Over the past three months, NASA’s New Horizons spacecraft has been taking images to measure the brightness of its next flyby target — a Kuiper belt object nicknamed Ultima Thule (also known as 2014 MU69) — and how the brightness varies as the object rotates. Even though New Horizons team members determined in 2017 that Ultima Thule 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 astronomers refer to as a light curve.
An artist’s impression of New Horizons encountering Ultima Thule. Image credit: NASA / Johns Hopkins Applied Physics Laboratory / Southwest Research Institute / Alex Parker.
“We are flying by Ultima Thule to see a Kuiper Belt object up close for the first time. Ultima is special for two reasons,” said New Horizons principal investigator Dr. Alan Stern, a researcher at the Southwest Research Institute.
“First, based on its orbit type, we know that Ultima Thule was formed way out where New Horizons is, 4 billion miles away. That means Ultima was formed in the middle of the Kuiper Belt, where temperatures are close to absolute zero.”
“Second, because of where it was formed and the fact that Ultima Thule is not large enough to have a geologic engine like Pluto and larger planets, we expect that Ultima is the most well-preserved sample of a planetary building block ever explored. In effect, Ultima should be a valuable window into the early stages of planet formation and what the Solar System was like over 4.5 billion years ago.
“What will Ultima Thule reveal? No one knows. To me, that is what’s most exciting — this is pure exploration and fundamental science.”
What could explain the tiny, still undetected light curve from Ultima Thule? Dr. Stern and colleagues have several ideas.
“It’s really a puzzle. I call this Ultima Thule’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,” Dr. Stern said.
“It’s possible that Ultima Thule’s rotation pole is aimed right at or close to the spacecraft,” added Dr. Marc Buie, also of the Southwest Research Institute.
“That explanation is a natural, but it requires the special circumstance of a particular orientation of Ultima Thule.”
“Another explanation is that Ultima Thule 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 Dr. Mark Showalter, from the SETI Institute.
“That explanation is plausible, but such a coma would require some source of heat to generate, and Ultima is too far away for the Sun’s feeble light to do the trick.”
“An even more bizarre scenario is one in which Ultima is surrounded by many tiny tumbling moons,” said New Horizons team member Dr. Anne Verbiscer, from the University of Virginia.
“If each moon has its own light curve, then together they could create a jumbled superposition of light curves that make it look to New Horizons like Ultima has a small light curve.”
“While that explanation is also plausible, it has no parallel in all the other bodies of our Solar System.”
