A Dawn-based Gravity Tractor and Kinetic Impactor Mission Concept Study
Wie, Bong¹; Lam, Q^2
1Iowa State University; ²Orbital Sciences Corporation

This paper presents the feasibility of using the Dawn spacecraft to experimentally validate the gravity tractor concept after the primary science mission at Ceres is complete in July 2015. This is contingent on Dawn having sufficient fuel for ion engines remaining at that time. If feasible, Dawn could travel to an accessible asteroid that is small enough for the perturbation on the asteroid due to Dawni's slow-pull thrust to be measurable before Dawni's remaining fuel is exhausted. It remains to be determined whether Dawn is likely to have enough extra propellant and whether a small, accessible asteroid can be found. Therefore, this mission concept study being conducted at this time may not be actually realized if in the early stages of the investigation it is determined to be infeasible. The mission feasibility of maneuvering the Dawn spacecraft to collide with a small asteroid after the primary science mission at Ceres is complete in July 2015 will also be described.

During its nearly decade-long mission the 1250-kg Dawn spacecraft will study the asteroid Vesta and the dwarf planet Ceres. These celestial bodies are believed to have accreted during the early history of the solar system. After launching on September 27th, 2007, Dawn will become the first spacecraft to ever visit Vesta or Ceres. It will also become the first spacecraft to ever rendezvous with a celestial body, orbit and study it, and then depart under its own power (without gravitational assist) to rendezvous with, orbit, and study a second body. Subsequent to a February 2009 Mars gravity assist, Dawn will arrive at Vesta in August 2011, depart in May 2012, and arrive at Ceres in February of 2015. Dawni's primary mission is scheduled to end in July 2015.

The Dawn spacecraft can be adopted as a baseline platform candidate for the GT/KEI systems design to perform NEO survey and deflection missions for four main reasons as described below.

1. The spacecraft is designed to rendezvous with, operate in the vicinity of, track, scientifically characterize, and navigate relative to asteroids.
2. The majority of the extant GN&C sensor and actuator suite for operations near asteroids is applicable and directly transferrable to a NEO survey and deflection mission.
3. The spacecraft design already includes ion thrusters for low-thrust interplanetary missions.
4. The spacecrafti's structure and mass budget are suitable for the GT/KEI spacecraft mission, even accounting for the inclusion of a new ion thruster (built by the same manufacturer, Boeing, and now L-3). The structure and mass will be optimized to achieve an acceptable amount of asteroid deflection so that a hazardous NEO can be successfully steered away from a collision course with Earth.