The Kremlin says its nimble new satellites are just for communications. But they look—and act—an awful lot like prototype weapons.

On Christmas Day in 2013, a rocket blasted off from the Russian Federal Space Agency's Plesetsk Cosmodrome, about 500 miles north of Moscow. The 95-foot-tall, 118-ton Rokot booster—an unarmed version of a Cold War nuclear-tipped missile—lanced into low orbit, shedding spent stages as it climbed.

Seventy-five miles above the surface the earth, the Rokot's nose cracked open and its payload spilled out. The rocket carried Rodnik communications satellites, according to Russian officials.

It's customary for Rodnik sats to deploy in threes, but in a notification to the United Nations, Moscow listed four spacecraft inside the Christmas Rokot.

The discrepancy was strange...and got stranger.

Rodnik sats, like most orbital spacecraft, don't have engines and can't move under their own power. So it came as a shock to some observers on the ground—a group including amateur satellite-spotters with radios and telescopes; radar-equipped civilian researchers; and military officials monitoring banks of high-tech sensors—when the Rokot's fourth satellite, designated Kosmos-2491, moved, propelling itself into a slightly different orbit.

Whatever Kosmos-2491 was, it wasn't some innocuous communications satellite. And over the next year and a half, Russia launched two more of the mysterious, maneuvering spacecraft, each time sneaking it into orbit as part of a routine commsat launch.

No one outside the Russian government, and perhaps the Pentagon, knows for sure what Kosmos-2491, -2499, and -2504 are for. But it's clear enough what the three mystery sats could do, in theory. Zipping across orbital planes hundreds of thousands of feet above Earth, the nimble little spacecraft—which are apparently the size of a mini-refrigerator—are able to get really close to other satellites. Close as in a few dozen feet away.

And if they can get that close, the little robots could spy on, hijack, and even destroy other sats.