NASA's New Horizons spacecraft zipped past it on New Year's Eve, flying within 3,540 kilometres of the space rock's rust-coloured surface.

The data it captured is now giving scientists a rare glimpse into the solar system's early days.

Ultima Thule has spent most of its 4.5 billion years frozen in time in the Kuiper Belt, a doughnut-shaped region beyond Neptune that contains remnants from the solar system's early days.

Its surface is barely heated by the sun, which is about 6.43 billion kilometres away, according to an initial analysis of New Horizons data published in Friday's edition of the journal Science.

"We had never seen something that was so primordial, so unchanged since the early formation days," said Alan Stern, the principal investigator for the New Horizons mission.

Ultima Thule is what's called a contact binary object, consisting of two lobes that formed separately through an accumulation of small particles of gas and dust. Only later did they fuse together, scientists believe.

The new report is based on only 10 per cent of all the data collected by New Horizons during its flyby. The full download won't be complete until mid-2020.

There are seven things learned about Ultima Thule so far:

It has been essentially undisturbed for more than four billion years

It is about 43 times further from the sun than we are and receives 900 times less sunlight.

During its 293-year orbit around the sun, some regions of Ultima Thule receive no sunlight for decades.

It's lobes - Ultima the larger and Thule the smaller - came together in a very gentle collision. It's lightly cratered because there are few things to crash into and when it does, it happens slowly.

It has no moons or rings.

It is extremely dark and reflects no more than 12 per cent of the light that strikes its surface but does have patches of brightness.

There's little water on its surface.

Australian Associated Press