A Nasa spacecraft has conducted a historic flyby of the farthest, and quite possibly the oldest, cosmic body ever explored by humankind.

The US space agency was hoping to take a closer look at a tiny, distant world called Ultima Thule early on Tuesday, hoping the frozen cosmic object will reveal some clues as to how planets took shape 4.6 billion years ago.

It rang in the New Year with a live online broadcast to mark New Horizons' zoom past the mysterious object located about four billion miles away in a dark and frigid region of space known as the Kuiper Belt.

The flyby at 12.33 am on Tuesday (5.33am GMT) took place about a billion miles beyond Pluto, which was until now the most faraway world ever visited up close by a spacecraft.

"Go New Horizons!" said lead scientist Alan Stern as a crowd including kids dressed in space costumes blew party horns and cheered at the Johns Hopkins Applied Physics Laboratory in Maryland to mark the moment when the spacecraft aimed its cameras at the distant space rock.

Real-time video of the actual flyby was impossible, since it takes more than six hours for a signal sent from Earth to reach the spaceship and another six hours for the response to arrive.