Fifty years ago, on 15 September 1971, a ship named the Greenpeace set out to confront and stop US nuclear weapons testing at Amchitka, one of the Aleutian Islands in south-west Alaska. Two years later a small boat called the Vega, crewed by David McTaggart, Ann-Marie Horne, Mary Horne and Nigel Ingram sailed into the French nuclear test site area at Moruroa, French Polynesia in the southern Pacific Ocean. Photographers had been using their images for years to publicise situations around the world. But Greenpeace was a young organisation pioneering a new kind of activism: this was the moment they began to realise that capturing images of what they were…