Ultima Thule Already Puzzling New Horizon Researchers, Before We’ve Even Arrived
If all continues as planned, New Horizons will rocket past the asteroid Ultima Thule on January 1, closing to within roughly 2200 miles of the asteroid on January 1 at 12:33 AM. Ultima Thule is a Kuiper Belt Object (KBO) roughly 19 miles in diameter, or 1/60th the diameter of Pluto. While New Horizons will be roughly 3x closer to UT than it ever got to Pluto, the difference in size between the two will have an obvious impact on just what our cameras will be able to see. One thing we do know is that scientists already have a mystery on their hands as we hurtle towards the rock.
Ultima Thule is an irregularly shaped object. It may be a binary pair of objects, including a contact binary: two distinct objects that may be superficially bound together by bits of rock, but with significant voids in between the spacing of the material). The comet 67P/Churyumov–Gerasimenko explored by Rosetta is a rubble pile, and the Martian moon Phobos is suspected to be one as well. This ought to mean that we see distinct differences in the light curve reflected by Ultima Thule as we approach it, and New Horizons is close enough to pick up these distinctions quite well. Since Ultima Thule isn’t a sphere, we ought to be seeing light curve differences caused by differences in how light reflects off its irregular surface. We aren’t.
“It’s really a puzzle,” New Horizons principal investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado, said. “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.”
There are several proposed explanations for this behavior. One potential explanation is that the pole of the planet is pointed directly at New Horizons. This would explain why the light varies so much less than it should. Another is that Ultima Thule is surrounded by a dust cloud, which obscures shifts in its light curve and prevents us from seeing what’s ‘really’ going on.
Researchers note that this is possible, but would require a stronger source of energy than what is available to Ultima Thule so far from the Sun. As a tiny object, there is no known source of interior heat that would provide the necessary energy. Finally, UT could be surrounded by a bunch of tiny moonlets that are collectively blocking shifts in its perceived light curve. New Horizons team member Dr. Anne Verbiscer, from the University of Virginia, notes that: “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.”
One way or the other, we should know in a matter of days. One of the advantages to focusing on Ultima Thule as a research target is that it should represent just about as close as we can get to a ‘pristine’ rock that’s been floating around since the solar system formed. By examining its characteristics, we should be able to see into the initial conditions of the solar system before there was a solar system.
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