How Ultima Thule Is Like a Sticky, Pull-Apart Pastry

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https://www.nytimes.com/2019/03/18/science/ultima-thule-new-horizons.html

Source:  By Kenneth Chang, The New York Times.

Excerpt: ...New Horizons traveled some 4 billion miles to take a close-up look at this 22-mile-long world in the solar system’s icy Kuiper belt beyond Neptune; it is officially designated as 2014 MU69 and nicknamed Ultima Thule. ...a small shard that has been frozen and almost unchanged since it formed 4.5 billion years ago. Ultima Thule was ... full of surprises. The shape turned out to be unlike anything seen in the solar system. ...two objects that at some stage touched and stuck together, like a snowman. Then the scientists discovered that the two lobes of Ultima Thule were not spherical, but more like lumpy pancakes. The larger lobe is also flatter than the smaller one. ...a change from older ideas of planetary formation: that objects called planetesimals grew slowly and steadily in size. Ultima Thule suggests an alternative scenario in which a bunch of similarly sized objects merged together over a shorter interval of time. ...With the flattened shape of the two lobes of Ultima Thule, another surprise was that the rotational axis of each were nearly parallel to each other. William B. McKinnon of Washington University in St. Louis, a co-investigator on the mission, said the finding strongly suggested that the two lobes formed out of one rotating cloud of icy particles. When the two finally touched, the collision was slow, about 2 meters per second, or 4.5 miles per hour. “Which, if you take a brisk walk into a wall, you’ll find out what that’s like,” Dr. McKinnon said. “But I don’t recommend you do that.”...

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