New Horizons then set out an extended mission, which centered on a close flyby of Ultima Thule, which is officially known as 2014 MU69. This object is about 21 miles (34 kilometers) long and lies 1 billion miles (1.6 billion km) beyond Pluto's orbit. (Ultima is currently 4.1 billion miles, or 6.6 billion km, from Earth.)
Initial imagery taken during New Horizons' approach suggested that Ultima Thule is shaped like a bowling pin. But that impression changed shortly before closest approach, which occurred just after midnight on New Year's Day and brought the probe within 2,200 miles (3,540 km) of the mysterious body. Photos snapped around that time indicated that Ultima Thule is composed of two lobes, both of which appeared to be roughly spherical.
A snowman, with a distinctly reddish hue.
The old (top) and new (bottom) shape-model views of the distant object Ultima Thule, which NASA's New Horizons spacecraft flew by on Jan. 1, 2019. Mission team members initially thought Ultima Thule resembles a snowman but now believe the object to be flattened. The dashed blue lines represent uncertainty, indicating that Ultima Thule could be either flatter than, or not as flat as, depicted in this figure.
But the newly released images have forced another rethink. New Horizons took the long-exposure photos about 10 minutes after closest approach; the central frame in the sequence was snapped from a distance of 5,494 miles (8,862 km), mission team members said.
The new views were captured from a different angle than the snowman-suggesting photos, and they show Ultima Thule's outline against a number of background stars. By noting which of these stars went dark as Ultima blocked them out, mission scientists were able to map out the object's (surprisingly flat) shape.
"This really is an incredible image sequence, taken by a spacecraft exploring a small world 4 billion miles away from Earth," Stern said. "Nothing quite like this has ever been captured in imagery."