Trajectory optimization with chemical propulsion for multiple debris removal missions

Today, space debris are a natural consequence of any space mission and are originated by launching, operative and end of life phases ranging from debris smaller than 10 cm to big rocket bodies and upper stages. With the great increase of space activities in the last few decades, the crucial concern is that space debris pose a problem not only for future missions but also for present space activities with an increasing risk of in-orbit collisions between a debris and an active payload that, happening at a speed of several km/s, would be destructive; another consequence of these impacts would be the formation of a multitude of other debris with the risk of an exponential increase of the number of objects according to what Kessler Syndrome has predicted. Thanks to some international regulations, today the space sector is trying to reduce the amount of space debris produced during space missions in their whole but these actions are not enough and the actual situation makes Active Debris Removal (ADR) a compelling need. This thesis presents a possible ADR solution being a chemical propulsion spacecraft that rendezvous with several LEO debris and makes them de-orbit into Earth's atmosphere where they will destroy; since there is an obvious advantage if a single ADR mission can remove more than a single object, this thesis analyses multiple debris removal missions and in particular gives a strategy to select the optimal debris sequence in order to minimize the propellant consumption. The transfers between the objects exploit the J2 perturbation to further reduce the propellant required and the sequences created, among which the optimal ones are chosen, are characterized by a variable start in time and a variable duration; the algorithm used to optimize the sequences is an Ant Colony Optimization (ACO) algorithm based on ants' foraging behaviour that has been chosen because of its great performances in solving the Travelling Salesman Problem (TSP) to which this thesis' problem can be mapped. The results obtained by first applying the ACO to the global problem and then applying it to the single missions show that this approach is effective in finding a good 4-missions sequence of multiple debris with a reduced consumption of propellant.