Four billion miles (6.4 billion kilometers) away, the unmanned spaceship, New Horizons, is poised to zoom by at 12:33 am (0533 GMT) on January 1, at a distance of just 2,200 miles (3,500 kilometers) from Ultima Thule.

That's more than three times closer than New Horizons came to Pluto when it zipped by the dwarf planet in 2015.

So what is this strange object, which is named after a mythical, far-northern island in medieval literature and has its own rock anthem performed by Queen guitarist Brian May?

"This is truly the most primitive object ever encountered by a spacecraft," said Hal Weaver, project scientist at the Johns Hopkins Applied Physics Laboratory.

Relatively small, scientists aren't sure about its exact size.

But they believe it is about 100 times tinier than Pluto which measures almost 1,500 miles (2,414 kilometers) in diameter.

Ultima Thule is also in a freezing area of space, suggesting it may remain well preserved.

"Really, it is a relic from the formation of the solar system," said Weaver.