Scientists have now released a more detailed image of Ultima Thule, the Kuiper Belt object visited by the New Horizons spacecraft on New Year’s Day 2019. (How to pronounce Ultima Thule: UL-ti-ma THOO-lee.) It’s the most distant object yet visited – and the only Kuiper Belt object yet seen – by an earthly spacecraft. New Horizons captured the images from as close as 17,000 miles (27,000 km) on approach. They reveal Ultima Thule to be a “contact binary,” consisting of two connected spheres. End to end, this little world is now known to measure 19 miles (31 km) in length, the science team said. The team has dubbed the larger sphere Ultima (12 miles/19 km across) and the smaller sphere Thule (9 miles/14 km across).
The team says that the two spheres likely joined as early as 99 percent of the way back to the formation of the solar system, colliding no faster than two cars in a fender-bender.
Signals confirming the spacecraft had survived the encounter and had filled its digital recorders with science data on Ultima Thule reached the mission operations center at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland almost exactly 10 hours later, at 14:29 UTC (10:29 a.m. EST).
It took that long for New Horizons to send back its data from this distant object, some four billion miles from our sun.
New Horizons’ path took it about 2,200 miles (3,500 km) from Ultima Thule (which, by the way, is officially designated 2014 MU69). The decision to take that path – instead of a hazard-avoiding detour that would have pushed it three times farther out – was made as recently as mid-December. Space scientists made the decision only after observations from New Horizons itself revealed no rings, no small moons, no potential hazards, in Ultima Thule’s vicinity.
Still, space scientists couldn’t be sure the spacecraft would survive the encounter. They looked tense at times while awaiting word from the spacecraft’s various systems. Then, one by one, all were confirmed as healthy and the mood turned jubilant.
New Horizons will continue downloading images and other data in the days and months ahead, completing the return of all science data over the next 20 months.
Principal Investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado, said:
New Horizons performed as planned today, conducting the farthest exploration of any world in history … The data we have look fantastic and we’re already learning about Ultima from up close. From here out the data will just get better and better!
Before the encounter, scientists had been puzzling over the light reflected from Ultima Thule. The spacecraft had been taking hundreds of images to measure Ultima’s brightness, but those recent measurements appeared to be at odds with a 2017 observation, made when Ultima Thule covered (occulted) a star as seen from Earth. In 2017, based on the occultation data, scientists thought Ultima Thule might be not one, but two bodies orbiting around each other. If there aren’t two objects there, the science team said in 2017, then this little Kuiper Belt object might have a pronounced elongated shape.
And indeed its shape is elongated. Flyby data have already solved one of Ultima’s mysteries, showing that the Kuiper Belt object is spinning like a propeller with the axis pointing approximately toward New Horizons. This explains why, in earlier images taken before Ultima was resolved, its brightness didn’t appear to vary as it rotated.
The team has still not determined Ultima Thule’s rotation period.
As the science data began its initial return to Earth, mission team members and leadership reveled in the excitement of the first exploration of this distant region of space. Johns Hopkins Applied Physics Laboratory Director Ralph Semmel commented:
New Horizons holds a dear place in our hearts as an intrepid and persistent little explorer, as well as a great photographer. This flyby marks a first for all of us – APL, NASA, the nation and the world – and it is a great credit to the bold team of scientists and engineers who brought us to this point.
Now, almost 13 years after the launch of New Horizons, scientists say the spacecraft will continue its exploration of the Kuiper Belt until at least 2021. Team members plan to propose more Kuiper Belt exploration.
Thus the New Year has brought with it a couple of astounding new space records. On December 31, NASA’s OSIRIS-REx spacecraft successfully moved into orbit around the smallest space body yet reached, a near-Earth asteroid called Bennu. Just hours later, in the early hours of New Year’s Day according to clocks in the Americas, the New Horizons spacecraft – also a NASA craft, launched from Earth in 2006 and made famous in 2015 for its once-in-a-lifetime encounter with Pluto – made history again with the most distant spacecraft encounter yet.
Bottom line: Since encountering Pluto in 2015, New Horizons has been heading outward. It’s now survived a sweep past its next target, Ultima Thule, the most distant object visited by a spacecraft from Earth. New Horizons images of Ultima Thule reveal it to be a “contact binary,” created when two small icy bodies in the early solar system stuck together.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.