NASA's Mission to Pluto and the Kuiper Belt
Following the flyby of Ultima Thule, New Horizons is scheduled to continue studying the Kuiper Belt through at least 2021, the limit of its currently funded extended mission.
It will take about 20 months, through late in the summer of 2020, just to return all the flyby data from Ultima Thule to Earth. But even while doing that, more Kuiper Belt Objects (KBOs) will be observed by the LORRI telescopic imager aboard New Horizons. These images will be used to study the rotation rates and surface properties of these KBOs, and also to search for satellite systems around them. New Horizons will also continue to use its space plasma and dust sensors to map out the charged particle radiation and dust environment in the Kuiper Belt, out to a distance 50 times as far from the Sun as the Earth is, just past the outer limit of Pluto's orbit. During this time New Horizons will also be mapping the interplanetary hydrogen gas that fills the Kuiper Belt from the solar wind. These studies will improve on what the Voyagers could do when they traversed this region because the sensors on New Horizons are much more advanced over 1970s-era Voyager technology.
It is also possible that another flyby target can be found and reached with the remaining fuel supply. And after that? Another exciting possibility is that we can dramatically augment New Horizons' capabilities by uploading new observing and onboard data reduction software once its flyby software is no longer needed. If NASA someday approves such a plan, New Horizons could survey the Kuiper Belt population in ways that no other mission or telescope on Earth or in Earth orbit can, and possibly even detect and hunt down its own next flyby target.
Future New Horizons extended missions, if funded by NASA, could explore even farther out. The spacecraft is on an escape trajectory from the Sun, traveling about 3 Astronomical Units from the Sun per year. (An Astronomical Unit, or au, is the distance from the Earth to the Sun.) Moreover, New Horizons and its payload sensors are healthy and operating perfectly. The spacecraft has enough power and fuel to operate for 15-20 more years, perhaps enough to reach the boundary of interstellar space.