I'd suppose your right 150 feet is not much of a distance but the balloon is rather slow and 150 ft is just my threshold. I likely wont even need anything closer than 300 ft or 400 even.
this is all assuming that the balloon is flying below...
the drone could fall off. I don't know how strong the balloon is but I'd figure if the drone hit the balloon and its big and strong enough it could damage and make a hole
oh yeah that's definitely illegal. unless if they had airspace authorization they could fly above that. LANDING ON THE BALLOON! I'd be surprised if they didn't receive any backlash from that I mean dang that's some skill but it could've ended badly.
Call the FAA. Ask them.
And if they say you can do it, contact the people who own the balloons and let them know you'll be doing it.
They might be delighted to see your video.
Remember, if you're doing it purely for entertainment, you don't...