While their fellow Earthlings counted down to midnight, they were ticking off time until 12:33 a.m., when the spacecraft would make its closest approach to its target. The Ultima Thule rendezvous was more complicated, given its 4 billion-mile (6.4 billion-kilometre) distance from Earth, the much closer gap between the spacecraft and its target, and all the unknowns surrounding Ultima Thule. So they had to wait until late morning before learning whether the spacecraft survived.
Cheers erupted at Johns Hopkins University's Applied Physics Laboratory, home to mission control. Instead, hundreds of team members and their guests gathered nearby on campus for back-to-back countdowns.
But the best colour close-ups will not be available until later in January and February.
"We set a record!"
Principal Investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado, noted that the data already collected looks "fantastic". As distant as it is, Pluto is barely in the Kuiper Belt, the so-called Twilight Zone stretching beyond Neptune.
It is the farthest away from the Sun any spacecraft has ever investigated an object, with New Horizons taking over 13 years to reach the rock.
If New Horizons sounds familiar, it's because this is the spacecraft that conducted a historic flyby of Pluto in 2015, sending back unprecedented images of the dwarf planet and revealing new details about Pluto and its moons.
Its seven science instruments were to continue collecting data for four hours after the flyby. This configuration limits the spacecraft's communication with Earth, commanding it to quickly address any technical issues on its own, then get back to science. Owing to the probe's great distance from Earth and the relative weakness of its signal, it took several hours for scientists to receive and process the image. Traveling at 31,500 miles per hour (50,700 kph), the spacecraft could easily be knocked out by a rice-size particle. The close encounter comes 3½ years after the spacecraft swung past Pluto.
"I can't promise you success".
"We're rendezvousing with something that's a mountain draped in black velvet in nearly pitch dark conditions and we're screaming up to it ... within 2 seconds of perfection", Stern said. "We're going to learn more very soon".
Tired from dual countdowns late Monday and early Tuesday, the New Horizons team members were visibly anxious as they reassembled in late morning.
The risk added to the excitement.
"We will never forget this moment", said May who led the New Years countdown. The amount of light it gives off is completely flat, and mission team doesn't know why!
It was discovered in 2014 with the help of the Hubble Space Telescope, and is believed to be up to 32km in size.
But much of Ultima's mystery will diminish in the next few days. The first fuzzy pictures of Ultima Thule show that it seems to be spinning and is about 22 miles long by 9 miles wide and shaped a bit like a peanut.
Based on these two factors, this object could be a pristine piece of the early solar system, which formed at its current location, and thus was not tossed around by the gravity of the other planets (looking at YOU, Neptune), and has possibly gone completely (or almost completely) untouched for over 4.5 billion years.
Ultima Thule was named for a mythical, far-northern island in medieval literature and cartography, according to Nasa.
Scientists speculate Ultima Thule could be two objects closely orbiting one another.
The exact shape and composition won't be known until Ultima Thule starts sending back data in a process expected to last nearly two years. "We will find out".