1 Introduction

1.1 Definition of Cislunar Space

Cislunar space refers to the gravitational interaction zone between the Earth and the Moon beyond geostationary orbit. It extends from approximately 36,000 kilometers above the Earth's surface to about 500,000 kilometers, encompassing the Moon and the Earth-Moon libration point orbits. There are two main differences between cislunar space and near-Earth space: first, the distance scale is more than 10 times larger and the volumetric space is more than 1,000 times greater; second, due to the combined gravitational influence of both the Earth and the Moon, orbital motion evolves from the basic "two-body problem" to the complex "three-body problem," transforming the deterministic classical Keplerian orbits of near-Earth space into the unstable chaotic orbits of cislunar space. This fundamentally changes humanity's traditional understanding of orbital motion and determines the characteristics of aerospace activities in cislunar space [1].

Figure: Schematic of Cislunar Space [2]

In recent years, with increasing attention from various countries to lunar exploration and deep space exploration, cislunar space missions have been growing. From the early Apollo program to modern lunar exploration missions, and to the future construction of lunar bases, cislunar space has become a new frontier for international aerospace competition and cooperation. Currently, the United States, China, Russia, Europe, Japan, India, and other nations and organizations are actively conducting cislunar space-related missions, including lunar exploration, lunar resource surveying, and cislunar space infrastructure deployment.

1.2 Strategic Value of Cislunar Space

Currently, human space economic and military activities are mainly concentrated in near-Earth space below geostationary orbit. In recent years, with the rapid increase in the number of artificial objects in near-Earth space, security issues such as debris threats, orbital resource competition, and space traffic management have become increasingly prominent, significantly constraining the sustainable development of the space domain. Therefore, attention has turned toward the vast expanse of deep space, with cislunar space being the first priority, becoming a new frontier hotspot for international competition. Cislunar space is vast, connecting to near-Earth space below and reaching other bodies in the solar system including Mars above. It guards the essential route from Earth to deep space, possesses sufficiently broad strategic depth, and is considered a new "blue ocean" for space resource development, economic growth, and military operations, with enormous economic and military potential.

The Moon possesses the necessary natural conditions and abundant natural resources to support human survival, making it an ideal forward and support base for humanity's advance into deep space. Cislunar space contains Earth-Moon transfer orbits, near-lunar orbits, and distant retrograde orbits (DROs) formed around the two gravitational centers of Earth and Moon, as well as halo orbits and near-rectilinear halo orbits (NRHOs) around the Earth-Moon libration points. Moreover, the energy cost for orbital maneuvers and transfers in cislunar space is very small, providing exceptional convenience for various cislunar spacecraft to maintain and transfer between orbits. In the future, cislunar space could become a "logistics hub" supporting an efficient transportation network between Earth and deep space. In this sense, cislunar space has unique geopolitical advantages and will play an important role in humanity's expansion into deep space.

In the future, whether for deep space economic or military applications, information infrastructure support such as sensing, communication, navigation, positioning, and timing will be essential, requiring the construction of new space-based information infrastructure beyond near-Earth space. The five Earth-Moon libration points maintain a naturally fixed geometric configuration relative to the Earth and Moon, with numerous stable periodic or quasi-periodic orbits in their vicinity. Spacecraft operating in these orbits require very little energy for long-term stable operations. It can be said that the Earth-Moon libration points are the inevitable choice for deploying cislunar space information infrastructure — whoever first occupies the libration point orbits will gain the initiative in deep space information infrastructure construction, and thus dominance in deep space economic development and military deployment.

Cislunar space possesses inexhaustible energy resources, providing ample energy security for building the future deep space economic ecosystem. In addition to the ultimate green and environmentally friendly solar energy, the Moon has abundant helium-3 resources — an ideal fuel for safe, clean, and pollution-free nuclear fusion. Just 30 tonnes of helium-3 could meet the energy needs of the United States for one year. It is estimated that the total helium-3 reserves on the Moon are nearly one million tonnes, enough to satisfy the entire Earth's energy needs for thousands of years. The Moon's ice deposits also contain abundant oxygen and hydrogen resources, which could serve as rocket propellant in the future. The Moon is also rich in rare-earth mineral deposits, including elements such as platinum and lithium frequently used in spacecraft, providing convenient conditions for manufacturing spacecraft on the Moon. Once lunar resource development reaches scale, it will generate incalculable wealth, and its strategic value goes without saying.

Welcome to the world of cislunar space exploration! Whether you are a curious beginner or a seasoned expert in this field, there is content here for you. Let us explore the mysteries of cislunar space together, understand its research frontiers, master the relevant terminology, access useful resources and tools, and jointly advance the development of cislunar space research and applications!

References

[1] Zhu Yanwei, Huang Huan, Cai Weiwei, et al. Introduction to Space Operations [M]. National Defense Industry Press, 2026.

[2] Baoyin Hexi, Zhang Nan, Wu Di. Multi-impulse trajectory optimization for routine Earth-Moon DRO round-trip transfer [J]. Flight Control & Detection, 2025.

[3] Yu Dengyun, Yin Jihao, Liu Siqi, et al. Current status and prospects of AI-empowered cislunar space perception technology [J]. Journal of Image and Graphics, 2025, 30(9): 2899-2910.

[4] Zhu Yanwei, Zhang Chengming, Yang Fuyunxiang, et al. A review of spacecraft orbital pursuit-evasion dynamics and control problems [J]. Journal of National University of Defense Technology, 2024, 46(3): 1-11.