[ODE] Collision objects

[symeg@tcd.ie]

Myself and some other guys are doing a project for college in which we are 
making a multiplayer space combat game. We are using ODE for the physics and 
are well-impressed with it.

I have one question though. Is it possible to insert a geometry into the space 
which will inform you whether objects have collided with it, but not cause any 
response to the collision? I was thinking that it would be a handy way to check 
which objects were going to be effected by an explosion.

Any Suggestions?

-Gordon