New Horizons isn't going to get much of a rest. Following on from its historic flyby of Pluto on July 14, NASA has selected the next potential destination for the unmanned spacecraft – a planetoid called 2014 MU69 that lies a billion miles beyond Pluto's orbit. The space probe will take over three years to reach this frozen remnant of the Solar System's earliest years.

Technically a small Kuiper Belt object, 2014 MU69 has been given the nickname "PT1" or "Potential Target 1" by the New Horizons team. According to NASA, it's less than 30 mi (45 km) in diameter and 10 times larger and 1,000 times more massive than a typical comet. It's this small size that makes it of interest to scientists, who regard PT1 as a leftover building block of the Solar System that's been preserved in the cold darkness of deep space and hasn't changed much in 4.6 billion years.

PT1 was one of several potential targets under consideration by the New Horizons team after a search going back to 2011. Earth-based telescopes weren't up to the job of choosing a candidate that was both interesting and within New Horizons' range, so the Hubble Space Telescope was used. It uncovered five candidates, which were then reduced to two, and now one.

Despite the fact that New Horizons won't reach PT1 until January 1, 2019, the team had to choose it early as a destination because the four orbital course corrections needed to reach it must be executed in late October and early November. Any later would require using up too much fuel to ensure the success of the mission.

New Horizons was launched on January 19, 2006, from Launch Complex 41 at Cape Canaveral Air Force Station, Florida atop a Lockheed Martin Atlas V-551 rocket. It's the fastest spacecraft ever launched, reaching the distance of lunar orbit in only nine hours.

After a February 2007 gravity assist from the planet Jupiter, it reached a speed of 83,600 km/h (52,000 mph) relative to the Sun. On July 14 this year, following a nine and a half year voyage through deep space, it passed Pluto at a distance of 7,750 mi (12,500 km) and a speed of 14 km/sec (31,000 mph), making it the final classical planet to be visited by a spacecraft. It is currently 3 billion mi (4.9 billion km) from Earth and will spend the next 15 months transmitting home the data it recorded during the flyby.

Like most deep space NASA missions, New Horizons was planned with the possibility of an extended mission after the main objective was achieved. It carries extra hydrazine fuel, its nuclear power unit is designed to continuing operating for many years, its communications system is designed to work well beyond the Plutonian orbit, and its instruments need less light than Pluto receives.

NASA points out that the decision to visit PT1 is still tentative and must gain formal approval even if the orbital maneuvers are carried out. By next year, the New Horizons team must submit a formal proposal, which must pass an independent review panel. If it gets the green light, the team can begin making preparations for the extended science phase of the mission.

"2014 MU69 is a great choice because it is just the kind of ancient KBO, formed where it orbits now, that the Decadal Survey desired us to fly by," says New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute (SwRI) in Boulder, Colorado. "Moreover, this KBO costs less fuel to reach [than other candidate targets], leaving more fuel for the flyby, for ancillary science, and greater fuel reserves to protect against the unforeseen."

View gallery - 3 images