A preliminary design of a 6U CubeSat platform is presented for an Earth Observation mission in Low Earth Orbit. Assuming a mission statement and analyzing the market optical payloads, this work aims to find the preliminary requirements of every sub-systems of a spacecraft that is compatible with most of these payload. These will be used to select a set of possible choices to define a platform baseline configuration useful to speed up the study and development process of a satellite in a future proposal phase of a similar mission. The combination of the mentioned choices will then provide to Argotec an approximate price and lead time of the total effort.