In the late 1970s a spacecraft may be sent on a 'tour' of four of the outer planets - Jupiter, Saturn, Uranus, and Neptune - using the gravity and orbital speed of each to boost it on to the next, and sending back photographs and scientific information. Patrick Moore discusses the chances of success for this ambitious project with Iain Nicolson, a British astronomer, and with Dr W.H. Pickering, head of America's unmanned scientific space programme.