TO PLUTO AND BEYOND

While the Pioneer and Voyager missions focused on the four giant outer planets, New Horizons whizzed past these with barely a glance. Its area of interest lay farther out, in the Kuiper Belt. This distant region of the Solar System, beyond the orbit of Neptune, was hardly known to astronomers when the earlier probes left Earth, but has attracted increasing attention since. For decades the only known Kuiper Belt object was Pluto. This was New Horizons’ first destination, which it reached in July 2015 after a journey of nine-and-a-half years.

A flyby of Pluto, rather than Saturn’s moon Titan, had been on the cards for Voyager 1, but it lost out because it was expected to be a dead world. The images and data sent back by New Horizons proved how mistaken this was. With an atmosphere, geological activity and possible cryovolcanoes, Pluto is as fascinating as any full-blown planet. Three-and-a-half years after its Pluto encounter, New Horizons passed a second – and much smaller – Kuiper Belt object. Called Arrokoth, it’s the most distant body to have been photographed at close range by a spacecraft.