The visual effect of constantly circling the globe is great and very well done!

I didn't understand the 'must lock to fire' mechanic at all.   Would not have figured it out if not for medicinestorm's comment.   Once I realized how it worked though, I kind of dug the mechanic.  It creates a nice tension as you track the enemy hoping to get a lock before they make it off screen.  A similar result could be achieved by giving the enemies a big health bar that depletes slowly as you shoot it.  That way the player has the satisfaction of being able to shoot whenever they like but they still need to track the enemies for a good bit in order to destroy them.

Looks good! I like the music, but there was no detectable sound effects. The controls are intuitive, but I found it nearly impossible to destroy more than a few opponents. I'm not sure if it is intended, but I found it frustrating that I am not able to fire at all unless I have a lock, which usually takes longer than the opponent is on the screen.

Though I realize this is to encourage me to match velocity with them before I can take them out, I was rarely able to match their speed and heading before they were out of detection range.