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NASA’s New Horizons spacecraft is about to fly past the most distant space rock we have ever visited. Since zooming by Pluto in 2015, the probe has been heading ever further from home, towards a tiny world called 2014 MU69. It is set to arrive on New Year’s Day.

The rock, nicknamed Ultima Thule, is about 6.6 billion kilometres from Earth. It was only discovered in 2014 during a search for potential targets for New Horizons, so we know very little about it.

We do know that it is a mere 30 kilometres or so across – less than 2 per cent of Pluto’s diameter – which has made getting there incredibly difficult. “It’s a lot harder than Pluto,” says mission leader Alan Stern. “Instead of being the size of the continental US, it’s the size of Boston. Being 100 times smaller means it’s 10,000 times fainter.”

That, combined with having had only four years to track the rock’s trajectory, makes it a much trickier target. If all goes well, New Horizons will hurtle by only 3500 kilometres from MU69’s surface at more than 14 kilometres per second.

Approaching at speed

Photographing the rock will be like taking a picture from a moving car of a gnat hovering beside the road – and with so little sun that it is essentially dark.

The difficulty doesn’t end there: the craft’s plutonium-based batteries have been degrading since the launch 13 years ago, so the team will have to be careful about which instruments to use. The battery can now only power the equivalent of three standard light bulbs at any one time, says Stern.

The team is already on the lookout for any dust, rocks or rings that might be around Ultima Thule. This is not just for the sake of scientific discovery, but also because they pose the greatest danger to the spacecraft. “If there is orbiting debris, even something literally the size of a rice pellet, at that speed it would shred New Horizons,” says Stern. “If it doesn’t signal on fly-by morning and say ‘I’m here and everything’s fine’, that’s probably what happened.”

But if everything goes well, the probe will send back a wealth of data in the first few days of 2019. As well as images, there will be information on MU69’s surface composition and its temperature.

The best images will be the hardest to get. As New Horizons passes by, it will take a string of high-resolution photos along its trajectory. These should look even better than the ones taken at Pluto because the craft will be more than three times as close to its target.

But if our estimates of Ultima Thule’s location are even a little off, it could end up out of the frame, leaving us with nothing but empty space. “If that Hail Mary pass works, it’s going to be spectacular,” says Stern.

One of the first things we will find out is whether Ultima Thule is one object with two lobes, shaped a bit like an unfinished snowman, or two rocks orbiting one another. It could even be several boulders trapped in a sort of floating rockslide.

Ultimately, the hope is that Ultima Thule will teach us about the beginnings of the solar system and its planets. Rocks like this were the precursors to Earth and the other planets. Because it is so far from the sun and too small to undergo geological activity, it will be the most pristine planetary building block we have ever visited.

“Never before have we seen something that’s this wild and woolly,” says Stern.