Scientists understood little about Ultima Thule before New Horizons arrived. We knew its orbit, its rough size (about 18 miles [30 kilometers] in length), its oblong or perhaps binary shape, its reddish color, and that it lacks large moons.

One key distinction about Ultima Thule is that its type, among the so-called cold classical KBOs, all but guarantees that it originated where it orbits now. So unlike most KBOs, it was not formed closer to the Sun and warmed. Unlike Pluto, Ultima Thule is too small to have sustained strong geologic activity for very long. Because of these two facts, Ultima Thule preserves its primordial nature better than every other object explored by spacecraft. Thus, it presented scientists their first look at what the original, primordial planetesimals of the outer solar system were like when they formed.

Flying to Ultima Thule thus became the planetary exploration equivalent of an unprecedented archaeological dig into the most ancient past of our solar system and the first building blocks of its planets. Although we knew this ahead of time, we didn’t know what the observations of it would reveal. That suspense was a big part of the scientific attraction of the planned flyby.

When you don’t know what you’re going to find, a good way to design your reconnaissance is to keep your eyes as wide open as possible. So we planned to fly much closer to Ultima Thule than we did to Pluto in order to get higher resolution observations. We also planned to use all the scientific instruments aboard, even those that probably wouldn’t find anything. Why? Because if they did surprise us with detections, those revelations would be incredibly valuable.

In addition to our plans to map Ultima Thule at higher resolution than Pluto — in color and in stereo — we also planned to map its surface composition, take its temperature, measure its radar reflectivity, and search for small moons, which are common among cold classical KBOs. We planned to search for an atmosphere as well, though there were sound scientific reasons why one should not exist, and to search for orbiting dust or rings, even though neither had ever been seen around any small KBO.

We even planned to search for Ultima Thule’s effect on the solar wind and charged particle environment, though computer models told us any such effects would be minuscule and undetectable. In effect, we planned to throw everything New Horizons had at exploring Ultima Thule.

What we found

Never before had such a pristine, unmodified object from the birth era of the planets been explored up close, and the results didn’t disappoint.