Sorry if I wasn't clear, but I brought up the question of floating with respect to nitrogen/CO2 atmospheres (thinking Titanlike or Venuslike) not hydrogen.

It's a moot point, but I still want to point out that "a gas lighter than hydrogen" is a thing: it is simply hydrogen at a higher temperature, i.e. a hot-hydrogen balloon, analogous to a hot-air balloon.

I do not want to ride in a hot hydrogen balloon :)

Why not? Hydrogen inside a hydrogen atmosphere is perfectly safe.

It's hydrogen inside an oxygen atmosphere that's the problem.

(So on Jupiter, you wouldn't want to ride in an oxygen balloon, ie you wouldn't want to ride in a hot air balloon there.)

Well, maybe if there's no oxygen nearby.

If you have a planet with a hydrogen atmosphere, it's a pretty good bet there is no (free) oxygen nearby.

Fair point

How do you "fill something with vacuum"?

With a vacuum pump ;)

> No. To float you would need a gas lighter than hydrogen, which isnt a thing.

The atmosphere gets denser further down. You just need a negative pressure vessel, or to heat the hydrogen, like a hot air balloon. At 1 (Earth) atmospheric pressure the gravity of most Gas giants is quite low.