Guardian: The wings of the experimental aircraft arch more than 63 metres, the same span as an Airbus A340, but they look frail, supported on the airstrip by wheeled struts. They are covered in a patina of 11,268 photovoltaic cells, which look dark blue in the grey predawn. The four 10-horsepower propellers they power now start to spin silently. Bertrand Piccard, a 55-year-old explorer and psychiatrist, dons his helmet and oxygen mask and completes his final checks. The Solar Impulse quietly taxies forwards....