The combined constraints of quantum mechanics and special relativity open new possibilities for quantum communications. This paper discusses a relativistic quantum key distribution (QKD) protocol over open space. The protocol does not require single-photon states, but rather attenuated coherent states of laser radiation. With a small average number of photons in coherent states, states with different relative phase values cannot be reliably distinguished. There is no need to use deceptive states, as in the decoy state method, which are mandatory for non-relativistic QKD protocols. This simplifies the optical design at the receiving end and eliminates the need for active elements — a phase modulator. An intensity modulator is not required at the transmitting end, which is necessary for implementing the decoy state method in non-relativistic systems. The protocol enables key distribution with high line losses of approximately 60 dB. The protocol uses only two information states, eliminating the need for base agreement. All registered packets are used in the key, increasing the speed of key distribution. The most suitable application for relativistic QKD systems is key distribution between stationary objects, such as cellular base stations or other objects between which laying fiber optic cables is expensive or practically impossible. Unlike the analysis of the cryptographic strength of non-relativistic QKD systems, which uses rather sophisticated mathematical apparatus, the analysis of the security of relativistic QKD is presented at a level that is intuitively understandable for a physics audience. It does not require prior specialized knowledge and is accessible to graduate students and senior university undergraduates familiar with the fundamentals of quantum optics, quantum information science, and quantum cryptography, as taught in standard university courses.