Voyager 2 is an unmanne interplanetary spacecraft launched on August 20, 1977. Identical in form to its sister Voyager program craft, Voyager 1, Voyager 2 followed a slower trajectory that allowed it to be kept in the ecliptic (the plane of the Solar System) so that it could be sent to Uranus and Neptune by means of gravity assist during the 1981 encounter at Saturn. Because of this trajectory, Voyager 2 could not see the moon Titan up close as its twin had, but the probe did become the first and only spacecraft to travel to Uranus and Neptune, thus completing the Planetary Grand Tour, a rare geometric arrangement of the outer planets that only occurs once every 176 years.