According to later reports there are more than 18000 artificial satellites in space, with more than 35000 objects larger than 10cm and another million objects large enough to cause serious damage to the satellites. With so many potentially dangerous objects in an orbit around the surface of the earth, a collision-avoidance system becomes often necessary. This problem is of greater concern with the ever-increasing number of satellites that are being planned with multiple new projects involving large constellations. There is an increasing need for the design and implementation of specific operations to to avoid collisions in space. These collision avoidance manoeuvres depend on having a good knowledge of orbit parameters of both the controlled and the colliding satellites. Another factor to be considered is the the impulse required and the available propellant to perform the collision avoidance manoeuvres. Optimization of propellant usage is a necessary step while performing these manoeuvres because the allocated resources for a potential collision avoidance manoeuvre are very low. We consider here a deterministic approach, neglecting uncertainities in the knowledge of the kinematic state of the colliders. The proposed optimization technique solves an eigenvalue problem for a case of non-direct approach. The effect of the maneuvre’s anticipation time with respect to the foreseen possible collision is highlighted, and specific detail is given to the in-plane and out-of-plane components of the impulsive manuvre indicated in order to avoid such an event. The cases investigated are relevant to formation flying in LEO. In addition some preliminary analysis about the feasibility of the maneuvres by means of propulsion syìubsystems currently avaialble for microsatellites is reported.