An asteroid deflector
An asteroid deflector was a device used to deflect asteroids.
The Preservers' obelisk on the planet Amerind was one example of such a device. (TOS: "The Paradise Syndrome")
When a large soap bar asteroid from the Bathnar Nebula was on a collision course for their planet, JR, Roo, and Sprocket used their asteroid deflector to beam a Romulan rubber ducky into the path of the asteroid. It simply bounced off of it. However, when they fired a tank of water at it, the asteroid was reduced to soap bubbles. (SCO: "Asteroid Blasters, Part 1")
On another occasion when their planet was in peril from an asteroid collision, JR, Roo, and Sprocket used the asteroid deflector once again in an attempt to stop it. This time, it was a meatball asteroid from the Noodle Nebula. They first fired a stop sign at it, but the asteroid didn't stop. Afterwards, they shot a fork at it, which cut the asteroid into tiny, meaty bits. (SCO: "Asteroid Blasters, Part 2")