Applause and cheers broke out at mission control when the signals from New Horizons were confirmed on Tuesday.
Scientists made a decision to study Ultima Thule with New Horizons after the spaceship, which launched in 2006, completed its main mission of flying by Pluto in 2015, returning the most detailed images ever taken of the dwarf planet.
After its flyby of Ultima Thule, New Horizons will continue its mission until at least April 2021, studying the environment and inhabitants of the Kuiper Belt, the most distant and mysterious zone of the solar system. However, it will take hours for the spacecraft to communicate its status with Earth and beam back the first images of the unexplored object.
By then, New Horizons will be on its way out of the solar system to roam the Milky Way galaxy for eternity, a fact that mission operations manager Alice Bowman said has stayed with her through the tension and excitement of the once-in-a-lifetime flyby. "In the space of one 72-hour period, Ultima will be transformed from a pinpoint of light - a dot in the distance - to a fully explored world". 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.
NASA scientists believe those objects, including Ultima Thule, could hold keys to understanding planetary formation. It's been a busy new year period for NASA: its Osiris Rex spacecraft just entered orbit around the asteroid Bennu on Monday, too.
Marc Buie, New Horizons co-investigator from the Southwest Research Institute (SwRI) in the USA, and members of the New Horizons science team discovered Ultima using the Hubble Space Telescope in 2014. Instead, hundreds of team members and their guests gathered nearby on campus for back-to-back countdowns.
Joining the celebrations of enthusiastic scientists was Brian May, who is a guitarist with rock band Queen and also an astrophysicist.
This tiny, icy body has remained largely unchanged since the solar system's birth roughly five billion years ago, providing astronomers a kind of time capsule of conditions from that era.
This flyby is the first exploration of a small Kuiper Belt object up close - and the most primitive world ever observed by a spacecraft.
But Alan Stern, a planetary scientist from the Southwest Research Institute who is principal investigator for the $800 million mission, which explored Pluto in 2015, said he was confident.
This time, the drama will unfold 4 billion miles (6.5 billion kilometers) from Earth, so far away it will be 10 hours before flight controllers in Laurel, Maryland, know whether the spacecraft survived the close encounter. From here out the data will just get better and better, " Stern added.
Scientists believe there should be no rings or moons around Ultima Thule that might endanger New Horizons.
Based on the early, rudimentary images, Ultima Thule is highly elongated - about 22 miles by 9 miles (35 kilometres by 15 kilometres).
Bowman takes comfort and pleasure in knowing that long after New Horizons stops working, it "will keep going on and on". Kudos to the science team and mission partners for starting the textbooks on Pluto and the Kuiper Belt. "This is exploration at its finest", said Adam L. Hamilton, president and CEO of the Southwest Research Institute in San Antonio.
For Ultima Thule - which wasn't even known when New Horizons departed Earth in 2006 - the endeavour was more hard.