The Enterprise's 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 Montgomery Scott'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 1,593 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 conjectured 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 radiation, 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 had been 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")