The race to decarbonize flight just picked up speed. Airbus and CFM International said this week they’ll collaborate on tests of an aircraft engine fueled by hydrogen. As races go, this is a long-distance one. The partners will go to work immediately, with the aim of performing the first tests around the middle of this decade. This demonstration could pave the way for an aircraft that, if all goes according to plan, would carry passengers in the mid-2030s while producing no carbon emissions during flight.