If future testing goes well and "if my wife allows, " Mr. Branson joked, he and his two children plan to be among the first six voyagers to ride the maiden flight of SpaceShipTwo to the edge of space, or about 60 miles above the Earth's surface, at a maximum speed of about 2, 500 miles per hour.

WSJ: Virgin Galactic Spacecraft Tests Its Rockets in Flight

Dragon's rendezvous systems had never been tested before in the harsh conditions of space, with the station and the capsule racing around the Earth in tandem at about 17, 000 miles an hour.

WSJ: SpaceX Craft Rehearses Moves

Organisers insist the aim of Earth Hour is to show that communities care passionately about climate change and want to keep up the pressure on governments to act decisively.

BBC: Cities dim lights for environment