A trio of mysterious Russian government satellites startled space experts when, shortly after blasting into low orbit between 2013 and 2015, they began dramatically changing their orbits, demonstrating a rare degree of maneuverability for small spacecraft.

Now after being idle for a year or more, two of the mystery-sats are on the move again. On April 20, 2017, one of them reportedly shaved hundreds of meters off its orbit in order to zoom within 1,200 meters of a big chunk of a defunct Chinese weather satellite that China smashed in a controversial 2007 test of an anti-satellite rocket.

By orbital standards, that's pretty close.

The Russian spacecrafts' impressive maneuvers have got observers scratching their heads. No one outside of the Russian government -- and probably the U.S. military -- seems to know for sure what the satellites are for.

Experts say the Russian satellites could be technology-demonstrators. They might also be precursors to orbital weapons.

Either way, the nimble spacecraft are "intriguing," Dr. Laura Grego, a space expert with the Massachusetts-based Union of Concerned Scientists, told The Daily Beast.

Those who know for sure ... aren't talking. The Russian space agency didn't respond to an email seeking comment -- and has barely mentioned the mystery craft at all since late 2014. The U.S. Air Force, which tracks all the world's satellites, issued the same boilerplate statement it released the first time the Russian satellites started moving around.

"U.S. Strategic Command's ... space component tracks Kosmos-2504 and -2499 ... as well as more than 23,000 man-made, earth-orbiting objects every day," Capt. Nicholas Mercurio, an Air Force spokesman, told The Daily Beast.

The original trio of satellites -- known by their Russian code names Kosmos-2491, Kosmos-2499 and Kosmos-2504 -- seemed to be maneuvering toward specific targets in space when they first began their orbital dances.

Several times in 2014, 2015, and 2016, the roughly 200-pound satellites moved closer and closer to spent stages of the rockets that had delivered them into orbit, approaching to within a few dozen feet of the old booster shells.

That implied that the Kosmos triplets could be inspection satellites capable of closely matching the orbit of another spacecraft and scanning it, or even physically interacting with it in order to repair, modify or dismantle it. The Pentagon calls these "rendezvous and proximity operations."

Indeed, Anatoly Zak, an independent expert on Russian spacecraft, claimed that the mystery-sats might match the dimensions and performance of a known Russian inspection satellite called Yubileiny.

The possible war-time applications of inspection satellites are obvious. “You can probably equip them with lasers, maybe put some explosives on them,” Zak said of the Kosmos triplets in 2015. “If [one] comes very close to some military satellite, it probably can do some harm.”

To be clear, inspection satellites are not new. The U.S. government operates several of them. But secret inspection satellites are rare and potentially problematic, considering how easily they could be converted into weapons.

Moscow took pains to obscure at least one of the Kosmos mystery-sats. The Russian space agency launched Kosmos-2491 aboard a single rocket that also carried three, non-maneuvering communications satellites.

The Russians announced the comms-sats in advance. They didn't mention Kosmos-2491 ... until after foreign and independent spacewatchers saw Kosmos-2491, which they had initially mistaken for debris, move under its own power.

In a brief statement in December 2014, Russian space agency chief Oleg Ostapenko insisted that the maneuverable spacecraft were peaceful in purpose and not, as some feared, “killer satellites.”

Kosmos-2491 has apparently been inactive since late 2014. Kosmos-2499 executed dramatic maneuvers in the spring of 2016 then fell idle until March 2017. Kosmos-2504 orbited like dead weight for nearly two years since performing a close pass on a spent rocket stage in October 2015. Around the same time Kosmos-2499 came back to life, Kosmos-2504 began moving closer to that chunk of old Chinese weather satellite.

The periods of idleness are not insignificant, Grego said, while stressing that she had not verified the details of the Russian satellites' recent movements. "I do find very interesting that the satellite would go dormant for two years and then come back to life to maneuver. That could help the satellite be stealthy."

"One strategy to keep maneuvering satellites stealthy is to pretend they are debris -- i.e., not to have them maneuver at all at first, and then come to life later. To be confident this works, you might want to be able to test if your equipment works after being idle for months or years."

Despite the weirdness of the Kosmos crafts' behavior, Brian Weeden, a space expert at the Secure World Foundation in Colorado, cautioned against assuming the mystery-sats are, or will ever be, weapons.

"In most cases, it's far easier to jam a satellite's communications or hit it with a missile than try and do some sort of destructive co-orbital rendezvous," Weeden told The Daily Beast. "The capability to do rendezvous and proximity operations ... has a whole bunch of applications -- civil, commercial and military."

It's worth noting that one of America's own highly-maneuverable spacecraft, the X-37B robotic mini-shuttle, returned to Earth in early May 2017 after spending 718 days in low orbit -- a record for the type.

The Air Force, which operates the two X-37Bs, has always insisted that the maneuverable mini-shuttles are strictly experimental -- but has otherwise declined to discuss the crafts' missions. Much like the Russians with their patient, maneuverable Kosmos sats.