China's experimental Shijian-21 and Shijian-25 satellites have concluded their apparent on-orbit refueling tests, separating in geosynchronous orbit after months of being docked. These tests, conducted with minimal public disclosure, have sparked interest and some concern within the international space community.
The two satellites engaged in rendezvous and proximity operations during the first half of the year, culminating in an apparent docking around late June or early July. Satellite-tracking analyses indicated that the pair became virtually indistinguishable from ground observations at this time, suggesting the commencement of planned refueling tests. Following this, the docked satellites performed fuel-intensive orbital plane change maneuvers, reducing their orbital inclination.
On November 29, S2a systems, a Swiss company specializing in optical space surveillance, observed the separation of the two satellites in geosynchronous orbit, near the geostationary belt (GEO) at an altitude of 35,786 km. The orbits of the satellites are now inclined by 4.6 degrees with respect to GEO. This separation might indicate the successful completion of a potentially world-first refueling operation in GEO.
Despite the evidence, neither China’s space authorities nor the satellites’ manufacturer have released any official statements about the mission since the launch of Shijian-25 in January. The lack of transparency surrounding the mission raises concerns. While on-orbit servicing has civilian applications, the ability to rendezvous, dock, and manipulate objects in GEO inherently carries military implications, especially considering the presence of communications, weather monitoring, remote sensing, and missile early warning satellites in GEO.
Further monitoring of the satellites' activities, especially the older Shijian-21, and any changes in their orbits could provide valuable insights into the success of the apparent refueling test operations. A successful test would significantly advance China's space capabilities. On-orbit servicing, encompassing refueling, orbit adjustment, and repositioning, offers a means to extend the lifespan or repurpose strategic spacecraft such as communications satellites.
Integrity ISR assessments in August noted that the orbital plane changes placed SJ-21 and SJ-25 in a similar inclination to other Chinese GEO satellites, including TJS-11 and TJS-19. These technology demonstration satellites were developed by the Shanghai Academy of Spaceflight Technology (SAST), the same institution responsible for developing SJ-21 and SJ-25. Yaogan-41, a military optical remote sensing satellite, and Shijian-23, another experimental satellite developed by SAST, also occupy similar orbits, potentially serving as future targets for RPO and refueling tests.
Other observers have highlighted that the inclination of Shijian-21 and Shijian-25 closely matches that of some U.S. satellites, suggesting the possibility of future close approaches. This raises the prospect of increased "cat and mouse" activities in GEO involving Chinese, American, and Russian satellites.
According to SAST, Shijian-25, launched in January, was designed to test on-orbit refueling and mission extension technologies in GEO. Shijian-21, launched in October 2021, notably towed a defunct Beidou-2 G2 navigation satellite into a graveyard orbit above GEO as its primary mission.
On-orbit servicing and refueling capabilities hold the potential to reduce costs, improve the sustainability of space operations, and minimize debris. The People’s Liberation Army is also reportedly developing technology and training tools for on-orbit satellite refueling for both peacetime and wartime scenarios.