Hints about Ultima Thule’s shape were gleaned during occultation observations in 2017 when the Kuiper Belt body passed in front of the star seen here. Researchers will finally get a close up view 1 January when NASA’s New Horizons probe flies past. Image: NASA/JHUAPL/SwRI
Over the past three months, NASA’s New Horizons spacecraft has been racing toward a New Year’s Day flyby of a Kuiper Belt object known as Ultima Thule, snapping hundreds of photos to measure the body’s brightness and rotation.
But the images do not show any hints of rotation, even though observations in 2017 showed Ultima Thule is not shaped like a sphere. Rather, it is an elongated body or perhaps made up of two asteroid-like objects in direct contact or very close together. One would expect such a body to be rotating and the light reflected from it to oscillate.
“It’s really a puzzle,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute. “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 ...