空间地球科学是利用空间观测的手段，研究地球系统、各子系统之间以及各要素和过程之间的相互作用、变化机制及其发展演化的一门综合性、交叉性学科，为科学家开展地球系统科学研究提供新手段、新思路和全新视角。空间地球科学的发展推动航天技术、遥感和测绘科学、地球系统科学包括气象、海洋、水文、生态等一系列学科的发展，是全球变化背景下促进全球治理和“人类命运共同体”构建的重要基石。本文主要回顾空间地球科学的产生和发展历程，以全球水循环为例阐述空间观测在地球系统关键循环过程研究中的作用，并对中国未来空间地球观测进行展望，以期盼中国空间地球科学事业迈向新篇章。

The space Earth science is a comprehensive and interdisciplinary discipline that studies the interactions, mechanisms and evolutions of the Earth system and Earth’s subsystems through the means of space observation (satellite remote sensing). It brings new means, new ideas and new perspectives to scientists for Earth science studies. The development of space Earth science will promote the development of a series of disciplines including aerospace technology, remote sensing science, Earth system science, meteorology, hydrology, ecology etc. It is an important cornerstone for the cooperation to jointly build a community of common destiny for all mankind. This article mainly reviews the development of space Earth science, and discusses the role of space observation in the study of key subsystems (e.g., water cycle) of the Earth system. It also looks forward to the future development of space Earth science in China.Space Earth science has emerged as a powerful tool to investigate the Earth as a system, expanding the research of Earth science from local to the global dimensions, from static to dynamic queues, and will continue to expand our knowledge about how the Earth has changed. The current space Earth sciences are showing some new development trends, such as (1) paying more attention to the key cyclic processes in the Earth system, and (2) Combining of satellite remote sensing and Earth system model. Taking the observation of Earth’s water cycle from space as an example, the key elements and process variables related to the global water cycle include precipitation, evapotranspiration, runoff, soil moisture, sea temperature and salinity, surface-water bodies, glaciers, snow, frozen soil, sea ice, polar ice caps and ice sheets, atmospheric water vapor, and groundwater etc. Satellite remote sensing has the unique advantage to provide information on the water status, migration, exchange, and phase change processes, which are essential information involved in the global climate change. At present, meteorological, oceanic and Earth observation satellites that have been launched internationally and have been able to measure many water cycle elements in the atmosphere, ocean, and land. These space observation data have greatly enhanced scientists’ knowledge and understanding of the global water cycle process.The outlook for future space Earth observation in the field of water cycle is as follows.(1) Develop satellites for detecting surface state variables of water cycle, including the synthetic aperture microwave radiometer technology, active and passive integrated detection technology, to enable multi-element, high-accuracy, high-resolution simultaneous observation of key state variables such as soil moisture, soil freeze/thaw, snow properties and sea surface salinity, etc.(2) Develop satellites for estimating land-atmosphere water fluxes of water cycle (precipitation and evapotranspiration) by enhancing the atmospheric detection abilities to distinguish between rainfall and snowfall, and combining the thermal infrared and microwave measurements to improve the quantities and qualities of variables related to evapotranspiration.(3) Develop thematic satellites of the cryosphere (water in solid form), including the development of long-wavelength (such as P-band) and wide-band microwave radiometer for the measurement of ice density and temperature profile of ice sheets, the measurement of glacier thickness, and the observation of the thickness of permafrost active layer, etc., and the development of synthetic aperture radar interferometry and lidar for high-precision surveying of ice sheet surface elevation and ice volume.