Modern air travel is a marvel. It's also a source of endless delay, annoyance and planet-killing greenhouse gases. A proposed hydrogen-powered hypersonic airliner could change all that. The plane is Reaction Engines's A2 concept, a Mach-5 (3,400mph) craft for 300 passengers funded in part by the European Union's Long-Term Advanced Propulsion Concepts and Technologies project (Lapcat). Lapcat wants an airliner that can fly from Brussels to Sydney in less than four hours. If built, the A2 will do just that—without producing a trace of carbon emissions.

When a Concorde jet on its way from Paris to New York crashed on July 25, 2000, killing all 109 people aboard and four on the ground, the event was not simply a tragedy -- it seemed a metaphor for the sorry state of supersonic air travel.