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The last picture of Ultima Thule shows a surprisingly smooth face.

Ultima Thule received a new shot from the mug.

The nearest image of the ancient Kuiper Belt object, taken during the flight of the New Horizon spacecraft by January 1, shows a relatively smooth face, not spoiled by the impact craters.

"This thing is simply not covered by craters," says Kelsey Singer, a planet scientist at the Southwestern Research Institute in Boulder, Colorado, on a January 24 photo.

The absence of impact scars suggests that in the Kuiper belt, the reservoir of ancient cosmic rocks beyond the orbit of Neptune, there are fewer smaller objects than scientists had expected. If this is true, it may mean that the precursors of the planets grew rapidly, without leaving behind many protoplanetary crumbs.

Pictures taken when New Horizons flew past Pluto and its moons in 2015 showed that these bodies are also remarkably smooth. Many of Pluto's craters could have been covered by geological activities on the planet of dwarves, such as the movement of glaciers (SN Online: 10/15/15). But it is believed that Charon’s largest satellite of Pluto is less active and therefore should not erase its craters (SN Online: 07/13/18). Singer and her colleagues argue that the lack of craters meant that there were not so many small objects available for hitting Pluto and Charon.

“If you can't get geology to erase craters on Charon, in particular, this is a kind of inevitable conclusion that you should simply not make craters in the first place,” says Singer.

A real test of this idea was a flyby of the Ultima Thule from New Horizons, the official name of which is MU69. If MU69 did not have small craters, which would mean that there are a relatively small number of small objects in the external solar system that it could encounter, Singer and his colleagues argue in an article published on in December.

The last image of MU69 shows only a few small craters along its upper edge – where the shadows make the edges of the crater in relief – probably left by objects about 100 meters wide. A strong indentation in the smaller of the two petals of an object can be an impact crater left by an object about 700 meters wide.

The lack of small items that could press bodies, such as MU69, could exclude some theories about how the planets and their predecessors were formed. One idea about this time in the early solar system is that the dust particles slowly stuck together to gradually build larger bodies. Another theory suggests that larger objects collided and smashed each other. But both scenarios would probably have caused many small objects to remain around the Kuiper belt today.

According to Singer, if protoplanets froze directly out of the nebula from gas and dust that preceded the formation of the solar system, they could relatively quickly grow to tens and hundreds of kilometers. This means that there would remain some small space pieces and pieces.

Researcher Alessandro Morbidelli from the Cote d'Azur Observatory in Nice, France, considers it premature to draw any conclusions. Crater expects Pluto and Charon to be unreliable, he says. And although he agrees that the MU69 is a “final test,” he says that higher resolution images are needed to see if his barely damaged image can withstand a closer look. The current best image was taken seven minutes before the nearest New Horizons approaching MU69, when the spacecraft was still at 6,700 kilometers. Better images are on the computer of the spacecraft, awaiting transmission to Earth.

New Horizons will continue to send data on MU69 until September 2020.

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