So the "never have to come down" is ~100 days record which is held by project Loon?

~6 months is the current best, actually. [1] This is a development project; there's no reason to assume that the current record is a hard limit.

[1]https://en.m.wikipedia.org/wiki/Loon_(company)#Timeline

Loon balloons are helium, and helium gas is famously hard to contain, so eventually leaks out and the balloon comes down.

I don't see any way to make a balloon stay up forever unless you have a particle accelerator to manufacture new helium to make up for leakage...

> Loon balloons are helium, and helium gas is famously hard to contain

It's not like there are any better options? The only two practical lifting gases are hydrogen and helium. Hydrogen leaks even more than helium and is flammable, even though it is much cheaper and abundant.

my interest was piqued, but the wikipedia page is actually quite substandard compared to other articles on wikipedia: while it does explain the problem of non SPB and the requirement to have a SPB, it does not state how this is arranged for: are they literally using a much higher pressure? apart from weight issues of stronger sking, so assuming the same weight for a stronger skin, doesn't this cause the balloon to be heavier (since more gas in the same volume) such that it has a lower ceiling? or is that the design tradeoff?

Yes - they have a much lower ceiling. But if you want controllable altitude, that's what you need.

Do you know what materials they use to make these balloons? Weather balloons can go as high as 100,000 feet and then they'd blow up.

AFAIK they only blow up because they're designed that way.

Bursting is not a goal in itself - it is just a side-effect of using cheap and simple latex or rubber balloons.

You fill the balloon so it has slightly positive buoyancy. As it rises, it encounters lower outside air pressure, and expands to match it. Eventually it will rise to an altitude where the pressure is so low, that the latex is stretched beyond its limits and bursts. Typically at about 40km and about 100x expansion in volume.

To prevent bursting, you need a more complex material that will expand many times at first (to allow the balloon to rise to operating height), but will then stop expanding without breaking.

Or perhaps some sort of venting system that engages at a certain altitude or pressure preventing the ballon from over expanding. Like a hot water tank has a pressure release it the tank gets too hot will vent releasing pressure.

Venting does not work. For a given altitude, there is no amount of helium which will make your craft stable unless the volume of your craft is non-linear with pressure (usually done by having something that stretches to a limit, and then cannot stretch further)

Or just use zero pressure balloons.

https://sites.wff.nasa.gov/code820/spb_differences_between_z...

Usually some kind of plastic foil, kind of like mylar.