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Starts With A Bang

New Horizons To Encounter Ultima Thule This New Years, And Here’s Why It Matters

Sometimes, the biggest answers to the deepest questions come in the most unexpected of places.

In the Solar System, anything out past Neptune is generally considered to be the outer reaches of our local neighborhood. The only mission we’ve ever sent out with the express purpose of imaging a world beyond the Solar System’s final planet is New Horizons, which famously flew past Pluto in 2015. The data that it took was unprecedented, and reshaped our view of not only Pluto, but its moons, surface, atmosphere, and the Kuiper belt in general.

Three years later, the New Horizons mission is now one billion miles (~1.6 billion km) farther away, and is rapidly approaching its new target: a small Kuiper belt object formally named 2014 MU69, but nicknamed Ultima Thule. On New Years Day, New Horizons will fly past this distant, little-understood object, imaging it with the full suite of its equipment. It’s a mission unlike any other, and will teach us about how our Solar System came to be.

Asteroids and planetesimals in the early Solar System were more numerous, and cratering was catastrophic in those early stages. Once the protoplanetary disk and the surrounding proto-stellar material evaporate away, the growth of the Solar System’s overall mass ceases, and can only decrease from that point on. It takes many generations of stars in order to arrive at a planetary system that can have an Earth-like planet with the right levels of heavy element abundance to support life-as-we-know-it. The outer portions of solar systems contain the greatest density of relic worlds. (NASA / GSFC, BENNU’S JOURNEY — HEAVY BOMBARDMENT)

On January 1st, at 12:33 AM Eastern Time, New Horizons will make its closest approach to Ultima Thule, a Kuiper belt object that hadn’t yet been discovered when New Horizons was first launched. When the Solar System first formed, there were many different regions:

  • an inner, charred region where any light gases or volatile ices would be boiled/sublimated away,
  • an outer, cool region where ices can stably form,
  • and a region beyond all the known planets, where this cold, distant material can coalesce into the objects comprising the outer Solar System.

While the inner two regions gave rise to the planets, moons, and asteroids we know today, the outermost region is relatively pristine.

Protoplanetary disks, which all solar systems are thought to form with, will coalesce into planets over time, as this illustration shows. It’s important to recognize that the central star, the individual planets, and the leftover primordial material (which became, for example, asteroids and Kuiper belt objects in our Solar System) may all have variations in ages on the order of tens of millions of years. (NAOJ)

Sure, you have the largest and best-known worlds that make up the Kuiper belt and — beyond that — the Oort cloud. Many of these worlds have moons and other satellites, thought to result when collisions between large bodies occur. Pluto is a spectacular example, with a total of five satellites: Charon, Styx, Nix, Kerberos and Hydra.

Even though New Horizons imaged all of these worlds to unprecedented detail and resolution, they don’t quite tell us as much as we’d like about the material that the Solar System formed from billions of years ago. The reason why is simple to understand: the large impacts that have occurred over the 4.5 billion years of our Solar System’s history have polluted anything we might try to learn from the survivors.

Based on their orbital parameters, most of the objects from beyond Neptune fall into some well-known categories like the Kuiper belt or the scattered disk. The known detached objects are few in number, with Sedna perhaps the single most exceptional object of all for both its size and its orbital parameters. Outside of Neptune, but still in the Kuiper belt, are the objects that are the earliest, most pristine remnants from the period of planet-formation in our Solar System.(WIKIMEDIA COMMONS USER EUROCOMMUTER)

To understand how our Solar System first formed, and later evolved to become the way it is today, one of our great scientific goals is to find those pristine materials. In the asteroid belt, the small, rubble-pile asteroids are the most numerous of all, and also the most difficult to find, visit and examine.

In 2005, the Hayabusa mission landed on a sub-kilometer-sized asteroid named Itokawa, and measured a large number of its properties. It found a potato-shaped world-in-miniature, with evidence for two major components of extremely different densities and compositions, with a large amount of surface-rubble. Even though there were interactions that clearly occurred over the 4.5 billion-year history of our Solar System, Itokawa was an ancient relic.

A computer-generated rendering of a rubble-pile asteroid and a debris field of surrounding rubble. Based on a 3-D model of asteroid Itokawa by Doug Ellison, and with data from NASA-JPL. (KEVIN GILL / FLICKR)

The small, icy objects of the Kuiper belt should provide a similar experience for us, and viewing the first one may show us whether “ice-pile” objects can exist or not.

That’s what makes Ultima Thule such an incredible opportunity for anyone interested in how the Solar System formed, evolved, and grew up. When Earth-and-space-based telescopes combined to scour the post-Pluto flight path of New Horizons for suitable targets, there was a special set of criteria in mind. The ideal target would be:

  • reachable in just a few years,
  • in an orbit that required the expenditure of minimal fuel,
  • and discovered in sufficient time to make the required maneuvers.

