Just days ahead of its New Year's 2019 close encounter, NASA's New Horizons spacecraft has detected an anomaly related to its next flyby target -- an icy world a billion kilometres past Pluto and more than 6.5 billion km from Earth.

The New Horizons probe, which flew past Pluto in 2015, is set to encounter the Kuiper Belt object, referred to as 2014 MU69 -- nicknamed Ultima Thule.

New Horizons will flyby it on New Year's Eve and January 1.

Among its approach observations over the past three months, the spacecraft has been taking hundreds of images to measure Ultima's brightness and how it varies as the object rotates.

The spacecraft has not been able to detect the repeated pulsations in brightness during every rotation produced -- what scientists refer to as a light curve -- that accompanies all celestial objects in orbit near a bright star.

"It's really a puzzle," said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, a US-based non-profit. 

"I call this Ultima's first puzzle -- why does it have such a tiny light curve that we can't even detect it? I expect the detailed flyby images coming soon to give us many more mysteries, but I did not expect this, and so soon."

The team has various explanations that could explain the tiny, still undetected light curve.

"It's possible that Ultima's rotation pole is aimed right at or close to the spacecraft," said Marc Buie, also from the Institute. 

"Another is it may be surrounded by a cloud of dust that obscures its light curve, much the way a comet's coma often overwhelms the light reflected by its central nucleus," said Mark Showalter, from the SETI Institute, another US-based research non-profit. 

However, the spacecraft, which would fly about three times closer to MU69 than it did to Pluto in July 2015, would allow cameras to provide a more detailed look that would help decode the anomaly.

New Horizons extended mission also includes observations of more than two-dozen other Kuiper Belt objects, as well as measurements of the plasma, gas and dust environment of the Kuiper Belt.