NASA's New Horizons spacecraft is about to overtake the farthest space rock we've ever visited. Since Pluto's zoom in 2015, the spacecraft has been moving ever further from home, toward a small world called 2014 MU69. You are scheduled to arrive on New Year's Day.
The rock is about 6.6 billion miles from Earth. It was only discovered in 2014 during a search for possible targets for New Horizons, so we know very little about it.
We know that they are a mere 30 kilometers in diameter – less than 2% of Pluto's diameter – which made it incredibly difficult to get there. "It's much harder than Pluto," says mission leader Alan Stern. "Instead of being the size of the continental United States, it's the size of Boston. Being 100 times smaller means that it is 10,000 times weaker. "
This, combined with just four years to track the trajectory of rock, makes it a much more complicated target. If all goes well, New Horizons will cover only 3500 km of the surface of MU69 at more than 14 kilometers per second.
Approaching the speed
Photographing the stone will be like taking a picture of a moving car from a mosquito hovering alongside the road – and with so little sunshine it is essentially dark.
The difficulty does not end there: the ship's plutonium-based batteries have been degrading since the launch 13 years ago, so the team will have to be careful with the instruments to be used. Now the battery can only power the equivalent of three common bulbs, says Stern.
The team is already looking for dust, stones or rings that might be around the Ultima Thule. This is not only for the sake of scientific discovery, but also because they pose the greatest danger to the spacecraft. "If there are debris in orbit, even something literally the size of a rice ball, at this rate, would destroy New Horizons," says Stern. "If it does not signal on the morning of the flight and say" I'm here and everything is fine, "it was probably what happened."
But if all goes well, the probe will send back a large amount of data in the early days of 2019. As well as the images, there will be information about the composition of the surface of MU69 and its temperature.
The best images will be the hardest to come by. As New Horizons passes, a sequence of high-resolution photos will be needed along its path. These should look even better than those made on Pluto because the spacecraft will be more than three times closer to the target.
But if our estimates of Ultima Thule's location are even slightly off, this may end up off the frame, leaving us with nothing but empty space. "If this Hail Mary passes, it will be spectacular," says Stern.
One of the first things we will discover is whether Ultima Thule is an object with two wolves, in the form of an unfinished snowman, or two rocks orbiting one another. It could even be several boulders stuck in a kind of floating rock slide.
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 of Earth and other planets. Because it is so far from the sun and too small to undergo geological activity, it will be the most untouched planetary building block we have ever visited.
"We've never seen anything so wild and woolly before," says Stern.
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