But a new, though still blurry image released Tuesday showed its oblong shape resembles something like a bowling pin or a peanut, and its dimensions are about 22 miles long and nine miles wide (35 by 15 kilometers).
This sequence of three images, received on 31 December 2018 and taken by the LORRI camera on board New Horizons at 70 and 85 minutes apart, illustrates the rotation of Ultima Thule.
Ultima Thule is the first destination to be reached that was not even known until after the spacecraft's launch.
Since temperatures this far from the Sun are barely above absolute zero - mummifying temperatures that preserves Kuiper Belt objects - they are essentially time capsules of the ancient past, the United States space agency said.
New Horizons, which is the size of a baby grand piano and part of an $800 million mission, was expected to hurtle to within 2,200 miles (3,500 kilometers) of Ultima Thule, considerably closer than the Pluto encounter of 2015.
Guests cheer as the New Horizons team members receive signals from the spacecraft that it is healthy.
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.
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.More news: Syria is sand and death, says Trump
"The Kuiper Belt is just a scientific wonderland", Alan Stern, principal investigator of the New Horizons mission, said on Sunday. The Ultima Thule rendezvous was more complicated, given the distance from Earth, the much closer gap between the spacecraft and its target, and all the unknowns surrounding Ultima Thule.
Tired from dual countdowns late Monday and early Tuesday, the New Horizons team members were visibly anxious as they reassembled in late morning.
The flyby took place at 12.33 EST today, but because New Horizons is so far away it takes around six hours to send data back to mission control at Johns Hopkins University's Applied Physics Laboratory. Just hours into the first day of 2019, the New Horizons spacecraft was confirmed to have made its historic flyby of the mysterious Kuiper Belt object Ultima Thule without issue. However, because the image is so blurry, it is still possible that it is actually two objects orbiting one another. By this evening, the team should have a 100-pixel image in hand; higher resolutions will follow in the next couple days, with the best shots not arriving till February.
Ultima Thule as an elongated blob.
As a preserved relic from that original time, Ultima Thule also promises to shed light on the so-called Kuiper Belt, or Twilight Zone, in which hundreds of thousands of objects reside well beyond Neptune.
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"We're already learning about Ultima from up close", Stern said.
Bowman takes comfort and pleasure in knowing that long after New Horizons stops working, it "will keep going on and on". The mission was launched in 2006 and took a 9½-year journey through space before reaching Pluto.