In a further attempt to make the operation of the bomber force safer, attempts were made to develop stand-off weapons, with which capability the bombers would not have to penetrate Soviet airspace. However, efforts to do so had only limited success. The first attempt involved the Blue Steel missile (in service: 1963–1970). It worked, but its range meant that bombers still had to enter Soviet airspace. Longer-range systems were developed, but failed and/or were cancelled. This fate befell the Mark 2 of the Blue Steel, its replacement, the American Skybolt ALBM and the ground-based Blue Streak programme.