Last month, Airbus announced plans for the world’s first zero-emission commercial aircraft models that run on hydrogen and said they could be flying by 2035. Boeing, meanwhile, has done several tests of hydrogen-powered aircraft — both manned and unmanned.

But how far away are hydrogen passenger planes that you could step aboard? Decades.

The idea is, of course, not new. One of the great early catastrophes of flight was due to hydrogen — the Hindenburg dirigible was effectively a giant balloon full of the highly flammable gas that caught fire and fell from the sky in 1937, killing 36. But the hydrogen wasn’t actually the Hindenburg’s fuel — it just provided buoyancy.

These days, there are two ways one can use hydrogen to fly planes. First, you can burn it (on purpose).

“You can run hydrogen through an existing engine,” said Mike McCurdy, a managing director with aviation consulting firm ICF. With a few engine modifications, you could actually use hydrogen as a fuel like petroleum jet fuel and burn it to power an engine.

The other option is to use the gas to fuel a chemical reaction. “You load liquid hydrogen on the airplane and run it through a fuel cell to generate electricity,” McCurdy said. That would power electric engines.

Europe-based Airbus, which did not respond to repeated interview requests, has announced its future concept planes would use both combustion and fuel cell technology.

Both technologies work, but have serious problems to overcome.

Dan Steingart, an associate professor of chemical metallurgy at Columbia University, explained that “in a single hydrogen molecule, there is somewhere between one-fourth and one-eighth the total energy available in a single typical jet fuel molecule.” So, for the same amount of fuel, “planes would go one-fourth the distance.”

Obviously, that’s not ideal. So large aircraft meant to go any substantial distance would need much larger fuel tanks. And those tanks would need to be frozen to minus 420 degrees Fahrenheit — close to absolute zero — to liquefy the hydrogen. That is almost as cold as anything in the universe can get.

“Yes, as far as hydrogen-powered, the planes will definitely look different,” said Bhupendra Khandelwal, an associate professor at the University of Alabama.

In today’s planes, wings are where the fuel is stored, and they are in no way large enough to store the hydrogen that would be needed for a long flight.

So the hydrogen planes of the future could have extra-large fuselages, but more likely they will be what’s called blended wing, in which the planes are shaped like large triangles. This would allow them to store more fuel, “but also reduce fuel consumption to make the aircraft aerodynamics even better,” Khandelwal said.

Whatever the planes look like, they would need infrastructure to produce, transport and store hydrogen. That would be expensive and doesn’t exist at scale yet.

“There are airports starting to look at converting or adding hydrogen-storage capabilities,” ICF’s McCurdy said. ”It’s really focused on the ground fleet right now,” he said, referring to the assortment of earthbound wheeled vehicles one might see crisscrossing the tarmac carrying luggage, fuel or people.

It’ll be sometime in the 2030s that the cost of hydrogen becomes comparable to jet fuel, according to ICF, and the infrastructure to help make that happen would probably develop outside of aviation, said Eliot Lees, ICF’s vice president for clean transportation. “What you’ll see is the infrastructure for hydrogen becoming important for the on-road fleet of heavy- and medium-duty trucks, and from there it will make its way into aviation.”

When hydrogen planes do arrive, they will not arrive en masse. Only certain types of planes will fly on the fuel at first, Lees predicts. “The most immediate focus is on the medium-range market, the regional aircraft market, the turboprops — that’s what can be converted much more quickly,” he said. They will likely use fuel cell technology to power electric propellers, whereas longer-range planes, down the road, would more likely combust the hydrogen for thrust.

Boeing did actually fly a manned hydrogen-powered plane back in 2008. It was a two-seater with extra-long wings and flew for about 20 minutes. The company has flown five hydrogen aircraft since.

Brian Yutko, Boeing’s vice president and chief engineer of sustainability and future mobility, said that at this point, hydrogen is still mostly made using energy from fossil fuels.

As time goes on, that is expected to change. But there’s an easier way to cut total emissions in the near term: “There are technologies called sustainable aviation fuels, or SAF,” he said.

Sustainable aviation fuels are essentially regular jet fuel with minor chemical differences but are recycled from carbon that would have otherwise been released into the atmosphere as pollution. “These can be things like municipal solid waste,” Yutko said. “They can be things like forestry residues or other agriculture residues, a variety of feedstocks for which carbon is already out in our environment.”

It appears that this is what will power the bulk of commercial long-haul jets long before hydrogen ever does, with a comparable amount of greenhouse gas savings.

“We’re able to reduce up to 80% of the total life-cycle carbon emissions by using those various pathways,” Yutko said. SAF may also help reduce global warming caused by contrails — clouds of ice crystals that form around soot particles emitted in plane exhaust.

Boeing’s planes will be capable of running on 100% sustainable aviation fuel by 2030, per Yutko, an increase from the 50% blend permitted today.

The company is investing in scaling up production of such fuels with Dutch firm SkyNRG. They’re still much more expensive than normal jet fuel, so the economics need to be worked out.

Airbus’ goal of hydrogen-powered planes by 2035 notwithstanding, long-range jets powered by zero-emission hydrogen used at scale globally are, by some estimates, 30 or 40 years away.