The Double Asteroid Redirection Test (DART) mission electric propulsion trade

Mitigation of a hazardous NEO can be accomplished by deflecting it so that it misses the Earth. Strategies to deflect an asteroid include impacting it with a spacecraft (a kinetic impactor), pulling it with the gravity of the mass of a spacecraft (a gravity tractor), using the blast of a nearby nuclear explosion, and modifying the surface or causing ablation by various means including lasers or particle beams. None of these approaches has been tested on a NEO. The AIDA mission is a proposed international collaboration to demonstrate kinetic deflection, the most mature technique for mitigating the impact hazard of a Near Earth Object (NEO). AIDA consists of two mission elements, the NASA Double Asteroid Redirection Test (DART) mission and the ESA Asteroid Impact Mission (AIM). The main objectives of the DART mission, which includes the spacecraft kinetic impact and an Earth-based observing campaign, are to: • Perform a full scale demonstration of the spacecraft kinetic impact technique for deflection of an asteroid, by targeting an object large enough to qualify as a Potentially Hazardous Asteroid (that is, larger than 100 m); • Measure the resulting asteroid deflection, by targeting the secondary member of a binary NEO and measuring the period change of the binary orbit; • Understand the hypervelocity collision effects on an asteroid, including the long-term dynamics of impact ejecta; validate models for momentum transfer in asteroid impacts, inferring physical properties of the asteroid surface and sub-surface. The DART target is the secondary member of the binary asteroid 65803 Didymos, with the impact scheduled to occur in September, 2022. The DART impact on the secondary member of the Didymos binary at ∼7 km/s will alter the binary orbit period by at least 4 minutes, assuming a simple transfer of momentum to the target. The period change may be significantly greater, as the momentum transferred to the target asteroid may exceed the incident momentum of the kinetic impactor, possibly by a large factor. The AIM spacecraft will characterize the asteroid target and monitor results of the impact in situ at Didymos, but the period change can be determined accurately solely with ground-based observatories, an approach that is only feasible because of the choice of a binary system as target. DART held its Mission Concept Review on May 21, 2015. At MCR, the DART concept had only expensive and potentially risky launch options. During Phase A, the project explored the possibility of being a secondary payload on a commercial Geosynchronous Transfer Orbit (GTO) launch with electric propulsion (EP) as an approach to reduce mission cost, eliminate the launch vehicle risk, and demonstrate the NASA Evolutionary Xenon Thruster (NEXT) engine. NASA determined that DART would use this approach, and the EP-based concept was presented at the DART System Requirements Review on Aug 30, 2016. This paper summarizes the trade that resulted in adoption of the new design.