The New Horizons team did an incredible job of finding Ultima Thule (full story here), but got even luckier than they expected.

A series of NASA/Hubble images, all stitched together, shows the motion of the world 2014 MU69, now better known as Ultima Thule. This 30-kilometer-wide Kuiper belt object will be the target of New Horizons’ next fly-by on January 1st, 2019.(NASA/JHUAPL/SWRI/ALEX PARKER)

If you want to study the relics left over from the early stages of our Solar System, your best bet is to find a small object in a stable, nearly-circular orbit around the Sun. They should be far enough away from Neptune that it never gravitationally affected their orbits, and yet close enough that they are still composed of the same material that formed the majority of our Solar System.

Objects that possess all of these properties are known as the “Cold Classical” Kuiper belt objects: a population of bodies that remains relatively unchanged from when the Solar System first formed from a disk more than 4 billion years ago. It was data from Hubble that determined its orbit, with an assist from Gaia in calibrating the background stars, paving the way for our discovery of such a world. This is exactly what Ultima Thule, the target of New Horizons, is anticipated to be.

Pluto and its five moons, including the monster moon Charon and the four smaller satellites, are highly processed due to the collisions and other interactions that gave rise to the Plutonian system in its current form. Ultima Thule, on the other hand, should be a leftover relic that’s largely unchanged over the past 4.5 billion years. (NASA/JHUAPL/SWRI)

It’s also exactly what Pluto and its moons aren’t. When you form from a massive collision, all sorts of changes occur. Atmospheres change or get stripped away entirely, which is why none of Pluto’s satellites — even the massive Charon — have a substantial atmosphere. Objects differentiate, implying that Pluto’s satellites are made of the primeval material that largely comprised Pluto’s outermost, pre-collisional layers. All told, its worlds experienced violent interactions and the materials of those worlds is now processed, rather than pristine.

Which is why something like Ultima Thule is so important.

This illustration is based on a modern model of objects in the outer solar system, including Pluto and 2014 MU69/Ultima Thule, and the trajectory of New Horizons (yellow). The orbits of the planets are illustrated with cyan rings, and both asteroids and Kuiper Belt objects illustrated as points. Cold Classical Kuiper Belt objects are drawn in red. (ALEX PARKER)

We have an opportunity like never before with this fly-by. A close examination of Ultima Thule should provide us with a window into our past that we’ve never peered through before: a window into the earliest stages of planet formation in our Solar System. For the first time, we’ll view a leftover planetesimal from before the first planet in our Solar System ever formed, and we’ll view it from up close.

A speck of a world, one whose existence was only revealed a year before New Horizons had its encounter with Pluto, is about to shed light onto what our Solar System was made of back in its infancy.

The Kuiper belt is where the greatest number of known objects in the Solar System resides, but the Oort cloud, fainter and more distant, not only contains many more, but is more likely to be perturbed by a passing mass like another star. Note that all Kuiper belt and Oort cloud objects move at extremely small speeds relative to the Sun, and consist of largely unprocessed material that hasn’t changed since before the Solar System’s planets formed. (NASA AND WILLIAM CROCHOT)

Ultima Thule is a much smaller target than Pluto was, and although New Horizons will get close to it (just 2,200 miles away, or 3,500 km), it’s quite tiny on its own. Estimated to be approximately 30 kilometers (19 miles) in diameter, it’s anticipated to be slightly larger than Styx or Kerberos, but much smaller than either Nix or Hydra.

Unlike any of those worlds, though, we managed to observe Ultima Thule pass in front of (or occult) a background star. This revealed its non-spherical shape, and one model has it looking similar to Itokawa in at least one way: like it has two distinct regions joined together.

New Horizons is now so distant it takes a signal nearly half a day to make the round trip from Earth. At these distances, it will take around 20 months for the full suite of fly-by data to be downloaded. But the first full image of New Horizons’ target, this time, should arrive after just a few days.

Ultima Thule, as imaged during New Horizons’ final approach to the distant object, promises to reveal the most pristine look at the material that formed the overwhelming majority of solid bodies in the Solar System. (NASA)

The pre-programmed set of commands for the fly-by has already begun. If the Federal Government remains shut down, NASA TV, and other media will remain offline, but the New Horizons mission and the Johns Hopkins Applied Physics Laboratory YouTube channel will continue to broadcast the mission and its updates live.

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

What will we find? Will Ultima Thule have any tiny, closely-orbiting satellites within 2000 km of its surface? Will it have multiple mass centers? Will it have a uniform color and density, or will it have resolvable surface features? What will it be made out of? Will there be an atmosphere on a world this tiny at such great distances?

For the first time, we’re going to get a glimpse of the Solar System as it was before any of the planets formed. By time all is said and done, we’ll have a better answer to the cosmic question of where we came from than ever before.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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