A NASA spacecraft travelling nearly 6.4 billion kilometres from the Earth has sent back its first close-up pictures of the most distant celestial object ever explored. It looks like a reddish snowman.

Ultima Thule, as the small, icy object has been dubbed, is an elongated body about 31km long, consisting of two fused-together spheres.

NASA's New Horizons, the spacecraft that sent back pictures of the Pluto three-and-a-half years ago, swept past the ancient, mysterious object early on New Year's Day. It is more than a billion kilometres past Pluto, and 6.4 billion km from the Earth.

The pictures from Ultima Thule were revealed on Wednesday.

"This flyby is a historic achievement," Alan Stern, New Horizons principal investigator said. "Never before has any spacecraft team tracked down such a small body at such high speed so far away in the abyss of space. New Horizons has set a new bar for state-of-the-art spacecraft navigation."

Scientist Jeff Moore says that the two spheres formed when small, icy pieces coalesced in space billions of years ago. Then the spheres slowly spiralled closer to each other and stuck together.

"Studying Ultima Thule is helping us understand how planets form - both those in our own solar system and those orbiting other stars in our galaxy," Moore said.

"New Horizons is like a time machine, taking us back to the birth of the solar system," he added. "We are seeing a physical representation of the beginning of planetary formation, frozen in time."

Ultima Thule has a mottled appearance the colour of the light brick. Scientists say no impact craters could be seen in the latest photos.

An earlier, fuzzier image made it look like a bowling pin.