NASA's New Horizons mission has taken detailed images of the most distant object mankind has ever explored — a big rock called Ultima Thule.

“This flyby is a historic achievement,” NASA New Horizons Principal Investigator Alan Stern 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.”

Ultima Thule is currently about 6.5 billion km from the Sun. The spacecraft took images from as close as 27,000 kilometres on approach, revealing the cluster to be a “contact binary”, consisting of two connected spheres.

End to end, it measures 31 kilometers in length.

The experts say the two spheres probably joined in the very earliest days of the formation of our solar system, colliding at a speed no faster than two cars.

“New Horizons is like a time machine, taking us back to the birth of the solar system. We are seeing a physical representation of the beginning of planetary formation, frozen in time,” said Jeff Moore, New Horizons Geology and Geophysics team lead.

“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.”

Data from the New Year's Day flyby will continue to arrive over the next weeks and months, with much higher resolution images yet to come.

“In the coming months, New Horizons will transmit dozens of data sets to Earth, and we'll write new chapters in the story of Ultima Thule — and the solar system,” said Helene Winters, New Horizons Project Manager.

Live updates and links to mission information are available here.