Weirdly shaped, very dark: Ultima Thule is revealing the outer solar system’s secrets


NASA’s New Horizons spacecraft successfully flew past the most distant world ever explored by humans on New Year’s Day. Now, a clearer picture of the 22-mile-long object known as Ultima Thule is beginning to take shape — and it has scientists baffled.

“We’ve never seen anything like this,” said Alan Stern, principal investigator for the New Horizons mission. “It’s a mystery.”

The New Horizons spacecraft zoomed past Ultima Thule on Jan.1 at a speed of 32,000 mph, furiously collecting data as it went. At the moment of closest approach it was about 2,200 miles from the small world’s surface, or roughly the distance from Los Angeles to Washington, D.C.


Ultima Thule is so small and far from the sun that before New Horizons hurdled past it, scientists had little idea what it might look like. The spacecraft revealed that it is what is known as a contact binary object, which means it is made of two distinct lobes that fused together long ago.

“It looks like two things stuck together, which is exactly what a contact binary is,” Stern said.

The first images sent back by New Horizons suggested that it resembled a snowman, but as more data have trickled in from the spacecraft, that initial impression has shifted.

The New Horizons team now thinks that Ultima Thule looks more like a snowman that has been tipped on its side and smushed. Imagine if a kid made two balls out of clay, stuck them next to each other and then pressed them both down with the palm of her hand.

“These are three dimensional bodies, but they are not spheres,” said William McKinnon, a New Horizons investigator from Washington University in St. Louis. “And they are stuck together in a very specific way — end to end.”


The researchers have also determined that the larger lobe known as Ultima is significantly flatter than the smaller lobe — Thule.

“We still don’t know the answer why, but we are working on it,” McKinnon said.

Part of what makes Ultima Thule so fascinating to scientists is its pristine condition. It is 4.1 billion miles from Earth and receives so little light and warmth from the sun that its chemistry and structure have remained frozen in time since it formed 4.5 billion years ago, Stern said.

In addition, it is part of the slow-moving, sparsely populated region of the solar system known as the Kuiper Belt, where impacts are infrequent and gentle when they do happen.

That low-frequency, low-intensity environment has helped ensure that Ultima Thule has remained intact and essentially unchanged for billions of years.

“It provides a treasure trove of information about the birth of the planets,” Stern said.

The geographical features of Ultima Thule, including the lack of stress fractures on its surface, indicate that the two lobes were initially in orbit around each other and came together gently at a speed of no more than 7 mph, scientists said.

They believe the two objects were once in orbit around each other, but that orbit shrank and shrank until the two distinct bodies merged into one.


Both Ultima and Thule have lumpy-looking surfaces that resemble monkey bread. In fact, the New Horizons team thinks the two may have formed in a similar way with individual space rocks sticking together, just like how you put individual balls of dough next to each other in a pan to form one loaf of the sweet bread.

Scientists have also determined that Ultima Thule’s surface is extremely dark and very red. The darkest areas reflect just 7% of the dim sunlight it receives while the brightest areas reflect about 14% of that light.

It appears to be composed of water ice, organic compounds and methanol, and its topography includes rolling hills, troughs and pits.

“It’s a small world with mouthwatering geology,” said Kirby Runyon, a New Horizons science team member from the Johns Hopkins Applied Physics Laboratory in Laurel, Md.

The New Horizons spacecraft will continue to send data from its Ultima Thule close encounter for at least another year. Scientists are hoping that somewhere in that data they will find evidence of a satellite around Ultima Thule, which would help them resolve the object’s density.

“We know that 35% of cold classical Kuiper Belt objects have satellites,” Stern said. “We haven’t seen one yet, but we have more sky around this object to explore.”


In the meantime, the mission team is already thinking about what New Horizons might explore next.

“We will be in the Kuiper Belt until the late 2020s, so we have almost a decade to search for another object and hopefully find something,” Stern said. “In that happens we will be probing accretion not 4 billion, but 5, 6 or 7 billion miles from the sun.”

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