Lead scientist Alan Stern of Southwest Research Institute in Boulder, Colorado, expects the New Year's encounter to be riskier and more difficult than the rendezvous with Pluto. The spacecraft is older, the target is smaller, the flyby is closer and the distance from us is greater. New Horizons NASA launched the spacecraft in 2006, and it's about the size of a baby grand piano. It flew past Pluto in 2015, providing the first close-up views of the dwarf planet. With the wildly successful flyby behind them, mission planners won an extension from NASA and set their sights on a destination deep inside the Kuiper Belt. As distant as it is, Pluto is barely in the Kuiper Belt, the so-called Twilight Zone stretching beyond Neptune. Ultima Thule is in the Twilight Zone's heart. The New Horizons spacecraft on its initial Pluto venture. Credit:AP Ultimate Thule

This Kuiper Belt object was discovered by the Hubble Space Telescope in 2014. Officially known as 2014 MU69, it got the nickname Ultima Thule in an online vote. In classic and medieval literature, Thule was the most distant, northernmost place beyond the known world. When New Horizons first glimpsed the rocky iceball in August it was just a dot. Higher quality close-up pictures should be available the day after the flyby. Are we there yet? New Horizons will make its closest approach on the afternoon of January 1, Australian Eastern Daylight Time. The spacecraft will zoom within 3,500 kilometres of Ultima Thule, its seven science instruments going full blast. The coast should be clear: scientists have yet to find any rings or moons around it that could batter the spacecraft. New Horizons hurtles through space at 50,700 kilometres per hour, and even something as minuscule as a grain of rice could demolish it. "There's some danger and some suspense," Mr Stern said at a fall meeting of astronomers. It will take about 10 hours to get confirmation that the spacecraft completed - and survived - the encounter. Possibly twins Scientists speculate Ultima Thule could be two objects closely orbiting one another. If a solo act, it's likely 32 kilometres long at most. It could even be two bodies connected by a neck. If twins, each could be 15 kilometres to 20 kilometres in diameter.

Loading Mapping mission Scientists will map Ultima Thule every possible way. They anticipate impact craters, possibly also pits and sinkholes, but its surface also could prove to be smooth. As for colour, Ultima Thule should be darker than coal, burned by eons of cosmic rays, with a reddish hue. Nothing is certain, though, including its orbit, so big that it takes almost 300 of our Earth years to circle the sun. Scientists say they know just enough about the orbit to intercept it. Comparing flybys New Horizons will get considerably closer to Ultima Thule than it did to Pluto: 3500 kilometres versus 12,500 kilometres. At the same time, Ultima Thule is 100 times smaller than Pluto and therefore harder to track, making everything more challenging. It took 4 hours each way for flight controllers at Johns Hopkins' Applied Physics Lab in Laurel, Maryland, to get a message to or from New Horizons at Pluto. Compare that with more than six hours at Ultima Thule.

What's next? It will take almost two years for New Horizons to beam back all its data on Ultima Thule. A flyby of an even more distant world could be in the offing in the 2020s, if NASA approves another mission extension and the spacecraft remains healthy. At the very least, the nuclear-powered New Horizons will continue to observe objects from afar, as it pushes deeper into the Kuiper Belt. There are countless objects out there, waiting to be explored. AP