Linear accelerators producing relativistic (5 MeV) electron beams are now down to a size that allows them to be flown on spacecraft and sounding rockets. This opens up new opportunities for atmospheric/ionospheric modification experiments where the mesosphere and lower thermosphere regions can be perturbed down to 40-km altitude. In this paper the relativistic electron beam injection process is investigated by means of three-dimensional particle-in-cell simulations to determine the initial interaction of the beam with the spacecraft and the ambient plasma. The results indicate that relativistic beams are more stable than keV-energy beams investigated in the past, allowing the injection and propagation of beams with currents several orders of magnitude higher than those for keV-energy beams. The superior stability of relativistic beams is the result of a combination of effects including the higher relativistic electron mass, a lower beam density, and a smaller effect from spacecraft charging. Relativistic beams injected downward from spacecraft are therefore expected to deposit a large fraction of the energy in the middle atmosphere. In the high-current limit (I > 100 A) the beam self-fields are strong. In this regime a beam may propagate in the ion-focused regime, where beam electrons expel ambient electrons to create a channel of ambient ions that space charge neutralize the beam. The establishment of the ion channel, however, creates significant turbulence and scattering.