The last blobby picture sent back before the flyby - the best available so far - showed that Ultima Thule is 35 by 15 kilometer, with a blurry peanut shape, so it is either "bi-lobate", with a different size for each lobe, or it could be two objects whose images blurred together.
Scientists did not want to interrupt observations as New Horizons swept past Ultima Thule - described as a bullet intersecting with another bullet - so they delayed radio transmissions.
"There's a bit of all of us on that spacecraft", she said, "and it will continue after we're long gone here on Earth".
The flyby comes 3½ years after New Horizons swung past Pluto and yielded the first close-ups of the dwarf planet.
New Horizons principal investigator Alan Stern, center, of the Southwest Research Institute in Boulder, Colo., celebrates with other mission team members after they received signals from the New Horizons spacecraft that it is healthy and collected data during the flyby of Ultima Thule, at the Mission Operations Center at the APL in Laurel, Md, January 1, 2019.
But now it's probably more famous as the moniker of a faraway object orbiting at the furthest frontier of our solar system in a region called the Kuiper Belt.
Engineers celebrated after receiving confirmation that the probe had successfully performed its manoeuvres and sped past Ultima Thule, yet one key mystery about the space rock remained unanswered for several hours yesterday.
New Horizons space probe set to fly by furthest object ever explored
As for New Horizons, scientists say the probe will continue studying the Kuiper Belt through at least April 2021, the end of its now funded extended mission operations.
Ultima Thule - an uncharted world over 4 billion miles away - is coming into view. Although NASA's Voyagers crossed the Kuiper Belt on their way to true interstellar space, their 1970s-era instruments were not almost as sophisticated as those on New Horizons, Weaver noted, and the twin spacecraft did not pass near any objects known at the time.
"Congratulations to NASA's New Horizons team, Johns Hopkins Applied Physics Laboratory and the Southwest Research Institute for making history yet again", NASA Administrator, Jim Bridenstine, said in a statement.
"We finally have reached the outskirts of the solar system, these things that have been there since the beginning and have hardly changed - we think".
Expect never-before-seen high-resolution images and new science to come streaming in Wednesday and over the next few days.
In 2014, astronomers found Thule using the Hubble Space Telescope and the following year selected it for New Horizon's extended mission.
"The data we have look fantastic, and we're already learning about Ultima from up close", Stern said.
Ultima Thule is named for a mythical, far-northern island in medieval literature and cartography, according to NASA. An artist's impression at right illustrates one possible appearance of Ultima Thule, based on the actual image at left. "There are no second chances for New Horizons".
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