2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/36660We suggest an explanation of the "Pioneer effect" based on the interaction of the spacecraft with a long-range scalar field, $ϕ$. The scalar field under consideration is external to gravity, coupled to the ordinary matter and undergoes obedience to the weak equivalence principle. In the weak fields limit it result a long-range acceleration $a_{P}$, asymptotically constant within the region of the solar system hitherto crossed by the spacecraft.Latex, 23 pages with 3 Postscript figures, submitted to Physical Review DGeneral Relativity and Quantum Cosmologytext