Voyager 2 is a space probe launched by NASA on August 20, 1977, to study the outer planets. A part of the Voyager program, it was launched 16 days before its twin, "Voyager 1", on a trajectory that took longer to reach Jupiter and Saturn but enabled further encounters with Uranus and Neptune. It is the only spacecraft to have visited either of these two ice giant planets. "Voyager 2" was the fourth of five spacecraft to achieve the Solar escape velocity, which allowed it to leave the Solar System.