The New Horizons spacecraft, which was first launched in January 2006, captured imagines of Ultima Thule, after it flung past the oddly-shaped asteroid, which was travelling at more than 36,000mph.
Images of Ultima Thule are resolved at 110ft per pixel, the highest resolution pictures New Horizons has received of the asteroid.
Ultima Thule is the most distant object humanity has ever seen close-up, according to Johns Hopkins University Applied Physics Laboratory.
The frozen space rock, known as 2016 MU69, is up to 4.1 billion miles from Earth.
John Spencer, a deputy project scientist for New Horizons told Space.com: “Whether these features are craters produced by impactors, sublimation pits, collapse pits or something entirely different is being debated by our science team.”
The principal investigator at New Horizons, Alan Stern, also told Stern.com: “Getting these images required us to know precisely where both tiny Ultima and New Horizons were as they passed one another at over 32,000mph in the dim light of the Kuiper Belt, a billion miles beyond Pluto.
“This was a much tougher observation than anything we attempted in our 2015 Pluto flyby.”
The Southwest Research Institute said: “The science, operations and navigation teams nailed it, and the result is a field day for our science team!
“Some of the details we now see on Ultima Thule’s surface are unlike any object ever explored before.”
The asteroid is almost 20 miles long and is thought to be unchanged since the early days of the Solar System.
The first batch of pictures showed the asteroid to appear as a lumpy, misshapen space peanut composed of two touching lobes called a contact binary.
But as more pictures have emerged the rock to appeared more like a pancake than a lump.
The pictures revealed strange light freckles and dark dents on the asteroid that scientists are trying to understand.
John Spencer said: “Whether these features are craters produced by impactors, sublimation pits, collapse pits, or something entirely different, is being debated in our science team.”