Inspired by Is one permitted to fly a kite on Shabbat?
That question asks about a situation where there is an eiruv. I assume that the concern is carrying the kite to the place where it will be flown and not whether "flying" is the same as "carrying" (though I may be wrong). It also begs the question if whether the kite strays by dint of its string's length over an area outside the eiruv if that is a problem but I am wondering something different:
Can I walk outside (where there is no eiruv) on Shabbat holding a string attached to a helium balloon?
While I am securing it, I am not keeping it UP and defying gravity (carrying) but holding it down. Is that hotza'ah? If I were to be holding 10 or 15 balloons, then controlling them and keeping them from escaping would take effort, but does that become a melacha?
If the string unties, the balloon flies away (no fixing) but then I am carrying the string, so is the balloon actually keeping me from doing the melacha? If so, can I attach other things to the balloon and "not carry" them because the balloon is doing the work?