The concept of a photonic satellite where optical subsystems replace the microwave and digital electronics on-board communications spacecraft is considered very attractive for its potential to reduce the mass and volume of a payload requiring increased processing bandwidth. Use of on-board photonic subsystems could therefore render advanced Satcom systems highly cost-competitive with respect to alternative technologies. In this invited paper, the status of onboard photonic subsystems such as phased-array antenna beam forming and steering, high-speed signal distribution, control, and processing, millimeter wave (mmW) signal generation, and intersatellite link, and relevant photonic devices and technologies is reviewed. Issues of network architecture, technology feasibility, reliability, and redundancy requirements, and their impact on the mass, power, and size of the payload are discussed. Also, a time line of critical optical technology, onboard implementation issues, and the cost-benefits achievable over advanced RF subsystems are presented.