Novelists have “It was a dark and stormy night.” For planetary scientists, the equivalent cliche is, “We expect to be surprised.” The story of every new space mission seems to begin that way. No matter how intensely researchers study some solar-system object, no matter how they muster the best resources available on Earth, they are inevitably caught off-guard when they get to study it up close for the first time. And no matter how worn and familiar that cliche may sound, it also rings true every time. Nature’s creativity surpasses human imagination, time after time.
Even by those standards, the flight of the New Horizons probe past Ultima Thule tonight is something special. In the words of Alan Stern, the mission’s principle investigator and spiritual leader, “We’ve never, in the history of spaceflight, gone to a target we know less about.” It’s a type of object never seen up close before, a small (30 kilometers wide) member of the Kuiper Belt. Even more exciting, it belongs to the so-called “cold classical” region of the Kuiper Belt, meaning that it probably has remained largely unchanged for more than 4 billion years, frozen in deep storage 6.5 billion kilometers from the Sun.
Will it look battered from ancient collisions? Will it be covered with organic molecules from the early solar system? Will it resemble Pluto’s moons, or look like a fresh comet, or like something else entirely? Nobody knows.
