In the summer of 2014, a search conducted with the Hubble Space Telescope identified 5 small Kuiper Belt objects as potential second destinations for the New Horizons spacecraft after its flyby of Pluto, which would take place on July 14, 2015. This field of candidate objects was subsequently winnowed to 3 and then 2. On August 28, 2015, NASA announced that it had selected one of these objects, formally designated 2014 MU69 and informally known as PT1 (for Potential Target 1), as the potential next destination of New Horizons, subject to a detailed assessment before official approval of the mission extension by NASA. New Horizons will perform a series of 4 maneuvers in late October and early November 2015 that will set it on course to fly by PT1 about January 1, 2019, nearly 13 years after the launch of the space probe. New Horizons will then be 43.4 astronomical units (au) from the Sun, about 11 au (1.6 × 109 km or 1.0 × 109 mi) further from the Sun than it was during the Pluto flyby. See also: Hubble Space Telescope; Kuiper Belt; New Horizons at Pluto; Pluto
PT1 is 30–45 km (19–28 mi) in diameter. It has an approximately circular orbit, close to the plane of the ecliptic. This is in contrast to Pluto, which has an inclined and elliptical orbit, and is locked in resonance with Neptune, such that Pluto orbits the Sun twice for every three times that Neptune does so. Bodies like Pluto are termed resonant Kuiper Belt objects, and their orbits are believed to have been changed as Neptune migrated outward in the early solar system. However, bodies like PT1, which are termed classical Kuiper Belt objects, are believed to have been unaffected by this migration, and are thought to be primordial planetesimals that have managed to preserve the original orbits that they acquired during the formation of the solar system. Unlike asteroids, such bodies have been heated only slightly by the Sun, and are thought to represent well-preserved deep-freeze samples of what the outer solar system was like following its birth 4.6 × 109 years ago. The observation of such an object by New Horizons thus provides a unique opportunity to gain new insights into the formation of the solar system. See also: Planet; Solar system