In a few years, it might be possible to fly from Washington, D.C., to Paris in four hours — instead of eight — or from San Francisco to Tokyo in just six hours aboard a new crop of supersonic jets.

Why it matters: High-speed air travel promises to shrink the planet, putting far-away vacation destinations within closer reach and enabling business travelers to attend meetings on another continent and return the same day.

The big picture: Affordable, sustainable, high-speed flight has the potential to revolutionize commercial air travel, just as the jet engine did in the late 1950s.

• Yes, but: Engineers must first solve technical, business and environmental challenges that doomed previous efforts at supersonic travel, most notably the transatlantic Concorde.

Flashback: British Airways and Air France flew the Concorde from 1976 to 2003 on international routes like New York to London in under three hours.

• But the "great white bird" was terrible for the environment and couldn't make a profit, even with round-trip fares averaging $12,000.

Fast forward: A handful of startups hopes a fresh approach — lightweight materials, efficient engine technologies and cleaner fuels — will make supersonic jets much cheaper to operate and thus economically viable for everyday travel.

Boeing-backed Aerion is developing a supersonic business jet that will begin production in Florida in 2023 and should be ready for customer delivery by 2027.

• NetJets, Warren Buffett's fractional jet company, this week ordered 20 of the planes for $120 million each.
• Also this week, Aerion teased a concept for its next plane: a larger 50-seat commercial airliner that would fly at Mach 4-plus speed — more than four times the speed of sound — "enabling flight between LA and Tokyo in less than three hours."

Boom Supersonic, backed by Japan Airlines, American Express Ventures, Emerson Collective and others, will begin production of its Overture supersonic aircraft in 2023.

• It will seat 65–88 people and begin commercial flights in 2029, founder and CEO Blake Scholl tells Axios.
• His goal, he says, is "four hours to anywhere, for $100." That's a long way off, but the company aims to start by offering fares comparable to today's business class.
• A handful of other startups, including Hermeus and Spike Aerospace, are also developing supersonic jets.

Sustainability, not just speed, is a core goal. The new planes will be less harmful to the environment, the companies say, because they're designed to run on sustainable aviation fuels (SAFs).

• SAFs are made from sustainable feedstocks like household solid waste, algae or used cooking oil, and producers like BP claim they can produce up to 80% lower carbon emissions than traditional jet fuel.
• But SAFs are in short supply, so production needs to scale significantly in the next few years to meet the demand for cleaner-flying planes.

Reality check: For all the promising developments, supersonic commercial flights are still limited by regulatory hurdles, cautions Iain Boyd, an aerospace engineering professor at the University of Colorado.

• "This is one of the very few high technology things where it feels like we have gone backward," he said.
• "You used to be able to fly supersonically if you had a lot of money. Today, you cannot ... We've been flying the same speeds for 50–60 years."

The biggest problem, he says, is the noise associated with supersonic flight, on both takeoff and landing and at 60,000 feet.

• Planes that fly past the speed of sound — about 760 mph — create shock waves that hit the ground with a startling thud, or "sonic boom."
• That's why the FAA won't let them fly over land, limiting them to transoceanic routes.

What to watch: NASA researchers, along with Lockheed Martin, are designing a new supersonic plane that turns the sonic boom into "a gentle thump."

• "It sounds more like a car door slamming, or maybe distant thunder," said NASA project manager Peter Coen.
• NASA plans to fly the plane over select U.S. communities to see how people react. That data will then help regulators set noise standards for supersonic flight.

Editor's note: Emerson Collective is an Axios investor.