The agile satellite systems for the Earth observation missions are expected to provide more precise information with the most reactivity possible, which is to reduce the delay between the image requests submitted to the system and the availability of these image products. Most of the current agile Earth-observing satellites are controlled from the ground, and their reactivity depends mostly on the mission operation or task programming. This paper presents in detail all the elements or the factors contributing to the system reactivity. After analyzing the various contributors of the timeliness, this paper describes the way to achieve the objective of the best reactivity, in particular with the constraints by the consideration of a near real-time and fast response requirement. Different ways to program satellites are introduced by explaining how to improve the system reactivity essentially from the point of view of the acquisition and downlink delay, such as Routine Programming, Rolling Horizon Programming, Urgent Programming and finally Direct Tasking Programming. It is summarized that the system need to leverage mission operation and task planning aiming to have the best possible reactivity while keeping the image acquisition capacity.