Discovering Ultima Thule

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Ultima Thule, with its reddish-brown surface, may be a perfectly preserved specimen from the dawn of our solar system.
Wikimedia Commons

Four billion miles from the Earth on New Year’s Eve, a spacecraft traveling at 32,000 miles per hour zoomed past a tiny object roughly the size of Denver. It would be humanity’s first glimpse of the vast cloud of icy bodies that populate the space beyond Pluto, a fossil record of the solar system’s formation billions of years ago. Scientists from Boulder had discovered this object only four years earlier, and were ringing in the new year by monitoring the encounter from a control room at the Johns Hopkins Applied Physics Laboratory in Maryland. The dramatic flyby was a sequel for NASA’s New Horizons mission, which sent back the first close-up images of the dwarf planet Pluto and its moon Charon a few years ago. Launched in 2006, New Horizons was the speediest spacecraft in history, and it still took nearly a decade to reach the edge of the solar system. Like an express train with no stops, New Horizons snaps photos and collects other data as it zips past its targets, with no second chances. At this distance from the Earth, commands sent from the control room take six hours to reach their destination, and a response from New Horizons takes another six hours to be returned. It’s not possible to maneuver in real time, so the planning must be flawless.

“We knew we wanted to look for objects beyond Pluto,” says John Spencer, a staff scientist at Southwest Research Institute in Boulder and Deputy Project Scientist for the New Horizons mission. “There weren’t any known objects that we could reach, so we had to find them ourselves.”

Spencer came to Boulder in 2004 to focus his efforts on the New Horizons mission. He had previously been working at Lowell Observatory in Arizona, where Pluto was discovered in 1930, and he began thinking about this encore for the mission in 2002. Starting in 2011, he led the effort to secure telescope time and scan the region of sky that New Horizons could reach with its remaining fuel after the Pluto flyby. The team succeeded a few years later, using the Hubble Space Telescope to discover an object formally known as “2014 MU69,” more recently nicknamed “Ultima Thule.”

Final preparations for the recent encounter started last August, when New Horizons began taking regular images of Ultima Thule to refine the trajectory of the flyby and to survey the surrounding area for possible hazards, like moons or rings. At first the images were obtained weekly, but they gradually became more frequent as the spacecraft got closer to the target. By mid-December, no hazards had been identified and the team chose a trajectory that would take New Horizons within 2,200 miles of the surface. The results were spectacular.

By now you have probably seen the first close-up images of Ultima Thule, showing two roundish blobs physically connected like a giant snowman. The color version of the image reveals a reddish-brown hue, an indication that its surface might be perfectly preserved from the dawn of the solar system. Ultima Thule is part of a population of objects, known as the Kuiper belt, which has remained mostly unperturbed for billions of years.

A fraction of these icy bodies might eventually dive into the inner solar system as comets, destroying their pristine state as they approach the Sun and heat up. The lumpy structure of Ultima Thule encodes important information about how our solar system formed.

“We’ve been saying for years that we would be able to use the shape of this object to help understand the formation of the planets, but I don’t think I ever expected it to be so obvious,” says Spencer. “It’s so obviously two objects that formed separately and then came together, which is essential if you’re going to build planets and not just produce a bunch of debris.”

Stars are formed from gravitational collapse within giant molecular clouds, like the famous “Pillars of Creation” image from the Hubble Space Telescope. Collisions within the massive disks of gas and dust that surround young stars gradually build larger orbiting bodies over time. As the objects grow larger, collisions become less frequent, and building planet-sized objects requires city-sized chunks of rock to stick together instead of fragmenting back into smaller pieces. The first image of Ultima Thule appears to be a snapshot of this fragile phase of planet formation.

In addition to the images, New Horizons gathered many other observations that will help scientists learn more about the properties of the Kuiper belt. Infrared data will probe the surface composition of Ultima Thule, searching for water ice, dry ice and frozen methane, while microwave radiation will be used to measure the surface temperature. Ultraviolet observations will measure any gas in the atmosphere, and an instrument built by students at the University of Colorado will detect any dust that surrounds the object.

New Horizons recorded many gigabytes of data during its 12-hour encounter with Ultima Thule, but it will take many months to transmit it all back to Earth with a connection that is slower than 1990s dial-up internet service. The transmitter operates on the amount of power needed to illuminate an old light bulb, but it is expected to keep working for another 15 years as the spacecraft continues its journey outwards. The snowman image was just our first glimpse of the outer solar system; much more is sure to come in the months and years ahead.

Travis Metcalfe, Ph.D., is a researcher and science communicator based in Boulder. The Lab Notes series is made possible in part by a research grant from the National Science Foundation.