As a result of human space activities for over sixty years, there exist numerous man-made debris objects in the Earth orbits. Such objects not only jeopardize current operations of important space assets, but can also seriously hinder future space missions through a potential chain reaction of colliding space debris. This research addresses mitigation and remediation of the space debris environment. The thesis entails research in four critical subject areas:​ i) characterization of the debris environment, ii) assessment of the active debris removal methods proposed in the literature, iii) detailed engineering of a novel removal concept using a CubeSat spacecraft, called Deorbiter CubeSat, for sizable debris objects in low-Earth orbit, and iv) design of attitude and orbit controllers for the proposed spacecraft.

The research first develops a probabilistic method for the prioritization of debris objects to be consid- ered in near-future removal missions. Then, a comparative study and in-depth analysis is conducted on the removal methods proposed in the literature to investigate their viability, through a number of multi- criteria assessment techniques. A Monte Carlo analysis is used in the study to quantify the intrinsic uncertainty associated with the space debris population. Next, a new debris removal mission utilizing Deorbiter CubeSats is conceptualized, and the design of CubeSat subsystems is detailed. A mothership spacecraft carries and deploys a number of Deorbiter CubeSats into designated orbits near their target debris. Each CubeSat uses an eight-unit form factor, and consists of commercially-available components with substantial space heritage. The actual performance specifications of the components are used to examine the proposed space debris removal approach. Finally, control schemes are synthesized for the critical maneuvers in the mission, using a unilateral low-thrust propulsion and a three-axis reaction wheel systems onboard the CubeSat, namely, i) concurrent rendezvous and attitude synchronization maneuver for approaching and attaching to the debris, ii) detumbling maneuver for stabilizing the debris attitude motion, and iii) deorbiting maneuver for transferring the debris from its original orbit to a deorbit altitude along a time-optimal trajectory. Several numerical simulations verify and validate the proposed approach as well as the control schemes.