The ESA Living Planet Programme includes two types of complementary user driven missions: the research-oriented Earth Explorer missions and the operational service oriented Earth Watch missions.
One class of the Earth Explorer missions, the Opportunity missions, are smaller, but also address the research objectives set out in the Living Planet Programme. All Earth Explorer missions are proposed, defined, evaluated and recommended by the scientific community.
In response to a call for Opportunity mission proposals in 2001, which resulted in 25 full proposals being submitted by early 2002, three mission candidates, ACE+, EGPM and Swarm, were also chosen for feasibility study. The Swarm candidate mission is based on the mission proposal co-written and submitted in 2002 by a team lead by Eigil Friis-Christensen, Hermann Lühr, and Gauthier Hulot.
Based upon the recommendations made by the Earth Science Advisory Committee and the ESA Director of Earth Observation, the choice of Swarm for full implementation was unanimously agreed upon as a result of its scientific excellence.
(Cited from ESA documents or web pages)