"It's two completely different objects that are now joined together", said S Alan Stern, principal investigator for the mission. "This flyby is a historic achievement".
New Horizons was launched from Cape Canaveral nearly exactly 13 years ago in 2006, and previously performed flybys of Jupiter and Pluto in 2007 and 2015 respectively.
Planetary scientists have never before seen a close-up of an object like Ultima Thule.
Moreover, its world measures 19 miles (31 kilometers) in length.
The new imagery, dramatic though it may be, is just the tip of the Ultima Thule iceberg. The smaller is Thule at 9 miles across.
Since New Horizons sent its first post-flyby message, the mission team slowly but surely has been receiving a trickle of data on Ultima Thule (pronounced TOOL-ee, a Latin phrase meaning "a place beyond the known world"), which is located a staggering 4.1 billion miles (6.6 billion km) from Earth. If the two pieces were moving fast when they collided, they would have bounced off each other.
Prior to today, images of Ultima Thule were only a handful of pixels across, leaving the object's true physique ambiguous.
We saw similar colors when New Horizons gave us stunning up-close views of Pluto. As more data comes in, scientists will have a better handle on what's causing it. On Pluto, the culprit was tholins - hydrocarbon molecules formed from the interaction between sunlight and methane.
According to Jeff Moore, New Horizons' geology and geophysics team lead, Ultima Thule's two lobes-now named Ultima (the larger) and Thule (the smaller)-might have formed from a throng of small, icy bodies.
"We think what we're looking at is perhaps the most primitive object that has yet been seen by any spacecraft, and may represent a class of objects which are the oldest and most primitive objects that can be seen anywhere in the present solar system", Mr Moore said.
Helene Winters, New Horizons Project Manager, said: "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".
The insane thing is, Ultima Thule might not be all that special in the grand scheme of things. It was picked because it was bright enough to be discovered and it just happened to be along New Horizons' path. It will now continue to speed onward to observe other Kupier Belt objects. According to Stern, the team has far less than one percent of all the data now onboard New Horizons in hand. "What you're seeing is the first contact binary ever explored by a spacecraft", Stern said today at a press conference here at the Johns Hopkins University Applied Physics Laboratory.