We describe the concept of a proposed mission, called Multi‐Angle Solar Sources Explorer (MASSE´), that would observe the Sun and the inner Heliosphere from an orbit at 0.72 AU over all solar longitudes. It would, in coordination with observations from Earth’s side, investigate the sources of solar activity from their origin deep within the Sun, their emergence onto the photosphere, and their ejection into the Heliosphere. It carries a Doppler‐magnetic imager, and in situ energetic particle, solar wind, and magnetic field detectors. Three‐dimensional views of the convection zone, where solar activity originates, are reconstructed by correlating MASSE´ and earth‐side Doppler signals from acoustic wave packets traversing deep solar layers. Magnetic images reveal the evolution of active regions over their life‐time and allow the study of emerging fields from deep layers. Particle, plasma, and magnetic field data provide information on the sites and mechanisms of acceleration of hazardous high‐energy particles produced by coronal mass ejections. © 2003 American Institute of Physics