We report the design of OPTEL-D which is an optical communication system for direct data transfer from deep space to earth. OPTEL-D extends the link distances of current optical communication systems to the deep space. The design covers the electrical, thermal, mechanical and optical challenges that such a link distance imposes to both the ground and the space segment. The detailed conceptual system design is presented together with its trade-offs and performance analyses. The space terminal is based on a modular approach which groups the fiber-optical, the opto-mechanical, and the electrical functions into separated modules for independent accommodation. The transmitter beam is provided by an amplitude modulated MOPA system operating at 1.5um. The opto-mechanical unit accomplishes beam forming and steering of the transmit beam as well as reception of the uplink channel. An electrical unit comprises power-conditioning, data-handling, and terminal control through its interfaces to the spacecraft. It is concluded that the OPTEL-D space terminal mass and power are 34 kg and 116 Watts. Depending on the ground station configuration, a data rate of several Mbps is reached over distances of >75 Mio kilometers. Critical assumptions on the receiver sensitivity were verified by tests and the results will be presented. The design of OPTEL-D was derived in collaboration with the European Space Agency to serve the Asteroid Impact Mission. Due to its modular approach using commercially available technology, it can be realized in a competitive schedule and cost frame providing flexible yet powerful data downlinks to most deep space missions.