In January China was observed using its new SJ-21 mobile satellite, equipped with a robotic arm, to tow a dead navigation satellite from a stationary geosynchronous orbit about 35,000 kilometers from earth. The defunct satellite was towed to a graveyard orbit several hundred kilometers away. This keeps choice geosynchronous orbit locations available for active satellites. Those in lower orbits can be towed closer to earth where they will accelerate towards earth and burn up in the atmosphere. Current satellites are supposed to be equipped with a propulsion mechanism that will move them to that lower orbit once the satellite is no longer operational.
More mobile satellites with robotic arms are also a welcome new development because they can be used to inspect satellites in trouble and help diagnose the problem and perhaps fix it. Chinese and Russian satellites similar to SJ-21 have already been spotted getting close to satellites belonging to other nations and, in some cases, apparently practicing disabling them. That was not unexpected.
What inspired the concept of repairing satellites in orbit was the American Space Shuttle program, which put a manned spacecraft into orbit where it could maneuver and perform tasks like inspecting and repairing satellites in orbit. In some cases, a small enough satellite could be brought back to earth for repairs and upgrades and launched again. The Space Shuttle program flew 133 manned missions between 1981 and 2011, with two shuttles lost in fatal accidents. That was enough to end the program, which was already in trouble because of the cost, about $100 million per launch.
Even before the Space Shuttle program ended NASA and the Air Force began a program of developing satellites that could do the same work. The NASA program began in 2007 and continues. The Chinese SJ-21 was not a surprise, just China catching up. Another solution is also about to fly as a Space Plane aircraft similar to the Space Shuttle that can take off like an aircraft and, under its own power, reach orbit. There the crew can inspect and repair satellites in lower 0rbits. The Space Plane then leaves orbit and lands like the Space Shuttle did.
Technically it is illegal to use satellite repair satellites or space planes for offensive missions. No one believes that agreement will survive the start of a war and even before then, inspecting other nation’s satellites is not a friendly act and may cause some escalation. There is already work underway to equip high-value satellites with defensive systems.