Word of success came 10 hours after the middle-of-the-night encounter.
An unmanned NASA spacecraft sent a signal back to Earth Tuesday that it successfully made it through a risky flyby past the most distant planetary object ever studied, the US space agency said.
"We have a healthy spacecraft," said Alice Bowman, missions operations manager for the New Horizons spacecraft, which zipped by Ultima Thule at 12:33 am (0533 GMT) on New Year's Day.
"We have just accomplished the most distant flyby," she said.
The "phone home" signals took about 10 hours to reach Earth following the flyby, which took place four billion miles (6.4 billion kilometers) away.
Images and data will start arriving later Tuesday, "science to help us understand the origins of our solar system," Bowman said.