Science Highlights from NASA's New Horizons Mission

NASA's New Horizons (NH) mission was the first to explore Pluto and the Kuiper Belt in situ. Launched on January 19, 2006, the spacecraft successfully executed close flybys of Jupiter on February 28, 2007, Pluto on July 14, 2015, and the small Kuiper Belt object (KBO) Arrokoth on New Year's Day 2019. NH transformed Pluto from a barely resolved astronomical target into a geologically complex and diverse world with hints of a subsurface ocean. NH data showed Charon to be very different from Pluto, with a surface dominated by water ice but sprinkled with ammonia-bearing ices near some craters, a giant chasm nearly encircling its equator, and a reddish polar hood. Pluto's small satellites were resolved for the first time by NH, and spectra of Nix and Hydra showed deep absorption bands of water ice, strengthening the hypothesis that these small satellites formed similar to 4.5 billion years ago in the aftermath of the same cosmic collision that produced the Pluto-Charon binary. NH observations revealed Arrokoth to be a "contact binary" with two highly flattened lobes and an organic rich surface showing traces of methanol ice but essentially devoid of water ice. The NH Arrokoth data provide some of the first detailed evidence for the "pebble cloud collapse" streaming instability model of planetesimal formation in the solar nebula. NH returned unique scientific results during its flyby of Jupiter; data from its plasma instruments during the mission's cruise phases are revealing new insights into the solar wind and the outer heliosphere, and NH observations of distant (i.e., non-flyby) KBOs provide data at geometries unattainable from Earth-based facilities, enabling unique results on light scattering from KBO surfaces. The NH instruments and spacecraft are still as capable as they were at launch more than 17 years ago, and they are continuing their exploration of the outer solar system and beyond during a recently approved second extended mission phase.