If you want your New Year celebrations to be truly out of this world, then you might consider stopping by the New Horizons website. Following on from its phenomenally successful flyby of Pluto, the spacecraft will perform its closest flyby of a small Kuiper Belt object at just after midnight in the US Eastern time zone—the one where the operations center of the Johns Hopkins University Applied Physics Laboratory is located.

Indications that New Horizons survived the flyby—rather than running in to a small moon or ring—will have to wait for the roughly six hours it takes light to travel from the location of 2014 MU69, the target of its attentions. Nicknamed Ultima Thule, the object is a small ball of ice in the distant Kuiper Belt, which is a large collection of small bodies that froze out of a disk of gas and dust early in the Solar System’s history.

The full extent of our knowledge of Ultima Thule’s surface is that it’s about 30 kilometers across and not perfectly spherical, based on its occultation of a background star. It only reflects about 10 percent of the sunlight that reaches it, but imparts a reddish tint to what little light escapes. Beyond that, everything New Horizons sees will be a completely new discovery. NASA was still scanning Ultima for moons and rings in mid-December before committing to a flyby that will take New Horizons three times closer than it came to Pluto.

We can make some inferences based on what we know about the Kuiper Belt, though. Ultima Thule’s largely circular orbit means that it’s almost certainly spent its entire existence over six billion kilometers from the Sun. At that distance, there wasn’t as much of the material that formed the planets of the inner Solar System. Compensating for that somewhat is that the dim sunlight in the Kuiper Belt left it past the “snow line” for a variety of gasses, meaning those gasses froze out to form particles. The end result was a collection of small icy bodies, the vast majority smaller than Pluto.

New Horizons’ flyby of Ultima Thule is our first opportunity to see one of them up close, and will allow us to make inferences about the composition of other objects in the Kuiper Belt.

The mission will create a number of additional firsts, as this will be the most distant flyby we’ve attempted. The Voyagers’ cameras were shut down by the time they reached six billion miles from the Sun, meaning every photo New Horizons takes will be the most distant image we’ve ever captured.

As mentioned, the 6.5 billion mile distance means that signals from New Horizons take about six hours to reach Earth. The spacecraft will also be pointing its instruments at Ultima Thule, rather than its primary transmitter at Earth, meaning that we’ll have to wait for after the flyby to receive any high-resolution images of the object. If everything goes according to plan, we’ll have plenty of New Year’s presents awaiting us in 2019 as we get our first ever close look at a Kuiper Belt object.

https://arstechnica.com/?p=1433655