Rick Robinson

May 15th 2019

A Visit to Ultima Thule: Exploring the Kuiper Belt


Four billion miles from the sun, at the edge of the solar system, orbits an object — or pair of objects — that, according to Ars Technica, is known officially only as 2014 MU69. Until a couple of months ago, it was almost completely unknown, even to scientists.

Now, it has become the latest of our new space discoveries and is far better known as Ultima Thule, the most distant object ever visited by a human spacecraft. NASA’s New Horizons space probe flashed past these tiny distant joined worlds on New Year’s Day at a speed of more than 32,000 miles per hour. At closest report, says, the spacecraft passed within 2,200 miles of Ultima Thule, snapping high-resolution imagery as it hurtled past.

A “Snowman” in Space

Images and other data from this fast close encounter are still coming in. Ultima Thule orbits some 4.1 billion miles from Earth, according to NASA, and Sky & Telescope reports that it will take years for New Horizons to send back all the data it gathered in those few short hours on New Year’s Day.

In fact, according to Sky & Telescope, so far less than 1% of the data has been sent back, but even that opening teaser is startling. has described Ultima Thule as resembling a snowman, a pair of roughly spherical mini-worlds stuck together. Sky & Telescope reports that these two halves are about nine and 12 miles in diameter and have been nicknamed Ultima and Thule.

A Freeze-Dried Sample

Celestial objects orbiting each other so close that they touch are weird enough for science fiction, but this is not the first time that astronomers have seen such objects. It is, however, the first time they have seen them at the outer edge of the solar system, in a region beyond Pluto known as the Kuiper Belt.

What really interests astronomers about the Kuiper Belt is not just odd combinations such as contact binaries, but its history — as in, it doesn’t have much. This region of space has been relatively placid since the solar system formed some 4.5 billion years ago.

Uneventful may sound boring, but it also means pristine: worldlets like Ultima Thule have orbited the sun all but undisturbed since they formed — and still retain signs of their formation processes that were long ago obliterated in the more turbulent region of space where Earth and the other major planets orbit.

The latest images from New Horizons are giving us our first clear look at the subtle processes that shaped these worlds and left their marks for us to examine, 4 billion years later.

More Beyond

Four billion miles is a long way from home, and Ultima Thule is the farthest destination our deep space missions have yet directly reached. But new space discoveries and even planetary exploration are not confined to the solar system.

The Transiting Extrasolar Survey Satellite, TESS, orbits Earth not much farther away than the moon — but its four precision cameras are finding planets orbiting stars dozens or hundreds of light-years away. And it is paving the way for the James Webb Space Telescope, which will take deep space exploration to a new level.