Occurring some 6.5 billion km (4 billion miles) from Earth, the flyby will set a new record for the most distant ever exploration of a Solar System object by a spacecraft.
New Horizons will gather a swathe of images and other data over the course of just a few hours leading up to and beyond the closest approach.
This is timed for 05:33 GMT.
At that moment, the probe will be about 3,500km from Ultima’s surface and moving at 14km/s.
When its observations are complete, the robotic craft will then turn to Earth to report in and begin downlinking the gigabytes of information stored in its memory.
Mission scientists, gathered in a control centre at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, are excited at what lies in prospect.
“It’s electric. People across the whole team are ready. They’re in the game and we can’t wait to go exploring,” says New Horizons’ principal investigator Prof Alan Stern.
The probe is famous for making the first ever visit to the dwarf planet Pluto in 2015. To reach Ultima, it has had to push 1.5 billion km deeper into space.
Virtually nothing is known about this next target for New Horizons, however.
Telescopic measurements indicate it is about 20-30km across, although scientists concede it could actually be two separate entities moving very close to each other, perhaps even touching. The next couple of days will tell.
Ultima is in what’s termed the Kuiper belt – the band of distant, frozen material that orbits far from the Sun and the eight major planets. There are probably hundreds of thousands of Kuiper members like Ultima, and their frigid state almost certainly holds clues to the formation conditions of the Solar System 4.6 billion years ago.
“About one day out we’ll turn on all our instruments,” explains mission scientist Dr Kelsi Singer. “We’ll take black and white images; we’ll take colour images. And we’ll take compositional information… This is just such a new object because we’ve never been to an object like this before. It’s hard to predict but I’m ready to be surprised by what we find.”
Why is New Horizons visiting Ultima Thule?
Nasa wanted to explore something beyond Pluto and this object was reachable.
Remarkably, it was only discovered four years ago by the Hubble telescope.
Initially catalogued as (486958) 2014 MU69, it was given the more catchy nickname of Ultima Thule (Pronounced: Tool-ee) after a public consultation exercise.
It’s a Latin phrase that means something like “a place beyond the known world”.
Like many Kuiper belt objects of its size, it is likely to be composed of a lot of ice, dust and maybe some larger rock fragments, which came together at the dawn of the Solar System.
Theory suggests such bodies will take on an elongated or lobate form. Think potato or peanut.
Distant telescopic observations suggested its surface is very dark, with a bit of a red tinge. That darkness (it reflects only about 10% of the light falling on its surface) is the result of having been “burnt” through the eons by high-energy radiation – cosmic rays and X-rays.
New Horizons will study Ultima’s shape, rotation, composition and environment.
Scientists want to know how these far-off worlds were assembled. One idea is that they grew from the mass accretion of a great many pebble-sized grains.
What can we expect from the flyby?
Don’t blink, you might miss it. Unlike the encounter with Pluto in July 2015, there won’t be increasingly resolved images on approach to admire. Ultima will remain a blob in the viewfinder pretty much until the final hours of the flyby.
However, the much reduced separation between the probe and Ultima (3,500km versus 12,500km at the dwarf planet) means that finer detail in the surface will eventually be observed. Features as small as 33m across should be discernible if the pointing of the cameras is spot on.
Because New Horizons has to swivel to point its instruments, it cannot keep its antenna locked on Earth while also gathering data.
Controllers must therefore wait until later on New Year’s Day for the probe to “phone home” a status update and to start to downlink some choice pictures.
The “hey, I’m healthy and I’ve got a treasure trove of data” message should be picked up by Nasa’s network of big radio dishes at 15:28 GMT.
Just how big a challenge is this flyby?
In some ways, this event is more difficult than the pass of Pluto.
The object in the viewfinder is almost a hundred times smaller.
New Horizons will get closer than at Pluto, which is good for image detail; but it means that if the pointing is off, the probe could be sending back pictures of empty space.
And this really is a major concern. Because Ultima was only discovered four years ago, its position and movement on the sky are much more uncertain than the coordinates for Pluto.
Every image taken on approach has been used to refine the navigation and timing models that will be critical to the control of New Horizons during the flyby.
And, remember, all this is being done at a distance of 6.62 billion km (4.11 billion miles) from Earth.
At that separation, radio signals take six hours and eight minutes to reach home.
What is more, the data rates are glacial – around 1,000 bits a second.
It will be late on Tuesday before the first of a few choice images is downlinked, and it will be September 2020 until every last scrap of data from the flyby is pulled off New Horizons.
The BBC’s Sky At Night programme will broadcast a special episode on the flyby on Sunday 13 January on BBC Four at 22:30 GMT. Presenter Chris Lintott will review the event and discuss some of the new science to emerge from the encounter with the New Horizons team.