There will be no New Year’s Eve revelry for those working on NASA’s New Horizon’s space probe. They will need to be stone cold sober with their wits about them when, on New Year’s Day 2019, it whizzes past one of the fossil building blocks of the planets and one of the most primitive bodies in the Solar System.


2014 MU69, now renamed Ultima Thule from a medieval term meaning ‘beyond the known world’, had not even been discovered when New Horizons was launched towards Pluto in 2006. Discovered by the Hubble Space Telescope in 2014, it is a member of the Kuiper Belt – a swarm of icy bodies orbiting on the edge of the Solar System that are so sparsely distributed that they never accumulated into planets.

What excites planetary scientists about Ultima Thule is that it is pristine. Whereas Pluto (which New Horizons flew past in July 2015) has an active surface, Ultima Thule is expected to have remained unchanged since the birth of the Solar System 4.55 billion years ago, and therefore promises to tell us about the bodies that came together to make Pluto.

Ultima Thule, orbiting 1.6 billion kilometres beyond Pluto, or 43 times further from the Sun than the Earth, is believed to be about 32 kilometres wide.

“It’s less than one per cent of the size of Pluto and can’t sustain an atmosphere,” says Marc Buie, a member of the New Horizons science team. “It will be completely different.”

On 1 January 2019, when New Horizons flies within 3,500 kilometres of Ultima Thule, it will be the most distant object ever imaged in the Solar System. It should be possible to see features as small as a basketball court.

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Pluto proved a massive surprise. Far from being in suspended animation in the frigid wastes at the edge of the Solar System, it revealed itself to be an active world with flowing nitrogen glaciers and a possible subsurface ocean. The New Horizons’ team is hoping Ultima Thule will be as unexpected a body as Pluto.

By 2025, New Horizons will be almost 4 billion kilometres further away, and heading out into the wastes of interstellar space. “We will be continuing long-range observations of other KBOs,” says Buie. “Given the opportunity, New Horizons could remain busy and productive for quite a while.”



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Marcus Chown is an award-winning writer and broadcaster and a former radio astronomer at the California Institute of Technology in Pasadena. He is the author of Breakthrough: Spectacular stories of scientific discovery from the Higgs particle to black holes (Faber & Faber, 2021).