The "snowman" shape indicates that Ultima Thule is a "contact binary" - the object's shape is the result of two spherical rocks fusing together in the early days of the solar system.
"That has brought us back to the very beginning of solar system history, to a place where we can observe the most primordial building blocks of the planets". New Horizons's journey into the solar system's past has just begun.
Ultima orbits the Sun in a region of the Solar System known as the Kuiper belt. At left is an enhanced colour image taken by the Multispectral Visible Imaging Camera (MVIC), produced by combining the near infrared, red and blue channels.
An image of Thule, sent overnight and barely more detailed than previous images, deepens the mystery of whether Thule is a single rock shaped like an asymmetrical peanut or actually two rocks orbiting each other, "blurred together due to their proximity", Stern said. That means it is a single object, with two lobes, but the lobes are gently in contact.
That's where Ultima Thule comes in: It looks to be a partial conglomeration of uncooked space dough-a premature mashup that could have served as the basis for a planet, but didn't.
Though they do not appear to have impact craters, there could be hills and ridges, with the neck connecting the two lobes being one of the steepest slopes. The coming weeks will reveal more about the object's composition and terrain. Altogether, it will take almost two years for all of New Horizons' data to reach Earth.
Scientist Jeff Moore says that the two spheres formed when small, icy pieces coalesced in space billions of years ago.
Eventually, these two bodies remained, slowly spiralling closer until they touched, forming Ultima Thule. He joked that if they were cars, "you probably wouldn't fill out the insurance form".
"This is the first object we can clearly tell was born this way, and didn't evolve to look this way", Stern said.
The nature of New Horizons' mission provided NASA with a unique opportunity to explore more of the outer solar system after the 2015 main event. As a whole, the little world measures 19 miles (30.5 km) long. That color can be explained by irradiation of ices on its surface, said Carly Howett of the Southwest Research Institute. "We have far less than 1 percent of the data" that is stored on the spacecraft from the flyby, which at its closest - about 2,200 miles - hurtled past Ultima Thule at a speed of 32,000 mph at 10:33 p.m. MST on Monday.
"It's two completely different objects that are now joined together", said S. Alan Stern, principal investigator for the mission.
Stereo analysis and subsequent imaging will be available soon.
The snowman picture was taken a half-hour before the spacecraft's closest approach early Tuesday, from a distance of about 18,000 miles (28,000 kilometers).