The ocean-filled moon might hold the life we’ve long searched for in space. And scientists have one shot to reach it. ... Everything we know about life says that it needs water. Conversely, every place on Earth where water exists, life does too. The conventional thinking, then, is that if you want to find alien life, the first thing you look for is alien water. Europa is the wettest known world in the solar system. Life also needs food and energy. Europa scores there too: Its ocean might be nourished by a drizzle of organic chemicals and stirred by volcanic vents like the ones dotting the mid-Atlantic ridge. If any place in the solar system holds the answer to the “Are we alone?” question, it’s a good bet that Europa, not the Red Planet, does. ... Which is not to say that getting the answer will be easy—not by a long shot. To give you a sense of exactly how hard it will be, consider three more numbers: 600 million—the average flight distance, in miles, from Earth to Europa, meaning that the journey there could take at least six years; 500—the average radiation dose, in rem per day, on Europa’s surface, enough to fry unprotected spacecraft electronics within a matter of days; and 10—the average estimated thickness, in miles, of Europa’s ice shell, more than four times as thick as the glaciers covering Antarctica. Overcoming those numbers will test the limits of human ingenuity. But a growing chorus of scientists has argued that we must try. ... most of the liquid water in the solar system is found not on the surface of rocky worlds like Earth but inside icy bodies like Europa. That raises the stakes for NASA’s upcoming mission. If we find evidence of life on Europa, it would point to a whole new class of habitable worlds across the solar system, and probably across the universe.

Europa truly does represent a singular chance. Crossing 800 million kilometers with a sizable, robust payload will require vast sums of money—there won’t be a second chance. But Europa represents a gamble in another sense, too. No one knows whether NASA will discover a frozen, dead world far from the Sun or if the organization will make the most profound of discoveries just below the ice. ... During the last decade, NASA has recast its human and robotic space exploration programs around the search for life both in our Solar System and beyond. Much of this effort has focused on Mars, which is relatively close to Earth and may have harbored life in the past. Culberson has pushed the agency further to seek extant life. He and a lot of scientists believe the best place to find extraterrestrial life in the Solar System lies in deep oceans below Europa’s inhospitable surface. ... Notably, the JPL team thinks it has solved the vexing problem of planetary protection, the concern that any stray microbes from Earth could contaminate Europa’s ecosystem. The solution has come straight out of the pages of science fiction—the lander mission will be the first interplanetary spacecraft to carry a self-destruct mechanism.