Two Chinese satellites are about to rendezvous in geostationary orbit, docking with one another for an experimental demonstration that could extend the longevity of spacecraft.

Shijian-21 and Shijian-25 performed a series of maneuvers this past weekend, reaching a close distance of just two degrees of longitude from one another on Monday, according to space situational awareness company COMSPOC. The pair are set to meet on Wednesday for a docking procedure for China’s first on-orbit refueling test in geostationary orbit (GEO), acting like a space gas station to extend the lifespan of satellites. China’s satellites aren’t alone, however, as two U.S. surveillance spacecraft are lurking nearby to watch it all happen, with the Pentagon fearing the technology could represent a significant milestone in China’s space ambitions.

The two satellites were developed by the Shanghai Academy of Spaceflight Technology, a subsidiary of the China Aerospace Science and Technology Corporation. Shijian-25 launched in January while Shijian-21 lifted off earlier in 2021. Around a year after its launch, Shijian-21 docked with a defunct Chinese satellite, the Beidou-2 G2 navigation satellite, and towed it to the graveyard orbit above GEO, SpaceNews reported at the time.

Since then, Shijian-21 appeared to have run out of fuel—until it recently sprang back to life with maneuvers of its own to get closer to Shijian-25. It’s likely the target of the in-orbit refueling test, in which Shijian-25 will attempt to transfer about 313 pounds (142 kilograms) of hydrazine to extend the satellite’s life by eight years, according to Universe Today.

The refueling test has raised concerns in the U.S. about China’s growing space capabilities, prompting officials to closely monitor the situation. Two surveillance satellites, USA 270 and USA 271, part of the Geosynchronous Space Situational Awareness Program, performed maneuvers of their own to approach the vicinity of the Chinese satellites, according to COMSPOC. “While these U.S. assets aren’t directly involved, their proximity adds complexity to the local SSA picture — and raises strategic awareness concerns,” the company wrote on X.

The U.S. has previously carried out in-orbit refueling tests of its own. In 2019, Northrop Grumman refueled a satellite in space to extend its lifespan by five years, becoming the first company to do so. The U.S. Space Force is also preparing its own set of orbital refueling tests, known as Tetra-5 and Tetra-6, using hardware from Northrop Grumman and other commercial suppliers.

For now, the U.S. will stick to watching its space rival carry out the orbital maneuver from a safe distance.