Deflecting an asteroid with a kinetic impactor is the only planetary defense method for asteroid impact avoidance tested on a full scale. This thesis investigates the initial design of a kinetic impactor and its deployment architecture. During hypervelocity impact, momentum enhancement due to ejecta plays a crucial role. The impact efficiency factor, accounting for impact obliquity, ejecta asymmetry, and gravity-induced trajectory bending, is modelled. Influential parameters, including spacecraft momentum, are considered. Seven realistic design options are evaluated, with an emphasis on departure strategy from Earth, interplanetary transfer and high-level spacecraft architecture. The contribution to the research area is a parametric framework to evaluate a kinetic impactor, including a model of the momentum enhancement and a low-thrust transfer trajectory. The assembled impactor is selected based on a performance trade-off, and preliminary requirements are identified.