Turns Out "Space Snowman" Ultima Thule Is Flat Like a Pancake. But Why?

Avery Thompson
Photo credit: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute/National Optical Astronomy Observatory

From Popular Mechanics

On New Year's Day, NASA’s New Horizons spacecraft flew past an asteroid in the far reaches of our solar system, and the slow trickle of photos coming back to Earth is revealing the secrets of Ultima Thule.

At first, Ultima Thule appeared as a blurry, snowman-shaped smudge in the blackness of space. A few weeks ago, NASA released a higher-quality image showing craters and other terrain on the asteroid’s surface. This newest image carries a surprise: Ultima Thule appears to be much flatter than astronomers thought.

The new image is the final picture that New Horizons snapped of Ultima Thule. The spacecraft managed to capture a side view of the asteroid while flying past, and this edge-on view lets the New Horizons team calculate the asteroid's thickness. With this measurement, combined with the views from the previous images, NASA scientists can build a three-dimensional view of Ultima Thule.

They did this not by imaging the asteroid directly, but by noticing how the asteroid blocked the view of the stars as it passed by. Most of the asteroid is dark in the side view captured by New Horizons, so NASA had to guess at its shape by how it blocked starlight. Still, this technique let NASA astronomers create a pretty accurate 3D model.

Photo credit: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute

That model seems to indicate that Ultima Thule is strangely flat. Most asteroids are at least vaguely spherical, but according to the new data, the asteroid is “like a pancake,” as lead scientist Alan Stern said in a press release. “But more importantly,” said Stern, “the new images are creating scientific puzzles about how such an object could even be formed.”

A flat, pancake-like asteroid has never been seen before in our solar system, creating another mystery around Ultima Thule for scientists to solve. Is every distant asteroid pancake-shaped? Did something weird happen to Ultima Thule to make it look the way it does? There’s still plenty more information from New Horizons that will get sent back to Earth over the next few months, so perhaps some answers will be contained in a future update.

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