Pancakes Anyone? Ultima Thule Flatter Than We Thought

Departure movie of Ultima Thule made with 14 images taken after New Horizons made its closest approach on Jan. 1, 2019.

Farewell Ultima Thule! This evocative video was made just after closest approach to Ultima Thule on New Year’s Day when New Horizons was speeding past at over 31,000 miles an hour (50,000 km/hour). See all those stars? Not only do they provide a breathtaking backdrop, but it turns out that they were crucial to determining the object’s true shape. When the first photos were beamed back, it looked for all the world like two approximately spherical bodies joined at the neck. Some of us went so far as to call it a snowman. But further analysis of these latest photos reveal that Ultima, the larger of the two halves, is shaped more like a pancake, while Thule reminds me of a Milk Dud, a chocolate-coated caramel candy.

Scientists “traced out” their shapes by examining how they blotted out the background stars as New Horizons raced by. They then compared the observations to a model made from pre-flyby photos and telescope observations from the ground. Amateur and professional astronomers use the same method when trying to figure out an asteroid’s shape from Earth.

If you know the path of an asteroid, you can predict when it will occult (hide) occasional bright stars along the way.  Using software, you can predict where on Earth the occultation will occur. Usually it’s visible in a zone, say 100 miles wide, like the path of totality during August 2017 total solar eclipse. Amateurs within the zone set up telescopes and time how long it takes the asteroid to cover and uncover the star. Someone near the edge of the zone of visibility will see it blink out for only a fraction of a second because only the asteroid’s edge covers the star, while an observer at the “centerline,” where the center of the asteroid crosses the star, will see the star disappear for several seconds. If a lot of people at different places within the path participate, you can convert all the different “blink out” times to create a profile of the asteroid. Pretty incredible.

If you’re ever interested in observing an occultation, be sure to check out the International Occultation Timing Association’s (IOTA) pages. Scientists watched Ultima Thule do the same thing from nearby to give us a much better idea of its shape:

“We had an impression of Ultima Thule based on the limited number of images returned in the days around the flyby, but seeing more data has significantly changed our view,” said Alan Stern, the mission’s principal investigator. “It would be closer to reality to say Ultima Thule’s shape is flatter, like a pancake. But more importantly, the new images are creating scientific puzzles about how such an object could even be formed. We’ve never seen something like this orbiting the sun.”

I agree. Pancakes, Milk Duds — what next?