  Establishing secure communication links at a global scale is a major
potential application of quantum information science but also extremely
challenging for the underlying technology. While milestone experiments using
satellite-to-ground links and exploiting singe-photon encoding for implementing
quantum key distribution have shown recently that this goal is achievable, it
is still necessary to further investigate practical solutions compatible with
classical optical communication systems. Here we examine the feasibility of
establishing secret keys in a satellite-to-ground downlink configuration using
continuous-variable encoding, which can be implemented using standard
telecommunication components certified for space environment and able to
operate at high symbol rates. Considering a realistic channel model and
state-of-the-art technology, and exploiting an orbit subdivision technique for
mitigating fluctuations in the transmission efficiency, we find positive secret
key rates for a low-Earth-orbit scenario, while finite-size effects can be a
limiting factor for higher orbits. Our analysis determines regions of values
for important experimental parameters where secret key exchange is possible and
can be used as a guideline for experimental efforts in this direction.
