On 14 July 2015, NASA’s New Horizons spacecraft made the first-ever flyby of distant Pluto. Amid the excitement of this tremendous event, an oft-repeated and celebrated quote was “we have completed the initial reconnaissance of the Solar System.” When it comes to science, completed is a big word, and the Solar System an even bigger place. What does it mean to complete an initial reconnaissance, and are we really done?

The Solar System is full of mileposts. For most of the 85 years since Clyde Tombaugh discovered Pluto, it has been the most distant of those mileposts. Delivering a spacecraft to anywhere in space isn’t easy, much less after a 7.5-billion-km ultramarathon. Making it to Pluto and learning more about it is an historic scientific and technological accomplishment.

But there’s more to it than that. The Solar System is not just 8 (formerly 9) planets. Indeed, the Kuiper Belt (and the Oort Cloud beyond) is the most populous collection of bodies and contains other large objects, from Eris, comparable in size to Pluto, to the football-shaped Haumea. Pluto is the first to be reached, and after traveling another billion miles, New Horizons aims to fly by another object in the Kuiper Belt in early 2019. One thing that we have learned in exploring the Solar System is that planetary bodies are usually far more complex than we expect, and the data from New Horizons have reinforced that fact since the very first image came down. There is so much left to explore—and not just beyond Pluto.

The Voyager spacecraft provided first-look flybys of the gas giant planets in the outer Solar System, similar to how New Horizons explored Pluto. However, our knowledge of these worlds and particularly their orbiting moons is not complete, especially for Uranus and Neptune. Indeed, small new moons continue to be discovered. Yet, the Galileo mission to Jupiter in the 1990s and the Cassini spacecraft currently around Saturn have demonstrated that from the tectonics and oceans of Europa, Ganymede, and Enceladus, to the atmosphere and seas of Titan, the icy worlds of the outer solar system have a lot to teach us about how the Solar Systems works. Fortunately, new missions by NASA and the European Space Agency (ESA) to study Jupiter’s icy moons up close are on the (distant) horizon. However, with two other giant planets, there is much undiscovered country to explore.

While we have seen much more of the more Earth-like planets and moons of the inner Solar System, our exploration here is also not complete. Furthermore, the inner Solar System has other inhabitants. NASA’s Dawn spacecraft and its visits to Vesta and Ceres and ESA’s Rosetta mission to comet 67P/Churyumov-Gerasimenko have shown how reconnaissance of the much smaller and more numerous planetary residents and visitors of the inner regions of the Solar System must continue.

The breathtaking data from Pluto serve as a reminder that the Solar System has much to teach us; we just have to keep exploring.

—Steven A. Hauck II, Editor in Chief, Journal of Geophysical Research: Planets; email: [email protected]