LISA Pathfinder, a European Space Agency (ESA) satellite to which the Astroparticle and Cosmology Laboratory (CNRS/Université Paris Diderot/CEA/Observatoire de Paris) contributed, was launched on 3 December. It will serve as a demonstrator for the ambitious eLISA mission, a set of three satellites to track gravitational waves. The challenge: to detect these deformations of space-time which, a century after their prediction by Albert Einstein, still escape the instruments. But also to develop a new field of observations, gravitational astronomy, in order to study the astrophysical objects at the origin of these waves. This challenge also involves two ground instruments - the Franco-Italian Virgo and LIGO in the United States - in which French laboratories are heavily involved.