About a billion miles more distant than Pluto is Ultima Thule, a peanut-shaped object in the outer solar system that's the farthest place ever visited by humans.
NASA's New Horizons spacecraft zipped past Ultima Thule on New Year's Eve (Pacific time), flying within 2,200 miles of the space rock's rust-colored 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 4 billion miles 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% of all the data collected by New Horizons during its flyby. The full download won't be complete until mid-2020.
Here are seven things we've learned about Ultima Thule so far:
-- It has been essentially undisturbed for more than 4 billion years
Ultima Thule is about 43 times farther from the sun than we are, and as a result, it receives 900 times less sunlight than we do on Earth. Since it has never gotten warmer than about -350 degrees Fahrenheit, it has been well preserved since its formation shortly after the solar system was born.
During its 293-year orbit around the sun, some regions of Ultima Thule receive no sunlight for decades at a time, while others face the sun for decades straight. Scientists think variations in daily and seasonal temperatures have probably only affected a very shallow surface layer of the Kuiper Belt object, ranging from a few millimeters to a few meters.