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.

So this is how the war in space might begin: not with a bang but a clank. On April 15, more than 450 miles above Earth, an experimental NASA spacecraft called DART (Demonstration of Autonomous Rendezvous Technology) fired its thrusters and closed in on a deactivated U.S. military communications satellite–and then gently bumped into it.