Exclusive: New Horizons’ mission head on new discoveries at Ultima Thule

New Horizons recently flew by the most distant body ever explored. Alan Stern, who led the mission, discusses what we've learned so far.
By | Published: February 22, 2019 | Last updated on May 18, 2023
New Horizons captured this stunning view of Ultima Thule from a distance of 4,109 miles, just minutes before its closest approach. Several depressions pock the surface of the two lobes; scientists aren’t yet sure if they are impact craters, collapse pits, or a combination. Also note the bright “collar” where the two lobes join. The resolution of the image is about 110 feet (33 meters) per pixel.

Spending your New Year’s Eve at work doesn’t sound like much fun, I guess. Unless it involves the farthest exploration of worlds in history, a midnight countdown led by Queen rock star Brian May, more than 1,000 friends and colleagues along for the ride, and getting your party on the cover of The New York Times, above the fold! That’s how I spent my New Year’s Eve — as the New Horizons spacecraft conducted the first-ever flyby of a small, primordial Kuiper Belt object a billion miles beyond Pluto.

New Horizons is NASA’s first mission to explore the Pluto system and the ancient Kuiper Belt beyond. After being designed, built, and tested by a partnership between the Johns Hopkins University Applied Physics Laboratory and the Southwest Research Institute, where I work, New Horizons was launched January 9, 2006. After a 9.5-year journey at a speed of almost one million miles per day, New Horizons successfully explored the Pluto system in the summer of 2015, approaching the planet to within a distance of about 8,000 miles.

Then, after an additional journey of a billion miles and almost 3.5 years, the New Horizons probe flew past the Kuiper Belt object (KBO) 2014 MU69, nicknamed Ultima Thule, on New Year’s Day. That flyby, which took New Horizons more than three times closer to Ultima Thule than it flew past Pluto, was also fully successful, taking the first detailed images of any KBO.

New Horizons scientists created this video of Ultima Thule using 14 images captured by New Horizons’ Long Range Reconnaissance Imager (LORRI), taken just minutes before the spacecraft flew by the Kuiper Belt object at over 32,000 miles (51,500 km) per hour.


The kind of orbit that Ultima Thule is in indicates it belongs to a unique population of KBOs, called cold classical KBOs, that have been undisturbed from their present positions since the formation of the solar system.

The main objectives for our first KBO flyby were to map the object’s shape, geology, color units, and surface composition, and to search for satellites and rings. Secondary objectives included searching for a gas or dust atmosphere around it, searching for its interaction with the solar wind, making stereoscopic maps of the surface to discover its topography, and taking the temperature of Ultima Thule’s day and night sides. All of these objectives were met, and over the past month, the spacecraft has begun to return data from the flyby to Earth.

Although it could take 20 months, through August or September of 2020, for all of the flyby data to be sent back, key results have already been obtained from the first few data sets. As the mission’s principal investigator, I want to share those early results with you.

Perhaps most exciting is that New Horizons found Ultima Thule to be a primordial contact binary — something never before seen up close by a spacecraft. That binary is almost certainly a relic planetesimal that can provide valuable insights into how KBOs, comets, and dwarf planets formed in the Kuiper Belt.

As far as things that might surround the binary, no evidence for any gas, moons, dust, or rings has been detected in the New Horizons data sent to Earth so far. The absence of an atmosphere is not surprising because any surface volatiles (substances that vaporize at relatively low temperatures) should have escaped long ago. But many small KBOs have satellites and some KBOs even have rings, so in that regard, Ultima Thule may be unusual. Better ring and satellite search data are still on the spacecraft, however, so we will keep looking as these data come to Earth. The final verdict isn’t in yet!

Surprisingly to many of us, the two lobes of Ultima Thule (which we have dubbed “Ultima” and “Thule,” with Ultima being the larger lobe) initially appeared to have roughly similar shapes. Ultima and Thule have diameters of about 12.1 and 8.8 miles, respectively. From approach imagery, we have measured Ultima Thule’s rotation period at 15.9 ± 0.1 hours. Early color images show that Ultima Thule is red overall, with little color variation from place to place seen in the images sent back as of this writing in early February. Moreover, Ultima Thule’s overall color is similar to other KBOs in the cold classical group to which Ultima Thule belongs.

New Horizons took these parting shots nearly 10 minutes after it sped past Ultima Thule on January 1. Viewing the Kuiper Belt object as a crescent revealed new clues about the object’s shape. Based on approach images, scientists thought the two lobes were fairly spherical. The new image, however, shows the larger lobe (nicknamed Ultima, at left) resembles a giant pancake while the smaller one (nicknamed Thule, at right) looks more like a dented walnut.

The surfaces of both lobes show hills, scarps, and either craters or collapse pits or both. But most strange are the abutting, mounded terrain units, each a few miles across, that cover much of both lobes in a network of “geologic cells” bordered by more reflective material in what appear to be troughs between the cells. Significant albedo (reflectivity) variations from about 6 percent to 14 percent are seen on each of the two lobes. The most prominent of the higher reflectivity features on Ultima Thule is a narrow, bright, and rather cylindrically symmetric neck at the junction where the Ultima and Thule lobes are joined. How this neck formed and maintains its starkly brighter reflectivity is presently a puzzle.

The main task ahead of us on the New Horizons team is to come to understand how Ultima Thule formed. We also want to learn how its strange surface geology and its network of bright albedo features came to be, and to understand the origin of the bright neck between the two lobes.

With many more data sets — including higher-resolution images, comprehensive compositional spectroscopy, and more stereo imagery — coming to the ground over the next few months, I expect we’ll have many new discoveries to announce. So, stay tuned for news releases from the New Horizons team. You also can look forward to a complete article from me that details the exploration of Ultima Thule and the many more scientific findings sure to come in Astronomy magazine later this year!

Planetary scientist S. Alan Stern is the principal investigator of NASA’s New Horizons mission and a vice president at the Southwest Research Institute. Follow him on Twitter @AlanStern.