In 2015, a NASA spacecraft snapped spectacular photographs of Pluto, forever changing humanity’s view of that world. On Tuesday that same probe, New Horizons, will provide a closeup of the farthest object ever visited.

New Horizons will speed past an object nicknamed Ultima Thule at 31,500 miles per hour and pass within 2,200 miles of the surface, seeking clues to the earliest days of the solar system. Ultima Thule is four billion miles from the sun, in an area where many astronomers within recent memory believed there wouldn’t be much that was worthy of study.

[READ: How to follow the New Horizons flyby of Ultima Thule.]

It was once a common view that all of the solar system’s big, interesting things — the sun and the nine planets — had been found. When NASA’s Pioneer 10 spacecraft crossed the orbit of Neptune in June 1983, some newspaper headlines declared that it had left the solar system. (Pluto was still a planet then, but it was at the innermost part of its orbit and closer to the sun than Neptune.)

Thirty-five years later, the Kuiper belt — the region Pioneer 10 was just entering and that New Horizons continues to explore — and the spaces beyond are perhaps the most fascinating parts of the solar system. In their vast, icy reaches are clues about how the sun and planets, including ours, coalesced out of gas and dust 4.5 billion years ago.