Asteroid Shield is an ability for the Space Station Passenger in Plants vs. Zombies: Battle for Neighborville. When activated, Space Station Passenger protects the whole Space Station with an energy shield. The Asteroid Shield has 300 health, and the Space Station (along with its passengers) can still fire whilst the shield is up.
The Asteroid Shield is good for protecting the Space Station. If it starts to lose health, put it up to block attacks. Use it to cover your flanks and rear while the driver of the Space Station is moving. If multiple Space Cadets all use their shield in the same spot, they'll form an impenetrable wall and will be able to deal serious damage.
The Asteroid Shield is large, so it isn't particularly hard to destroy. Peashooters can go into Pea Gatling to destroy it easily, and Sunflowers can use their Sunbeam. Once the shield is destroyed, you can easily attack the Space Station.