Days ago, a leftover from the solar system's formation was minding its own business more than a billion miles beyond Neptune's orbit. Then, just as humans back on Earth celebrated the new year, a robotic explorer—NASA's New Horizons spacecraft—flew by the object at ten miles a second, snapping pictures and beaming them back home.
Today, the New Horizons team shared with the world the first full-fledged images of the space rock, which is officially called 2014 MU69 and nicknamed by the team Ultima Thule.
“It was just a day and a half ago, barely 36 hours, that New Horizons swept down over Ultima Thule in a technical success beyond anything ever attempted before in spaceflight,” New Horizons principal investigator Alan Stern said at a Wednesday press briefing. “We're basically chasing it down in the dark at 32,000 miles an hour.”
Way back in 2018, researchers had speculated that the object was shaped like a bowling pin. Now we know that they have a frosty, 21-mile-tall snowman on their hands. The latest imagery of MU69 reveals that it is actually made of two roughly spherical lumps of rock and ice, seemingly welded together in a gentle collision and pirouetting in space every 15 hours or so. The images also show that MU69's two lobes measure 12 and nine miles across and are a ruddy, mottled brown.
“This is like potting soil ... it's a very dark object,” Southwest Research Institute scientist Cathy Olkin, a deputy project scientist on the New Horizons team, said during a press briefing. “But it shows significant [brightness] variation—a factor of two—across its surface.”
The object's reddish tones are consistent with other primitive bodies in the solar system. It's thought that the color comes from carbon compounds called tholins, a reddish schmutz that forms as exotic ices of nitrogen and methane get pounded by high-energy particles from the sun. During a 2015 flyby, New Horizons saw similar hues on Pluto and its largest moon Charon, whose northern pole—nicknamed Mordor Macula—is covered with what look like bloodstains. (See behind-the-scenes pictures from the full New Horizons mission.)
Tip of the iceberg
The new images of MU69 don't yet reveal many surface features, as the sun's angle during the flyby didn't let the spacecraft see many shadows. But at a resolution of 150 yards per pixel, scientists can make out slight variations in MU69's surface texture, including peaks and troughs that extend more than a mile in height. Rounded brighter spots give way to darker streaks or ridges. Crimps atop MU69's smaller lobe may well outline the boundaries of a plateau.
And if MU69 is a snowman, it seems to be wearing some kind of necklace. The region where the two lobes meet is lighter in color and more reflective than most of MU69. It's possible that the difference is due to many tiny particles that built up along the circular ravine, or it could be because the “neck” is chemically different from the surrounding rock.
In 1930, Clyde Tombaugh discovered Pluto when he compared this image (with Pluto circled) with another taken six days earlier and noticed the bright speck had moved.
The two lobes' nearly spherical shapes and lack of craters give some clue to how MU69 formed. Within a few hundred thousand years of the solar system's formation, innumerable tiny pebbles swirled together, accreting as they went. Eventually, only the two lobes were left as separate bodies orbiting one another. As the two lobes kept losing flotsam and jetsam, they inched closer together to conserve the momentum of their dance. Then, at a mere walking pace, the two objects kissed, forming a union that has lasted some 4.5 billion years.
“These are the only remaining basic building blocks in the backyard of the solar system ... that everything else we live on, or see through our telescopes, or visit with our spacecraft were formed from,” New Horizons team member Jeffrey Moore, a scientist at NASA Ames Research Center, said during the briefing. “I think we should think of New Horizons as a time machine which is set to time zero.”
For now, though, New Horizons has transmitted back less than one percent of the data it stored from the flyby, and it will take 20 months for the spacecraft to complete the transfer.
“Everything we're going to tell you,” Stern said at today's briefing, “is just the tip of the iceberg.”