In 1610, Galileo peered through his telescope and spotted four bright moons orbiting Jupiter, dispelling the long-held notion that all celestial bodies revolved around the Earth. In 2024, when scientists expect to send the Europa Clipper spacecraft to investigate one of those moons, they too may find evidence that fundamentally alters our understanding of the solar system.

Europa is the sixth nearest moon to Jupiter and is roughly the same size as our own. Thanks to data retrieved by the Galileo space probe—launched in 1989 and named to honor the Italian astronomer—and the Hubble Space Telescope, scientists are almost sure that a salty, liquid ocean is hidden beneath Europa's icy surface, one so large that astronomers believe it could contain two times the water in all of Earth's oceans combined.

Europa itself has been around for 4.5 billion years, but its surface is geologically young, only about 60 million years old, suggesting that it has been continually resurfaced, perhaps through a process much like Earth's shifting plate tectonics. As Europa travels around Jupiter, its elliptical orbit and the planet's strong gravitational pull cause the moon to flex like a rubber ball, producing heat that's capable of maintaining an ocean's liquid state.

When 'Oumuamua, the first interstellar object ever observed passing through the solar system, was discovered in 2017, it exhibited some unexpected properties that left astronomers scratching their heads. Its elongated shape, lack of a coma, and the fact that it changed its trajectory were all surprising, leading to several competing theories about its origin: was it a hydrogen iceberg exhibiting outgassing, or maybe an extraterrestrial solar sail (sorry folks, not likely) on a deep-space journey? We may never know the answer, because 'Oumuamua was moving too fast, and was observed too late, to get a good look.

It may be too late for 'Oumuamua, but we could be ready for the next strange interstellar visitor if we wanted to. A spacecraft could be designed and built to catch such an object at a moment's notice. The idea of an interstellar interceptor like this has been floated by various experts, and funding to study such a concept has even been granted through NASA's Innovative Advanced Concepts (NIAC) program. But how exactly would such an interceptor work?