Supersonic air travel once was considered the transportation of the future. But the Concorde—the supersonic jet that flew at speeds of over 1,300 m.p.h. from 1969 to 2003—was grounded in part because of noise and environmental issues. Now, supersonic travel may be back, thanks to the XB-1 supersonic demonstrator, a 71-ft.-long, faster-than-sound jet developed by Denver-based Boom Supersonic. The jet, which tackles the Concorde’s problems with cleaner-burning fuel and noise-mitigation technology, can achieve speeds of Mach 1.7—allowing it to cover the distance from New York to London in 3.5 hours instead of six. What’s more, it can make that journey relying on 100% sustainable aviation fuel—an alternative to fossil-based fuels developed to be net-zero carbon. The company hopes to be flying full-size planes that can carry up to 88 passengers by 2029. —Jeffrey Kluger