Deep-space exploration, which serves as a pivotal avenue for uncovering the mysteries of the universe and fostering sustainable development, has emerged as the foremost strategic frontier in space technology. After decades of development, this technology has been widely used to explore various celestial bodies in the solar system. In February 2015, the first deep-space exploration satellite targeting Earth, the Deep Space Observatory, was successfully deployed at the Sun-Earth Lagrange Point 1, providing new perspectives and data for the study of Earth system science while also introducing new challenges to traditional satellite data research. This study comprehensively analyzed more than 100 related papers and conference summaries published on the official website of the National Aeronautics and Space Administration since the launch of the Deep Space Observatory. From three perspectives—basic research, applied research, and specialized research—this paper reviews the development status, advantages, and future directions of deep-space Earth observation. The findings reveal that deep-space Earth observations can be integrated with existing satellite-aircraft-ground systems, establishing a benchmark for multisource data fusion to create globally comprehensive, high-temporal-frequency, and multispectral datasets for an integrated Earth observation system. This approach provides temporally consistent, spatially continuous, and spectrally stable global observational data, demonstrating significant potential for studying largescale geophysical phenomena across the atmosphere, biosphere, hydrosphere, and lithosphere. Future advancements in sensor innovation, optimized detection technologies, and diverse observation points are expected to enable continuous, omnidirectional, and multidimensional Earth observations. These advancements will enhance our understanding of Earth's physical, chemical, and biological systems.