China’s satellite might have a companion.

The U.S. Space Force has detected a mysterious object orbiting in parallel with China’s new Shijian-21 spacecraft, according to an initial report from SpaceNews.

And, since it might be moving under its own power, we’re still not clear on what it is. But we have several theories about what it might be, from a new space junk device to the latest exhibition of space war tactics.

China’s space junk-cleaning satellite might have a companion

China’s Shijian-21 satellite was launched into space atop a Long March-3B rocket, back on Oct. 23. At the time, China’s state-run news agency Xinhua said its spacecraft “entered the planned orbit successfully”, and would “be mainly used to test and verify space debris mitigation technologies.” There’s not much to parse in this announcement with relevance to the newly-detected object, but China isn’t usually very forthcoming about space endeavors. But on Nov. 3, the U.S. military began to monitor an unidentified object orbiting in parallel with Shijian-21, and the Space Force’s 18th Space Control Squadron categorized the detection as an “apogee kick motor”, dubbing it 2021-094C.

An apogee kick motor is typically used to lift payloads into operational orbits, including geostationary orbits (GEO). When they’re finished with them, satellites sometimes kick their apogee kick motors away, but this is “pretty rare”, and “almost always done by launching to the GEO graveyard, ejecting the motor, and then lowering the payload into GEO proper,” said Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics, in a Gizmodo report. Graveyard orbits are where the lion’s share of space junk resides, where satellites go after they’ve outlived their usefulness. This is typically deliberate, so the risk of in-space collisions is reduced as the volume of blindingly fast bits of debris becomes more abundant in orbital space.

Unkown object’s motion ‘appears to be a deliberate synchronization’

GEO is a special case of orbital trajectories, where satellites assume an orbit that moves in synchrony with the geographical location on Earth below. This makes them appear to “stand still” to observers on the ground, despite how fast they are still moving through space. Satellites are typically installed in these comparatively high orbits to provide telecommunications or weather data for a specific region of Earth. But ejecting an object from a satellite in GEO “is a bad idea and very rare,” added McDowell in the report, since this heightens the risk of a subsequent collision with other satellites and equipment in GEO or lower trajectories.

However, nobody said it had to be an apogee kick motor, and the evidence suggests it’s actually something else. As of writing, it’s “currently unknown whether the object is an [apogee kick motor], an object possibly related to space debris mitigation tests, or part of potential counterspace operation tests,” wrote SpaceNews in their report. “The object could be used to test rendezvous and proximity operations, refueling experiments or manipulation using a robotic arm or other means.” Both Shijian-21 and 2021-094C are still orbiting the Earth roughly 50 miles (80 km) above nominal GEO, “which is well within the band” typically employed to relocate GEO satellites, added McDowell in the report. They’re roughly 37 miles (60 km) apart, and this “appears to be a deliberate synchronization,” said McDowell to Gizmodo, which doesn’t fit the Space Force’s apogee kick motor designation. “If you just ejected and said bye-bye, you’d expect a steadily increasing separation,” he added. And after renewed interest in monitoring space war tactics amid rising tensions between China and the U.S., we can be certain that the superpowers of the world are monitoring this situation very closely.