7 submarines are placed on an x, y grid - from negative infinity to positive infinity. They are located on whole numbered locations. Each submarine starts from a location with a given fixed speed and fixed direction, reaching another point on the grid every second. The submarines continuous simultaneously in the given direction and speed thus reaching potentially infinite points of the grid, at the same time each second, if not interrupted.
You are armed with a unique gun that kills a submarine with its bullets. You are allowed to use the gun every second, when submarine reaches a grid point. The gun is not limited with its range, the accuracy is absolute, and you are armed with infinite bullets.
Devise a strategy by which you will kill all 7 submarines in a finite time.