It's getting bigger.
Last week, NASA released photos of the space exploration craft New Horizons gradually approaching an ancient, little-known object in deep space, called Ultima Thule.
Ultima orbits the sun one billion miles past Pluto, and NASA expects to swoop close to the far-off object soon after midnight, on January 1, 2019.
It will be humanity's farthest-ever encounter with another world.
"What will Ultima reveal? No one knows," Alan Stern, the NASA planetary scientist leading the deep space mission, wrote last week.
NASA suspects Ultima is a type of icy mass formed some 4.5 billion years ago, during the inception of our solar system.
But since then, hovering in the profoundly cold outer reaches of the solar system, Ultima is presumed to have been preserved largely in its pristine, primeval state — allowing scientists to see the distant past.
"In effect, Ultima should be a valuable window into the early stages of planet formation and what the solar system was like over 4.5 billion years ago," said Stern.
Ultima is formally classified as a "Kuiper Belt Object," which is a ring of icy worlds that encircles the solar system beyond the last major planet, Neptune. It is a "region of leftovers from the solar system's early history," says NASA.
Ultima has already proven somewhat mysterious.
From previous images, scientists learned that Ultima probably has a weird, non-spherical shape. But as New Horizons travels closer, the pattern of light reflecting off off of Ultima, or its light curve, is inconsistent. With most other objects, these light patterns repeat as the objects spin.
"It's really a puzzle," said Stern in a statement.
Other New Horizons scientists mused that a dust cloud or moons "tumbling" around Ultima might be producing the strange light curve.
But, there is one thing that's almost certain.
On December 15, Stern's team concluded there were no obstructions between New Horizons — a triangular spacecraft 7 feet long and 9 feet tall — and Ultima Thule.
Stern told NASA that the deep space probe is now "Go" to closely approach Ultima.
In the summer of 2015, New Horizons flew by Pluto. It captured unprecedented detail of the dwarf planet, its mountains, cliffs, and icy plains. The exploration craft flew 7,000 miles from Pluto's surface.
But it will get much closer to Ultima Thule, swooping 2,200 miles above the little-known object.
The first images are expected back early on New Years Day, about 30 minutes after the ball drops in Times Square.
"The Ultima Thule flyby is going to be fast, it’s going to be challenging, and it’s going to yield new knowledge," said Stern.
"Being the most distant exploration of anything in history, it’s also going to be historic."
We'll be watching.
To watch the images come in during Ultima's approach, tune into a livestream from NASA TV beginning around 12:15 a.m. ET, Jan. 1, 2019.