RIS-Empowered Satellite IoT: Bridging the Coverage-Efficiency Gap of Last-Mile Access and Sensing

Satellite Internet of Things (IoT) is set to be a critical infrastructure for global connectivity, particularly in remote and under-served regions beyond the reach of terrestrial networks. However, the last-mile segment of linking satellite backhaul to ground IoT devices and sensors for ubiquitous integrated sensing and communication (ISAC) still suffers from three intertwined challenges: infrastructure deployment and economic constraints, complex satellite-terrestrial physical environments, and multi-dimensional resource optimization with interference management, which conventional active relays and base stations (BSs) fail to address simultaneously due to their high cost, power consumption, and infrastructural complexity. Reconfigurable intelligent surface (RIS) emerges as a transformative technology that bridges the coverage-efficiency gap through passive yet intelligent signal manipulation by dynamically configuring wireless propagation environments. This article begins by dissecting the last-mile bottlenecks in satellite IoT and introduces RIS fundamentals, including its hardware taxonomy and synergy with satellite architectures, followed by a complete RIS-empowered ISAC framework encompassing satellite-based signal formation, RIS-based channel reshaping with multi-domain resource allocation, and cloud-centric echo processing. A novel RIS-aided joint optimization scheme under realistic settings is discussed in case study. Finally, we outline promising research directions toward the green, ubiquitous sixth-generation (6G) networks.