© NASA/JHUAPL/SwRIUltima's shape was measured in July 2017 as its silhouette passed in front of a star – what's known as a stellar occultation.
NASA's
New Horizons spacecraft is bearing down on Ultima Thule, its New Year's flyby target in the far away Kuiper Belt. 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.
Those measurements have produced the mission's first mystery about Ultima. Even though scientists determined in 2017 that the Kuiper Belt object isn't shaped like a sphere - that it is probably elongated or maybe even two objects - they haven't seen the repeated pulsations in brightness that they'd expect from a rotating object of that shape. The periodic variation in brightness during every rotation produces what scientists refer to as a
light curve.
"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 this, and so soon."
What could explain the tiny, still undetected light curve? New Horizons science team members have different ideas.
"It's possible that Ultima's rotation pole is aimed right at or close to the spacecraft," said Marc Buie, also of the Southwest Research Institute. That
explanation is a natural, he said, but it requires the special circumstance of a particular orientation of Ultima.
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