UPDATE: 2014 MU69 is now officially named Arrokoth
A wealth of data was collected when the New Horizons' spacecraft flew past the small Kuiper Belt object, 486958 2014 MU69, in January. Since it will be a long time before we get such a close encounter with an object like this again, everything we have learned will be picked over in great depth. The first published study has confirmed suspicions this is a largely untouched relic of the early Solar System, the first time we have visited such a definitively ancient object with a spacecraft.
Somewhat controversially nicknamed Ultima Thule, MU69 has been in a sufficiently stable orbit since the Solar System formed 4.5 billion years ago that it has never come close enough to the Sun to get warmer than 60 K (-352 ºF) and has had few encounters with other substantial objects, leaving it little changed for billions of years.
In Science, a team of almost 200 authors describe their target as “A bilobed contact binary with a flattened shape [and] discrete geological units,” 30 kilometers (18 miles) in diameter. In other words, it is made up of two objects of unequal size that have become joined together. The flattened shape resembles that of Saturn's moons Atlas and Pan, but no object we've previously seen orbiting the Sun.
At its closest New Horizons was 3,538 kilometers (2,200 miles) from MU69's surface. Traveling at more than 14 kilometers a second (31,500 mph) it didn't get long to take photographs, particularly in such dim light.
New Horizons collects data quickly, but transmits it only slowly. We won't have access to everything it detected until the middle of next year, and the paper was written based only on the first 10 percent delivered up to March.
Rather than being smashed together in a high-energy collision, the two parts of MU69 appear to have come together so softly, probably at a rate of just a few meters per second, their encounter caused no fractures or deformation. Such a gentle merger is almost impossible today, and probably took place during the collapse of the cloud that eventually became the Solar System. Although there are a few craters indicating subsequent hits from smaller asteroids, these are rare compared to a similar sized object closer to the Sun, where space gets much more crowded. Even some of what look like craters are thought to be outgassing pits.
MU69 rotates once every 16 hours. Unfortunately, without observing any moons, we were unable to measure its mass, and therefore density.
MU69 has some curiously bright patches, particularly around the “neck” where the two lobes meet. The paper proposes several possible causes, acknowledging the neck may have a different explanation from the other patches, but treats an accumulation of finer particles as the favorite. We'll have to wait for the next batch of data to learn more about the farthest world we've ever visited.
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