For a comet rotating around the player, I'd try something like this:


//this function controls the rotation of the comet
function comet_turn()
{
var radius = 100;
var angle = 0;
var speed = 5;
while(my)
{
angle += speed * time_step;//rotate the angle
vec_set(my.x, your.x);
my.x += radius * cos(angle);//figure out x and y position
my.y += radius * sin(angle);
effect(comet_trail, 1, my.x, nullvector);//particle effect
wait(1);
}
}

// create a comet object from the player action: just insert this at the start of your player's action

ent_create("asteroid.mdl", my.x, comet_turn);


hope this works! you'll have to substitute your own files for the comet model and your own particle effect for the rotating comet to leave a trail.