HELSINKI — A classified satellite launched by China in June is operating in a creative orbit, giving it a unique vantage point over the geostationary belt.
Shijian-31 launched on a Long March 3B June 16 from Xichang spaceport, with navigation warnings and the launch vehicle type suggesting an insertion into geostationary transfer orbit. The satellite was later tracked in a specialized Molniya orbit, joining China’s two Shiyan-10 satellites and TJS-13 and TJS-21. However, the concept of a traditional Molniya orbit has been applied by China in a manner never seen before, according to outside analyses.
Analysis by Integrity ISR and a recent followup by space situational awareness (SSA) company ComSpOC show a utilization of the Molniya orbit concept to potentially provide a unique view of activities in geostationary orbit (GEO).
Molniya orbits use an inclination of 63.4 degrees to prevent a drifting of a satellite’s argument of perigee, caused by Earth’s asymmetric mass distribution. Traditionally, this inclined and highly elliptical orbit has been used to maintain an apogee, or farthest point from Earth, over the northern hemisphere, affording a satellite a long viewing time over an area of interest without the costly need to burn fuel to correct its orbit to maintain its vantage point. This means a pair or more of satellites can provide constant visibility over the northern hemisphere, making the orbit useful for high latitude communications and missile early warning. China’s aforementioned Shiyan and TJS satellites could, for example, detect telltale heat signatures over the Arctic.
Shijian-31, however, is using a Molniya type orbit to reach apogee over the equator, allowing it to perform sweeps of GEO. Integrity ISR first flagged Shijian-31 in a Molniya orbit in a June 26 post, using orbital data from SpaceTrack. ISR explained that the satellite is in a unique orbit, allowing Shijian-31 to loiter over the equator and monitor satellites orbiting in GEO, which is situated at around 35,786 km above the equator.
While satellites in Molniya orbits usually orbit every 12 hours, Shijian-31 orbits once every 12.5 hours, meaning it does not pass the same point of longitude every orbit. Instead, each swing sees Shijian-31 shift approximately 15.7 degrees in longitude. This means Shijian-31 can sweep the entire GEO belt around Earth roughly once every 23 days, though its apogee (approximately 39,900 km) takes it a few thousand kilometers beyond the GEO belt
ComSpOC also noted the activities of Shijian-31 in a July 21 post on X, describing it as a “satellite with an orbit we have never seen before.” The post includes a video illustrating Shijian-31’s orbit, also showing how it sweeps around GEO over time, gaining a unique view down on the entire GEO belt.
Shijian-31 is nominally part of a broad series of often classified experimental satellites. The China Aerospace Science and Technology Corporation (CASC) provided no images of the satellite, describing it tersely as “mainly used for space environment exploration,” in keeping with many satellites in the series. Shijian satellites have also been used for pioneering refueling tests and space situational awareness in GEO.
Integrity ISR states that it may be extremely challenging to get a good glimpse of Shijian-31 and ascertain its payloads and capabilities. Its perigee of just under 2,000 km is well above most low Earth orbit remote sensing satellites, and it passes thousands of kilometers away from spacecraft such as the U.S. GSSAP series in GEO. At the same time, if China wanted to intersect the GEO belt, this could be done with a relatively small fuel cost. A relatively small retrograde burn at perigee, bringing a velocity change of 50 meters per second, would “lower apogee to roughly 36,671 km, resulting in SJ-31 passing directly through the GEO neighborhood as it completes its orbit.”
“With SJ-31, Chinese space operators have displayed orbital mechanics mastery and more than a little creativity,” the Integrity ISR post stated.
Victoria Samson of the Secure World Foundation told SpaceNews that the mission is “pretty unusual” and hard to characterize with limited information. “SJ-31 has only been up in orbit for about a month so perhaps as time goes on, more details will be revealed via its behavior or activities,” Samson said.
The mission could be part of a “larger trend toward inspector satellites and an increased interest in having strong situational awareness of activities and hardware in GEO,” Samson added.
The U.S. has the GSSAP series of inspector satellites, while Russia has the Luch 2 satellite operating in GEO, following the demise of the original Luch (Olymp) satellite in January. China has Shijian satellites thought to be SSA spacecraft in GEO, and has earlier studied creative orbits to boost its GEO awareness, such as looping a satellite around the moon to be able to insert it into a retrograde geosynchronous orbit.