Balok's cube
While mapping beyond previously charted space, the Enterprise encountered Balok's cube.
The sensors registered the cube as entirely solid, each edge 107 meters long, and massing just under 11,000 metric tons. But neither the sensors nor Scotty's experience were able to explain how the device sensed the ship, moved to block it, and moved to remain in position.
The cube held a position 1593 meters in front of the Enterprise. Kirk elected to play a waiting game. Eighteen hours later, with no change in the cube's status, Kirk convened a briefing. Spock speculated that the device was either a space buoy, or "flypaper" – designed to hold ships in place, or to measure their capabilities. He opined that it would be unwise to appear too weak.
The ship attempted to pull away, using a spiral course. The cube maintained its position, but at half-speed, began to emit dangerous radiation, and started to close on the Enterprise. Kirk first ordered a halt; when the cube continued to close and emit radition, he retreated before it. It maintained its approach, closing even at multi-warp speeds, and emitting increasingly deadly radiation. It was finally destroyed at point blank range by a short burst of phaser fire.
Events that followed revealed that the cube was intended first to hold the ship, and later to threaten it, so that the alien Balok could learn what sort of reaction this would prompt. At one point, Balok used Kirk's destruction of the cube as "proof" that the Federation was a hostile organization; this was a ploy on his part to goad Kirk for the purpose of testing him. (TOS: "The Corbomite Maneuver")
- These buoys were seen again in a non-canon source, the video game Star Trek: Shattered Universe, this time belonging to the mirror universe Balok, used against the ISS Excelsior. They were each easily dispatched by a few shots from the Excelsior fighters.