On April 15, 2025, at the "Academic Seminar on Exploration and Research of DRO in Earth-Moon Region of Space," hosted by the Technology and Engineering Center for Space Utilization, Chinese Academy of Sciences (hereinafter referred to as CSU), the research team unveiled pivotal findings from the CAS Category A Strategic Pioneer Program titled "Exploration of the Distant Retrograde Orbit (DRO) in the Earth-Moon Region of Space."

The program has successfully developed two satellites, DRO-A and DRO-B, which, upon reaching their designated orbits, established an inter-satellite measurement and communication link with the previously launched DRO-L satellite in low-Earth orbit. This milestone signifies the establishment of the world's first three-satellite constellation in the Earth-Moon region of space, such development has yielded variety of original scientific and technology outcomes, laying a solid ground for the country's future development of the Earth-Moon region of space and frontier exploration of the space science.

Trio Satellite Network in the Earth-Moon Region of Space

01 The Earth-Moon Region of Space and Distant Retrograde Orbit (DRO): A Gateway for Space Exploration

Earth-Moon region of space, refers to the expanded domain extending outward from Earth's orbit, reaching up to 2 million kilometers from the Earth. Compared to Earth's orbital space, its three-dimensional volume expands by more than a thousand times.

The DRO is a unique, bounded periodic orbit in the Earth-Moon region of space. It revolves around the Earth in a prograde direction and around the Moon in a retrograde direction.

Located at a high-potential energy position relative to the Earth-Moon system, DRO serves as a transportation hub connecting the Earth, the Moon, and deep space. DRO has unique characteristics such as low-energy access, stable parking, and low-energy reachability throughout the domain, making it a natural harbor in the Earth-Moon region of space.

In 2017, the research team from CSU first explained the unique properties and strategic value of DRO in the Earth-Moon region of space. The team made a series of important theoretical breakthroughs, depicted the dynamic phase-space structure of DRO, and revealed its low-energy orbit characteristics. CSU also launched preliminary research and key-technology development.

In 2022, the Chinese Academy of Sciences initiated the Category A Strategic Pioneer Program "Exploration of the Distant Retrograde Orbit (DRO) in the Earth-Moon Region of Space."

02 Space Rescue Mission Saves Twin Satellites DRO-A/B

On February 3, 2024, the pioneering experimental satellite DRO-L successfully entered a Sun-synchronous orbit, initiating its planned experiments with precision. However, just over a month later, on March 13, the launch of the twin-satellite combination DRO-A/B encountered unexpected difficulties, failing to achieve the intended orbit.

In response to this sudden setback, Chinese scientists swiftly embarked on an exhilarating space rescue mission, aiming to salvage the twin satellites. Preliminary data analysis revealed that DRO-A/B was spinning rapidly and out of control. In the face of this dire situation, the engineering team developed emergency measures to stabilize the satellites.

The actual initial orbit of DRO-A/B had an apogee height of only 134,000 kilometers, falling short of the planned 292,000 kilometers. This significant discrepancy posed a severe challenge, as the satellites' orbit height was insufficient for their intended mission.

To address this critical issue, the research team made a strategic decision on March 15 to keep the twin satellites together and devised an innovative orbit-reconstruction strategy. They alternately utilized fuel from both satellites to gradually elevate their orbit height.

On March 18 and 23, the engineering team executed two crucial perigee orbit-correction maneuvers, successfully raising the height of DRO-A/B to 240,000 and 380,000 kilometers, respectively. These maneuvers helped the satellites cross the critical "death line," marking a significant step towards their intended mission orbit.

On April 2, DRO-A/B successfully performed a key lunar-transfer maneuver, entering a pre-determined low-energy Earth-Moon transfer orbit. After a meticulous journey of 123 days, covering approximately 8.5 million kilometers, the satellites finally executed a DRO-entry maneuver on July 15, accurately reaching their planned mission orbit.

With their successful insertion into the designated orbit, DRO-A/B now provide essential support for subsequent in-orbit testing of their payloads. This remarkable rescue mission not only showcases China's advanced space technologies but also underscores the resilience and ingenuity of its scientists and engineers.

03 Breakthrough in the Earth-Moon region of Space Exploration: World's First DRO-Based Three-Satellite Constellation Successfully Deployed

On August 28, 2024, a significant milestone was reached in the realm of  the Earth-Moon region of space exploration as the DRO-A/B satellite duo successfully separated and captured images of each other for verification. Technical assessments revealed that the twin satellites, often referred to as the "twin stars," exhibited balanced energy levels, with their platforms and payloads operating flawlessly.

Following this separation, a K-band microwave measurement and communication link over 1.17 million kilometers for inter-satellite and satellite-to-ground communications, overcoming key technological bottlenecks in constructing large-scale constellations in cislunar space.

Globally, for the first time, China has verified the new-quality capability of satellite-tracking-satellite orbit determination and navigation in the Earth-Moon region of space. With the successful networking of three satellites, a novel space-based orbit determination system utilizing satellite-tracking-satellite technology has been validated. Within just three hours of in-orbit inter-satellite measurements, the achieved orbit determination accuracy matched that of two days of traditional tracking and measurement data. This significant achievement has drastically reduced the operational costs of Earth-Moon spacecraft and paved new avenues for efficient spacecraft operations.