Locating a Harvester Orb in Modern Warfare 3's new zombies mode is one of the many missions you'll find yourself having to complete if you want to progress the story. These hovering orbs may seem slightly unnerving when you first encounter one, but don't be afraid, they're really just big loot pinatas—shoot them with your gun and see what they've got.
Since the Reaper mission—that requires you to find a Harvester Orb—isn't until Act Two, you've got quite a lot to do first, such as acquiring Cyphered Tablets and the Essence of Aether samples. Still, if you're already up to this mission, here's how to find Harvester Orbs in Call of Duty: Modern Warfare 3.
How to find Harvester Orbs in MWZ
Since Harvester Orbs spawn randomly across the Exclusion Zone, there's no guarantee you can find one in any specific place. However, they are big floating purple orbs that fire out lightning and make a fair bit of noise. You can't see them on the map until you're nearby, when they'll appear as a little labelled orb with a ring around it that looks a bit like Saturn.
I've had the best luck finding these orbs in the more open areas, so if you skirt the outskirts of the map, and the wide open fields of the low threat zone, you'll quickly find one before long. To get a Harvester Orb's loot, you need to shoot it. After the first shot, it'll try to escape and fly away, so chase after it and keep blasting. As you follow, you can pick up the Essence vials it drops if you want, but the real prize is destroying it.
Once it explodes, you can grab its loot from below, which generally includes an Aether Tool, some gear, and sometimes even a weapon. More importantly, it will count towards your completion of the Reaper mission, meaning you can keep progressing.