacecraft to a hypervelocity impact with the Didymos secondary. The DART spacecraft will carry an ion propulsion system with the NASA Evolutionary Xenon Thruster Commercial (NEXT-C)engine, which provides flexibility in trajectory design to achieve the desired Didymos arrival conditions. The trajectory maximizes the asteroid deflection with an arrival velocity of 6 km/s relative to Didymos-B, while maintaining a proximity to Earth that allows both observation of the impact, and sufficient communication gain to recover imagery of the target upon the approach. Additionally, NEXT-C allows the mission an opportunity to fly by another asteroid and characterize SMART Nav performance seven months prior to its use for the Didymos impact. The DART spacecraft carries 22 m
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solar arrays to generate the ~3.5 kW needed to power the NEXT-C engine. These long arrays introduce substantial flexible body motion to the spacecraft. This motion must be managed carefully to maintain the DART narrow angle camera on the target asteroid, even while performing the necessary autonomous ΔV maneuvers required to intercept the target. Guidance to Didymos B is further complicated by having to switch targets late in the approach, as SMART Nav must target the primary asteroid initially, as the secondary is too small to be resolved by the narrow angle camera until ~1 hour prior to impact. Shadowing of both primary and secondary set by arrival lighting makes it challenging to impact at the center of Didymos B. The spacecraft streams images back to Earth in real time, up until impact.