But despite the festive atmosphere - which included the release of a New Horizons song recorded by contributing scientist and rock guitarist Brian May of Queen, who was on hand for the event - the mission team had no way of knowing if their spacecraft was still in one piece and executing its reconnaissance of Ultima Thule.
"Go New Horizons!" said lead scientist Alan Stern as a crowd cheered at the Johns Hopkins Applied Physics Laboratory in Maryland to mark the moment at 12:33 am (0533 GMT) when the New Horizons spacecraft aimed its cameras at the space rock four billion miles (6.4 billion kilometers) away. Now, it is heading towards the edge of the solar system and will shortly reach Ultima Thule, where it will complete a historic flyby.
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.
Ultima Thule receives only a tiny amount of light from the sun...
"Ultima Thule is finally revealing its secrets to us", said project scientist Hal Weaver of Johns Hopkins.
With New Horizons out of contact with Earth from late Monday afternoon until Tuesday morning, Mission Control was pretty much empty, with all the festivities centered at a nearby campus building. The spacecraft zoomed within 2,200 miles (3,500 kilometres) of it, more than three times closer than the Pluto flyby.
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"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. As a result, Ultima Thule wasn't even discovered until 2014.
NASA already has a number of press conferences scheduled for the coming days, and we're sure to learn a lot about how well New Horizons performed and, hopefully, some new information about Ultima Thule as well.
As revellers watched fireworks exploding in the night sky, billions of kilometres beyond the spectacle, NASA's New Horizons probe quietly notched up another unbelievable first - making its closest approach to the most distant object ever visited by a spacecraft.
Dr. Stern added that while this week's images should be a dramatic improvement over what is now known about the Kuiper Belt, scientists will not have their best views downloaded until February.
"There's a bit of all of us on that spacecraft that will continue after we're long gone here on Earth", she said. "To me, this milestone for New Horizons is full of everything that NASA and NASA science is about". The mission team expects the data to be returned over the next 20 months, with an additional year of data analysis and archiving. The team hopes to get the mission extended and explore more mysterious Kuiper Belt objects in the 2020s.
Lead scientist Alan Stern of Southwest Research Institute in Boulder, Colorado, expects the New Year's encounter to be riskier and more hard than the rendezvous with Pluto: The spacecraft is older, the target is smaller, the flyby is closer and the distance from us is greater. In classic and medieval literature, Thule was the most distant, northernmost place beyond the known world. "We'll find out Tuesday".