A Journey Into the Solar System's Outer Reaches, Seeking New Worlds to Explore

It was the common view of the time: All of the solar system’s big, interesting things — the sun and the nine planets — were behind Pioneer 10. (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 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.

“The Kuiper belt object studies are revolutionizing all of solar system studies,” said Renu Malhotra, a professor of planetary sciences at the University of Arizona.

Even farther out might be bodies the size of Mars or Earth, or even a larger one some astronomers call Planet Nine, and technological advances could usher in a new age of planetary discovery.

On Tuesday, New Horizons, the NASA spacecraft that snapped spectacular photographs of Pluto in 2015, will provide humanity with a close-up of one of these mysterious, distant and tiny icy worlds.

Its target of exploration is believed to be just 12 to 22 miles wide, known as 2014 MU69 — its designation in the International Astronomical Union’s catalog of worlds — or Ultima Thule, the nickname bestowed upon it by the New Horizons team.

This will be the farthest object ever visited by a spacecraft.

New Horizons will speed past Ultima Thule at 31,500 mph and pass within 2,200 miles of the surface. What the probe finds could reveal much about the earliest days of the solar system and what else lies in the Kuiper belt.

“I’m more excited to see it than I was of Pluto,” said Harold F. Levison, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. “It’s going to be really cool.”

This article originally appeared in The New York Times.