Touchless Despinning of Asteroids and Comets via Neutral Beam Emitting Spacecraft

Automated surveys along with population models have determined that tens of thousands Near Earth Objects (NEOs) exist and regularly cross over the Earth's orbit. While none of the currently discovered NEOs are predicted to impact Earth, many have yet to be discovered. We propose using neutral beam equipped spacecraft to deflect an asteroid from an Earth impact trajectory. Neutral beams are created through global neutralization of ion beams via recombination reactions and contain no charged particles. This research focuses on the design and testing of a unique neutral beam thruster and its application to planetary defense. The first contribution of this research focused on the efficacy of using low-thrust propulsion to de-spin and deflect sub-kilometer sized asteroids. We have presented the range of asteroid sizes that could be deflected and/or de-spun by a 4 NBAC spacecraft system, considering a range of spectral types and spin rates, and assuming the orbit for the hypothetical asteroid 2017 PDC. NBAC can handle a variety of asteroid spectral classes with a maximum mass of 3.8×〖10〗^9 kg for a 420 day campaign, deflecting them to over 200 km from an Earth impact. Using multiple spacecraft is resilient in terms of providing adequate deflection and reduces the probability of system down-time during the deflection campaign. NBAC is also suited for slowing or arresting the rotation of some asteroids. Partial arrest increases the surface gravity thus increasing the asteroid strength for future missions to grapple the asteroid in order to tow it to another orbit. The propellant mass per spacecraft for total arrest of the asteroid is typically less than 60 kg per spacecraft which is reasonable considering the mission duration and thrust levels achieved with NBAC. For fast rotating asteroids (P ≤ 2 hrs), there are some solutions for partial arrest of these bodies within a half orbital period. Fast rotating C, B, and S-type asteroids can be partially arrested while Xc-type asteroids are unlikely to be partially arrested. The second contribution of this research demonstrated that neutral beams, a technology typically used to heat tokamak plasmas, can be scaled for keV electric propulsion for spacecraft. A system-level study of the neutral beam, altering the electric propulsion performance equations to account for changes brought upon by an additional gas flow rate for the gas diffusion neutralizer was conducted. The optimal background gas density necessary for the greatest global neutralization of an argon ion beam as a function of beam energy is calculated. The additional flow rate is derived using a slip, molecular flow relationship for flow in cylindrical tubes that accounts for the viscosity of the fluid. The mass utilization efficiency decreases as a result of introducing two flow rates into the system, but the specific impulse for high energy argon neutral beams maintains acceptable efficiencies for long-term operation in space. The third contribution of this research was that a method has been developed to conduct thrust measurements for sub-Newton propulsion on a hanging pendulum thrust stand during thermal drift. The thrust resolution of the VAHPER thrust stand at Marshall Spaceflight Center has been improved to ~ 10 µN. The 1 keV neutral beam technology demonstration produced ~100 µN of thrust over the range of neutralizer flow rates. The thrust measurements between grid on/off cycles match one another within one standard deviation of the measurement noise. For the fourth contribution of this research analytical expressions for a low power neutral beam were derived and simulations were conducted to design a unique gas diffusion neutralizer. The experimental performance was validated against both the analytical expressions and simulation data. A Faraday cup was used to measure the current drawn of the beam to determine the fraction of ions in the flow and thus the neutralization. Losses in thrust and thus performance were characterized by investigating the Faraday cup current draw differences from having a neutralizer in line with the ion source and removing the neutralizer. It was concluded that losses were caused by having a grounded metal gas diffusion neutralizer that was able to develop an image charge on the interior. This caused ions to defocus and impact the wall of the gas diffusion neutralizer. Using neutral beams for mitigating the hazard of sub-kilometer asteroids is possible within current and near-term technology. The gas diffusion neutralizer designed is applicable to other gridded ion thrusters with only minor modifications needed. This thesis presents the experimental methods for neutral beam characterization including assembly of the diagnostics. The improvements in testing on a hanging pendulum thrust stand allow for more rapid characterization of the thrust performance of sub-Newton thrusters. The analytical expressions and simulations developed by this work have been confirmed by experimental data and demonstrate the viability of neutral beam propulsion.