NASA's New Horizons spacecraft is headed toward a New Year's Day encounter with this mysterious object or two, 1.6 billion kilometres beyond Pluto on the fringes of our solar system.

Its official name is 2014 MU69. The mission team wanted a less technical and more inspiring name and, late last year, sought public input.

Announced on Tuesday, the nickname Ultima Thule (THOO-lee) comes from medieval literature and refers to a distant, unknown world.

"MU69 is humanity's next Ultima Thule," New Horizons' lead scientist, Alan Stern of the Southwest Research Institute in Boulder, Colorado, said in a statement. "Our spacecraft is heading beyond the limits of the known worlds."

New Horizons will fly past Ultima Thule on January 1, making it humanity's most distant encounter with another world.

Discovered in 2014 and more than 6.4 billion kilometres from Earth, Ultima Thule is considerably smaller than Pluto, explored by New Horizons in 2015.

Scientists estimate it's no more than 30km across if a single object. If it is two objects orbiting one another, each one might be 15km to 20km. There might also be a small moon.

The team will choose a more formal name following the flyby.

For the nickname, more than 34,000 suggestions were whittled down to a list of 37 for public online voting.

The top vote-getter was the name of Thor's hammer - Mjolnir - of Norse mythology, followed by Z'ha'dum, a planet in the 1990s TV series Babylon 5. Ultima Thule ranked seventh.

NASA and the New Horizons team had the final say. It came down to what seemed most appropriate to the mission and its objectives, said NASA spokesman Dwayne Brown.

Australian Associated Press