It is getting bigger.
Last week, NASA released photos of the New Horizons space exploration vehicle, which is gradually approaching an ancient, little-known object in space called Ultima Thule.
It will be the furthest meeting of humanity with another world.
"What will Ultima reveal, no one knows," wrote Alan Stern, NASA's planetary scientist, who led the space mission, last week.
NASA suspects Ultima is a kind of ice mass that formed about 4.5 billion years ago during the founding of our solar system.
But since then, hovering in the freezing cold outer reaches of the solar system, it is believed that Ultima has largely been preserved in its original, pristine state – allowing scientists to see the distant past.
"Ultima should be a valuable window into the early stages of planet formation and what the solar system looked like over 4.5 billion years ago," said Stern.
Ultima is formally classified as the "Kuiper Belt Object", which is an ice ring worlds that surround the solar system beyond the last great planet Neptune. It is a "region of remnants of the early history of the solar system," says NASA.
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Ultima has already proven to be somewhat mysterious.
From earlier images, scientists have learned that Ultima is likely to have a weird, non-spherical shape. But as New Horizons approaches, the light pattern reflected by Ultima or its light curve is inconsistent. For most other objects, these patterns of light are repeated as the objects rotate.
"It's really a mystery," Stern said in a statement.
Other New Horizons scientists speculated that a cloud of dust or moons that "crashed" around Ultima could create the strange light curve.
But there is one thing is almost certain.
On December 15, Stern's team concluded that there were no obstacles between New Horizons (a 7 foot long triangular spaceship at 9 feet high) and Ultima Thule.