Why explore the Kuiper Belt?

Ultima Thule is just one of thousands of objects that call the Kuiper Belt home, ranging from dwarf planets to comets.

The Kuiper Belt extends from around 4.5 billion kilometres from the Sun (Neptune) to 7.5 billion kilometres from the Sun.

The first small object — 1992 QB1 dubbed "Smiley" — was discovered in this region in 1992. Since then more than 2,000 objects have been discovered.

Some of these worlds rival Pluto in size, but most of the worlds are only tens to hundreds of kilometres across.

"We have a pretty good understanding of these worlds. Many of them have … moons, some even have rings," Dr Stern said.

These small worlds are ancient time capsules left over from the birth of the solar system 4.6 billion years ago.

"We know that Ultima Thule was born at this very great distance from the Sun and has always been in that region of the solar system."

At that distance temperatures are freezing — almost absolute zero or -273 degrees C.

"Those temperatures should preserve the record of the formation of Ultima Thule very faithfully over all those billions of years," Dr Stern said.

"No-one has ever been to anything like this, that's so well preserved from the birth of the solar system, and so we don't know what quite to expect."

Flying through the densest part of the region, New Horizons will pick up details never imagined by previous missions such as Voyager.

The Voyager spacecraft made their way above and below the Kuiper Belt in the 1990s, but were "blissfully unaware" of its existence.

"The Voyagers didn't even look at the Kuiper Belt because they didn't know there was a Kuiper Belt to look at," Dr Stern said.

"And of course they had, by today's standards, very primitive instrumentation based on 1970s technology."

"But never before New Horizons have any of these objects been studied up close with cameras and spectrometers and any of the gear we're taking along."