On January 1, 2019, the New Horizons spacecraft will make a close approach to MU69, also known as Ultima Thule. Astronomers worldwide are excited at the data to be collected, the first good insight we will have of the composition of what is known as the Cold Classical Kuiper Belt, thought to represent the remnants of the disk from which the planets formed. The story of how astronomers found this target, however, may be as impressive as anything the images will reveal.
When New Horizons was launched in 2006, the focus was on Pluto, then still considered a planet, with some interest in Charon and the smaller moons discovered the year before. As the encounter approached, NASA realized there might be enough fuel left to direct New Horizons to a new horizon, provided one could be found not too far from its post-Pluto trajectory.
There was little doubt that such an object existed. Pluto had been demoted to dwarf planet status because so many more distant objects, some of them only slightly smaller, had been found within the Kuiper Belt. If Pluto had been almost anywhere else in its orbit at the time, there would have been an abundance of targets to choose from.
However, as astronomer Dr Alex Parker of the Southwest Research Institute explained in 2016, there was one very large problem. Pluto's location at the time meant we were seeing it against a background of the galaxy's center. Photographs of that portion of the sky were filled with so many faint stars that finding objects in our Solar System was horrendously difficult.
Parker was part of a small team that raced against time to pick the Kuiper Belt needles from the haystack of background stars. The challenge was that even if they found a suitable object, the later it was detected, the sharper the course correction to redirect the plucky spacecraft, and therefore the more fuel required.
Earthbound telescopes struggled to distinguish targets from stars and determine their orbits. The Hubble Telescope could do both those things, but its small field of view meant it would take a ridiculous amount of its precious time to do the job on its own. Parker and colleagues first used techniques to eliminate the stars from images, and then found ways to use other telescopes to select the most promising areas for Hubble to look.
The effort yielded more than 50 new Kuiper Belt Objects, but only two of them had suitable orbits. At 30 kilometers (20 miles) across, Ultima Thule might be smaller than astronomers would have picked if they could have chosen any Kuiper Belt Object to visit, but it was almost the only choice available, and will be the oldest and most distant object we have visited.