What would it take for a balloon to reach an altitude of 65 km (214,000 ft) above the Earth's sea level?

Question

Kind of a follow-up question to Could a helium balloon on Mars and on Triton float at air pressures lower than it could on Earth due to the bodies' low gravities?. As stated in the linked question, the highest a helium balloon has gone above Earth is about 53 km (actually higher: 176,200 ft). For a balloon to reach the mesosphere, it needs to have very much volume, a tiny enough payload and must be of a certain material. Is it possible from a realistic point of view to reach 65 km or 214,000 feet from the Earth, and what requirements would such a balloon have to meet?

This question is about pushing the limits of balloon flight closer to the Kármán line and the threshold of space. Questions about balloons and balloon flight are on-topic here since they are routinely used as platforms for space exploration:

• near space vacuum balloon
• What cosmic ray sensor is attached to this Balloon?
• High pressure Helium tanks for the Vega balloon experiments
• Scientifically useful measurements from low-budget high-altitude balloon flights
• Around Venus in 8 days by Balloon?
• Maximum height for a Mars Balloon?

and sometimes discussed as launch platforms:

• Launching a rocket from a weather balloon for a very small payload

and for science education:

• Power supply for (30km stratosphere) school's High Altitude Balloon project

and for space suit development:

• Indian Manned Flight to 'Near Space' by ISRO?

Answer 1

High-altitude ballooning is kind of a gray area as far as space exploration goes, because they can't leave the atmosphere, but they do go high enough to experience space-like conditions (e.g. the pressure is blood-boiling low, and it's darn hot on the sunlit side and cold on the shaded side). Balloon experiments measure things that you might normally associate with satellites, and were used to test equipment and human performance in preparation for space flight (think Project Manhigh). Before Mercury, the US space program was pretty much high-altitude balloons.

NASA hit a recent record on August 17, 2018, at 48.5 km, which is below the limit for "space", but pretty darn high. That was attributed to its "enormous size and delicate skin." That pretty much sums up high-altitude balloons. This one was a polyethylene balloon of 60 million cubic feet, 20 acres of material, with a thickness that is a "little less than the thickness of kitchen plastic wrap."

The weight of the payload matters, but the weight divided by volume of the balloon is the theoretical limiter when you can assume a payload with zero mass. That means you have to make the balloon bigger, and make the material thinner, but still able to handle the stresses of carrying the payload AND taking wind shear, at low temperatures, without ripping. It's also probably worth noting that as the balloon ascends and atmospheric pressure drops, the helium gas inside will expand and could rip the skin if not regulated.

Edit: I had said,

Air density at 48.5 km is around 0.0010 kg/m³, at 60 km it is around 0.00031 kg/m³. So increase the size of the balloon by about a factor of 3.3.

But that didn't factor in the increase in material. Volume increases as radius cubed while surface area, and material, increases as radius squared, so it is an achievable extrapolation, but I don't know the areal density of the polyethylene off-hand. The cited article doesn't give it, or the weight.

Answer 2

Your flotation scales with the cubic of radius while surface area and weight scales with the square of radius. In other words, you need a big balloon.

Surprisingly, we have become incredibly good at building arbitrarily big balloons. Notably, relatively recently, NASA Big 60 (NASA announcement here).

the scientific balloon reached a peak altitude of 161,000 feet (49 kilometers), and with a volume of 60 million cubic feet (1.7 million cubic meters), was the largest balloon ever launched successfully.

At 50km, air is about one gram per cubic meter, so a 1.7M cubic meter balloon translates to about 1700kg flotation allowing it to carry a 690kg payload. It's quite possible it will go 15km higher if you reduce the payload.