  In this paper, we explore from a conceptual standpoint the possibility of
using natural astrophysical sources to accelerate spacecrafts to relativistic
speeds. We focus on light sails and electric sails, which are reliant on
momentum transfer from photons and protons, respectively, because these two
classes of spacecrafts are not required to carry fuel on board. The payload is
assumed to be stationed near the astrophysical source, and the sail is
subsequently unfolded and activated when the source is functional. By
considering a number of astrophysical objects such as massive stars,
microquasars, supernovae, pulsar wind nebulae, and active galactic nuclei, we
show that terminal speeds approaching the speed of light might be realizable
under idealized circumstances provided that sufficiently advanced sail
materials and control techniques exist. We also investigate the constraints
arising from the sail's material properties, the voyage through the ambient
source environment, and the passage through the interstellar medium. While all
of these considerations pose significant challenges to spacecrafts, our
analysis indicates that they are not insurmountable in optimal conditions.
Finally, we sketch the implications for carrying out future technosignature
searches.
