We’re about to find many more interstellar interlopers—here’s how to visit one

A long-period comet is an icy visitor from the farthest reaches of the Solar System that has spent little time getting blasted by the Sun’s heat and radiation, freezing its physical and chemical properties much as they were billions of years ago.

Long-period comets are typically discovered a year or two before coming near the Sun, still not enough time to develop a mission from scratch. With Comet Interceptor, ESA will launch a probe to loiter in space a million miles from Earth,…