European Space Agency (ESA) has a remarkable history of successful missions to explore comets, despite the many challenges associated with visiting these unpredictable and active bodies on highly eccentric orbits. They are high value targets for scientific exploration, as they preserve, to a greater or lesser degree, the ices formed at the birth of our Solar System. The Giotto mission was ESA’s first deep-space mission1. It returned the most detailed images of the nucleus of comet 1P/Halley2, as part of the armada of spacecraft that encountered it in 1986. Although Giotto was damaged by dust grain impacts during its high-speed (68 km s−1) close (600 km) encounter, it returned data that set the framework for our understanding of comets ever since: proving the nucleus was small and dark, with ‘jets’ of activity from discrete areas. Three decades later, ESA again excelled itself with the Rosetta mission to comet 67P/Churyumov-Gerasimenko; the first spacecraft to orbit a comet, providing an unprecedented two year investigation of its evolution as it approached and receded from the Sun, and the first soft landing on a comet nucleus, delivering the Philae lander to perform experiments on the surface3. Rosetta has led to a revolution in cometary science, and the processes of planet formation, with more than 1000 papers already published; some of the key results are summarised in early review articles [e.g. refs. 4,5,6].

Work on the rich dataset from this mission will continue for many years, but already it has raised new questions that will require a future mission to answer. Some of these, such as how the smallest building blocks were originally assembled, and where the constituent particles formed, will require an even more ambitious (and large) future mission to return a sample of a comet for study in laboratories on Earth. The comet science community is already planning such a mission, but it is many decades away7. One of the surprising results from Rosetta was the extent to which the nucleus surface had been altered, by erosion or fall back of material, as a result of 67P’s repeated close perihelion passages8. This motivates study of more pristine comets that have not undergone multiple cycles of activity. Comet Interceptor will be the first mission to visit a Dynamically New Comet (DNC), i.e. one entering the inner Solar System for the first time since it formed, and only beginning to experience activity due to being close enough to the Sun to raise the temperature above the sublimation point of its constituent ices.

DNCs are a subset of the long period comets that come from the Oort cloud, at the very edge of our Solar System, reaching half-way to the next star. Most comet missions to date have visited short period comets, like 67P, which come from the less distant Kuiper Belt region, and have experienced heating on many orbits near to the Sun. Oort cloud objects were scattered to this distant reservoir during the formation and early evolution of our Solar System, and have been preserved there ever since. They are therefore some of the most pristine ‘building blocks’ from the era of planet formation, and, when finally scattered back into the inner Solar System to feel the warmth of the Sun, produce some of the most spectacular bright comets. The high activity levels, and therefore brightness, of such comets means that they have been popular telescopic targets over the years, but are difficult to explore with spacecraft. However, the primary reason they have yet to be visited is that their appearance is fleeting—they are typically discovered only a few months to a year before they pass perihelion, before returning to the distant outer Solar System, with orbital periods of tens of thousands of years, which mean they will be seen only once by humanity. This has always been incompatible with the timescales to design, build and launch a space mission, which can take decades.