The two bodies came together at a very low speed, he said, on the order of a few kilometers per hour, slow enough to preserve each object.
"It's a snowman", mission principal investigator Alan Stern, a planetary scientist from the Southwest Research Institute, said during a news briefing here at Johns Hopkins University's Applied Physics Laboratory.
New Horizons flew past Ultima Thule at 12:33 a.m. EST (5:33 a.m. GMT) January 1, 2019, ushering in the era of exploration from Kuiper Belt, a collection of icy bodies left over from the Solar System's formation 4.6 billion years ago.
The Queen guitarist, who also happens to be an astrophysicist, wrote a song called New Horizons, his first solo work in more than 20 years. "The object that we rendezvoused with at midnight on New Year's Day wasn't even known until the summer of 2014".More news: U.S. ambassador visits suspected U.S. spy in Russian jail
The mission team has made a decision to call the larger lobe "Ultima" and the smaller lobe "Thule".
Another factor is that Ultima is small (about 35km in the longest dimension), and this means it doesn't have the type of "geological engine" that in larger objects will rework their composition.
As project scientist Hal Weaver told Space.com, "It just keeps getting better and better".
The flyby took place about a billion miles beyond Pluto, which was until now the most faraway world ever visited up close by a spacecraft. It's so far away the picture took six hours to reach Earth.
It's no simple matter to reach the outer solar system, so NASA is making the most of the opportunity it has with the New Horizons probe.More news: Reuben Foster's domestic violence charges dropped
If confirmed, this configuration could represent a precious snapshot of planetary formation in action, supporting the idea that the massive, orbiting bodies in our inner solar system assembled in part through the rapid coalescing of pebbles and dust.
The pictures from Ultima Thule were revealed on Wednesday.
"This thing was born somewhere between 99 percent and 99.9 percent of the way back to T-zero (liftoff) in our solar system, really unbelievable", Stern said. At right, the color was draped over the black and white image.
Stereo analysis and subsequent imaging will be available soon. The lobes, he said, were really only "resting on each other". "Unfortunately the approach images that came down first just aren't conducive to determining whether there are craters on the surface or not". The probe will now study the makeup of Ultima Thule's atmosphere and terrain for further clues about how solar system and planets form. It is 6.4 billion km (4 billion miles) from Earth.
Stern expressed surprise, and elation, that after picking the mission target "more or less" out of the hat, "that we were able to get as big a victor as this, that is going to revolutionize our knowledge of planetary science".More news: Farewell to a hard 2018 on Wall Street