Adapting Speed Particles Tutorial

I just watch the Video Copilot tutorial on Speed particles,
http://www.videocopilot.net/tutorial.html?id=114
and am wondering if the expression can be adapted for position values, and not the speed.

The example scene would be a meteor flying towards a laser wall and blowing up on impact; ie, particles are emitted when the meteor reaches the X position of the laser line and they continue at the meteor's velocity.

The expression Andrew used for parenting particular's emitter to the speed of the example text was:

S=thisComp.layer("text").transform.position.speed;

if(S>500){
500;
}else{
0;
}

I tried adapting it to:

S=thisComp.layer("Meteor").transform.position;

if (S=thisComp.layer("Laser").transform.position[0]){
500;
}else{
0;
}

But instead of 500 particles being emitted when the "Meteor" reaches the "Laser"'s X position, particles are being emitted all the time, but there's no warning that the expression is faulty.

At this point, I know I can just set a keyframe for the emitter to emit when it reaches the spot I want, but now I'm curious if it can be done through expressions in a more automated way.

Thanks
- S