NASA's Mission to Pluto and the Kuiper Belt
Pluto is the largest object in the Kuiper Belt, the vast swarm of hundreds of thousands of objects orbiting the Sun beyond Neptune. Many of these objects have been undisturbed and unaltered since the beginning of the solar system, and thus harbor many secrets about the formation of the planets. After the Pluto flyby, New Horizons headed deeper into the Kuiper Belt, providing an opportunity to explore other, smaller and more primitive Kuiper Belt objects (KBOs). An extensive search with ground-based telescopes and the Hubble Space Telescope resulted in the discovery, in summer 2014, of two KBOs that were close enough to the spacecraft's trajectory that the spacecraft could be diverted to perform a close flyby of one of them.
Shortly after the Pluto flyby, in August 2015, the team chose one of these, 2014 MU69 (officially named "Arrokoth" in November 2019) as the flyby target. In early November 2015 New Horizons fired its thrusters to alter the direction of its trajectory by 1/4 of a degree, to set a course toward a close flyby of Ultima at 12:33 a.m. EST on January 1, 2019.
On the way to MU69 and beyond, the spacecraft's path cuts through the densest part of the Kuiper Belt, passing within 1 astronomical unit (the distance between the Earth and the Sun, which is about 150 million kilometers or 93 million miles) of about a dozen other known KBOs. Though none of these will appear as more than a point of light in New Horizons' cameras, the spacecraft observes them from unique angles not possible from Earth to learn about their surface properties, and searches for moons and rings with greater sensitivity than the most powerful telescopes on or near Earth. New Horizons is also taking pictures of several larger but more distant Kuiper Belt objects, including several dwarf planets, to measure their brightnesses from its unique perspective.
The approach to Arrokoth began in August 2018, when the spacecraft took its first pictures of its target, which appeared as a faint dot barely visible against a crowded field of background stars. Through Fall 2018 the spacecraft continued to take regular images of Ultima as it got closer and brighter, using them to check that it was on the right course, and firing its thrusters to make course corrections as necessary. In early December, the spacecraft performed an intensive campaign of imaging to look for any dangerous rings or moons around Arrokoth -- which it did not find.
In the last few days of the approach, the navigation team analyzed the latest images of Arrokoth taken by New Horizons to refine estimates of the KBO's position relative to the spacecraft. The team uplinked the updated information to New Horizons, so that the spacecraft could more accurately time its observations and point its cameras.
Intensive science observations began 24 hours before the flyby. The spacecraft took frequent grayscale, color, near-infrared and ultraviolet observations of Ultima as it rotated, to investigate its shape, composition, and any possible degassing, on all sides of the object. Long-exposure images of the space surrounding Ultima searched for rings or moons and determine their orbits. The closest approach observations, taken during the hour or so nearest closest approach, needed to account for the fact that Arrokoth's position was uncertain. Observations thus consisted of a series of long scans to obtain color and grayscale images, and infrared spectra, of all the possible places where the KBO might have been.
After the closest approach, New Horizons pointed its ultraviolet instrument at the Sun to look for absorption of ultraviolet light by any gases being released by MU69 (though detection of outgassing was unlikely). It also made additional searches for rings around the KBO. Four hours after the flyby, the spacecraft turned briefly to Earth to report that the flyby was successful. A few hours after that it began downlinking the roughly seven gigabytes of data acquired during the flyby.