NASA's New Horizons spacecraft captured the world's attention when it buzzed Pluto in July 2015. In January 2019, it will set another record when it reaches another object in the outer edges of the solar system. Known as 2014 MU69 or by its nickname Ultima Thule, the ancient object will provide insight into the early life of the sun and its planets. Unlike other things explored by spacecraft, the tiny chip of ice and rock will be the first to be explored by a spacecraft launched before its discovery.
Both Pluto and 2014 MU69 lie within the Kuiper Belt, a collection of icy rocks that surrounds the outer reaches of the solar system. These objects are thought to be pristine samples from the early solar system, cast out into the boundary zone through gravitational interactions with the larger objects that would grow into planets. Examining them should reveal insight into what was happening in the solar system in the first stages of its lifetime.
As early as 2003, the National Academy of Sciences' Planetary Decadal Survey strongly recommended that the visit to Pluto include flybys of small Kuiper Belt objects (KBOs). Observing multiple targets would provide greater insight into the previously unexplored segment of the solar system. New Horizons launched in 2006 with extra fuel for such a flyby, and its power and communications systems are prepared to work at distances beyond Pluto's orbit for years to come.
In 2011, mission scientists used ground-based telescopes to begin searching for a second target, but none of the new discoveries lay within the reach of New Horizons. In 2014, the Hubble Space Telescope joined the search, locating five potential objects. One of them was 2014 MU69, which was labeled 1110113Y after its June 26, 2014, discovery and PT1 ("potential target 1") after its elevation to one of two possible destinations. In August 2015, the mission team selected 2014 MU69 as its next potential target.
"2014 MU69 is a great choice because it is just the kind of ancient KBO, formed where it orbits now, that the Decadal Survey desired us to fly by," New Horizons principal investigator Alan Stern, of the Southwest Research Institute (SwRI) in Boulder, Colorado, said in a statement. "Moreover, this KBO costs less fuel to reach [than other candidate targets], leaving more fuel for the flyby, for ancillary science, and greater fuel reserves to protect against the unforeseen."
New Horizons' final target lies about 1 billion miles (1.6 billion kilometers) beyond Pluto.
In 2017, the New Horizons team requested suggestions for nicknames from the public as part of an outreach campaign. The final decision, Ultima Thule, is a term used in medieval times to mean "beyond the known world." The nickname was submitted by about 40 different people, NASA officials said.
"MU69 is humanity's next Ultima Thule," Stern said.
Even with Hubble, details of Ultima Thule are difficult to make out. The tiny object is estimated to be just under 30 miles (about 45 km) across, less than 1 percent the size of Pluto. If the object is brighter, then it is likely smaller, while a darker object would be larger. Similar objects could have helped to build the dwarf planet in the past.
"There's so much that we can learn from close-up spacecraft observations that we'll never learn from Earth, as the Pluto flyby demonstrated so spectacularly," New Horizons science team member John Spencer, also of SwRI, said in the statement.
"The detailed images and other data that New Horizons could obtain from a KBO flyby will revolutionize our understanding of the Kuiper Belt and KBOs."
During an interview with the Planetary Society, New Horizons mission planner Mark Holdridge said that New Horizons' first images of 2014 MU69 will be taken about 80 days before the encounter, in October 2018. When the spacecraft makes its Jan. 1, 2019, flyby, it will pass even closer to 2014 MU69 than it flew by Pluto, though the exact distance is not yet known. At the time, the pair will be about 43.3 times as far from the sun as the Earth is.
Since the Pluto flyby, New Horizons has performed four maneuvers to intercept Ultima Thule, making an encounter inevitable. Less certain is the issue of whether or not the mission will receive funding approval to visit its second target.
"Even as the New Horizon's spacecraft speeds away from Pluto out into the Kuiper Belt, and the data from the exciting encounter with this new world is being streamed back to Earth, we are looking outward to the next destination for this intrepid explorer," said John Grunsfeld, astronaut and chief of the NASA Science Mission Directorate at the agency headquarters in Washington, D.C.
"While discussions whether to approve this extended mission will take place in the larger context of the planetary science portfolio, we expect it to be much less expensive than the prime mission while still providing new and exciting science."
Astronomers used the help of two different stars to study Ultima Thule before its arrival. When an object like a KBO travels between Earth and a star, astronomers can use the starlight to study the distant target. Ultima Thule traveled between Earth and several stars on June 3, July 10, and July 17 of 2017. Each event lasted only two seconds or so and was only visible from a narrow band of land and sea on Earth.
"This effort — spanning six months, three spacecraft, 24 portable ground-based telescopes and NASA's SOFIA airborne observatory — was the most challenging stellar occultation in the history of astronomy, but we did it!" Stern said. "We spied the shape and size of 2014 MU69 for the first time, a Kuiper Belt scientific treasure we will explore just over 17 months from now. Thanks to this success, we can now plan the upcoming flyby with much more confidence."
Measurements from all three of the 2017 observations revealed that Ultima Thule wasn't orbiting where it was expected. A small blip during the July 10 observation proved puzzling on its own. Together, the combined results suggest that MU69 isn't spinning through space alone: MU69 may have at least one moon, and could host even more natural satellites.
"This might be the harbinger," Stern said. "It might hint that there is actually a swarm of satellites from MU69."
The New Horizons team got one final glimpse of Ultima Thule before the New Horizons flyby. On Aug. 4, 2018, a final occultation, visible only in Senegal and Colombia, was visible from Earth.
"This occultation will give us hints about what to expect at Ultima Thule and help us refine our flyby plans," New Horizons occultation-event leader Marc Buie, of the SwRI said in a statement.
Scientists think Ultima Thule is about 20 miles (32 km) across if it's a single object. If it is two objects, each component is probably 9 to 12 miles (15 to 21 km) long.
The New Horizons team is studying the results of their final occultation as they buckle down for the Jan. 1, 2019 flyby of Ultima Thule.
"Our flyby of MU69 on New Year's Eve and New Year's Day 2019 will be an exciting sequel to the historic exploration New Horizons performed at Pluto in 2015," Stern said in a statement.