As the anniversary of their launch approaches, the two Voyager spacecrafts' last mission is to indicate to scientists where the edge of the solar system is. The two Voyager spacecrafts, which were launched on 20 August and 5 September 1977, were sent to return images and data on Saturn and Jupiter, with Voyager 2 continuing to Uranus and Neptune. But now they are heading for the edge of the solar system, or heliopause. Scientists on Earth are eager to find out where this begins or if, as they expect, it is a moveable object which changes with the strength of the solar winds. It has been estimated that this information will become clear in around three or four years time. Scientists are aware that they need this feedback from the Voyagers before 2020, when the spacecraft are expected to run out of electric power. Voyager 1 is 7.9 billion miles form Earth and is travelling at over 38,000 miles an hour and Voyager 2 is 6.3 billion miles from Earth with a speed of 35,000 miles an hour. They both weigh one ton. Information relaying from and to the spacecraft takes 23 hours for a round trip communication with Voyager 1 and 18 hours for Voyager 2. Although the Voyagers were not the first spacecraft to send back photos of the outer planets - Pioneer 11 did this in 1979 - they are regarded as having provided the most complete information. Bradford Smith, who led the Voyager photographic interpretation team, described the Voyager mission as the 'the most successful mission that NASA has carried out'.