'Something Weird Is Going On' as New Horizons Approaches Distant Asteroid

NASA's New Horizons spacecraft passed Pluto in 2015. But now it's getting strange readings while approaching its next destination, the Ultima Thule asteroid.
Popular Mechanics reports:
Ultima Thule appears to not have a light curve, which is perplexing scientists... Asteroids like Ultima Thule reflect sunlight -- that's why they're bright spots instead of dark spots -- but the amount of light they reflect depends on how much of their surface is facing the Sun. The bigger their surface area, the brighter they become. Small asteroids like Ultima Thule aren't perfectly round, which means how much of their surface is facing the Sun changes as they rotate....
Ultima Thule isn't changing its brightness at all. New Horizons has been watching Ultima Thule for three months and hasn't spotted any brightness change, which is really odd. Ultima Thule is definitely not spherical -- astronomers determined that a year ago -- so why doesn't its brightness change?
One theory is that the New Horizon's probe is perfectly aligned with the asteroid's axis of rotation, so it's only seeing Ultima Thule's north (or south) pole. Another is that the asteroid is surrounded by dust clouds that 'even out' its light curve. But that usually only happens when asteroids are near the sun and heating up, whereas Ultima Thule 'is cold and dark and shouldn't have any dust....'

'Fortunately, we might not have to wait long for an answer to this problem. New Horizons will fly by Ultima Thule on January 1, and should give us high-resolution photos of the entire system,' the article concludes. 'With any luck, those photos will solve the mystery.'

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