The most distant planetary exploration in history required a significant amount of careful planning and preparation, as well as a little bit of luck.
“We started panicking by the time we got into 2013, especially late 2013,” says Hal Weaver, project scientist on the New Horizons mission to Pluto and beyond. “We were realizing, ‘Oh my gosh, we haven’t discovered the next target for New Horizons yet.’”
In June 2014, the Hubble Space Telescope came to the rescue, spotting a small pinprick of light slowly moving across the sky in the region the New Horizons spacecraft was headed. The mission, with a primary objective of exploring and characterizing Pluto, presented a unique opportunity to seek out another planetary object in the distant “third region” of the solar system, the Kuiper Belt. Launched back in January 2006, New Horizons didn’t make its closest approach to Pluto until more than nine years later in July 2015. If the team couldn’t find a new target for the spacecraft, it would likely be decades before another spacecraft could be approved, built and flown to the outer reaches of the solar system.
“It’s going to take so long before another mission is out there, we feel some responsibility to make sure that we looked under every rock,” Weaver says.
Now, the target object, known by its designation from the Minor Planet Center, 2014 MU69, has been revealed for the first time. The distant planetary body is a bi-lobed contact binary, meaning it was formerly two objects that formed separately and then very gently collided with each other and fused together. The larger lobe is about three times the volume of the smaller one, and 2014 MU69 has a reddish hue, thought to be the result of radiation in the outer solar system. From the early images, the team believes the object may be covered in features such as hills, ridges and plateaus. 2014 MU69 rotates once about every 15 hours, and it appears to contain exotic ices such as nitrogen or methane, something that scientists will look to confirm as more data about the composition of 2014 MU69 reaches Earth.
Alan Stern, principal investigator of the New Horizons mission and Smithsonian Ingenuity Award Winner, said at a press conference at Johns Hopkins University’s Applied Physics Laboratory that the encounter with 2014 MU69 was “a technical success beyond anything ever attempted before in spaceflight.”
“It’s only really the size of something like Washington, D.C.,” Stern says of 2014 MU69, which is about 21 miles on its longest side. “And it’s about as reflective as garden-variety dirt, and it’s illuminated by a sun that’s 1,900 times fainter than it is outside on a sunny day here on the Earth. So, we were basically chasing it down in the dark at 32,000 miles per hour.”
2014 MU69 has been nicknamed Ultima Thule by the New Horizons team, a Latin phrase used by the Romans to describe unexplored regions to the north and, more generally, a region that lies beyond the known world. The phrase was used by Virgil in the poem Georgics, and the term “Thule” has a long literary history, appearing in works such as James Thompson’s 1730 poem “Autumn,” which is quoted in the first chapter of Charlotte Bronte’s novel Jane Eyre. Versions of “Ultima Thule” also appear in the poem “Dream-Land” by Edgar Allan Poe and in the works of Vladimir Nabokov.
However, the phrase and nickname for 2014 MU69 has drawn criticism because “Ultima Thule” was also a mythical region in early Nazi lore, used by the German occultist Thule Society to describe a lost land that was the birthplace of the “Aryan race.” “Ultima Thule” is an unofficial nickname for 2014 MU69, and now that the object has been explored and characterized, the International Astronomical Union can begin the process of giving the object an official name.
“The term Ultima Thule, which is very old, many centuries old, possibly over 1,000 years old, is a wonderful meme for exploration, and that’s why we chose it,” Stern said at the press conference when asked about the nickname. “And I would say that just because some bad guys once liked that term, we’re not going to let them hijack it.”
While the Pluto flyby revealed a remarkable world of active geology, with flowing glaciers of exotic ices such as carbon monoxide and methane, and towering mountains of water ice, 2014 MU69 is expected to provide a window into the history and evolution of the solar system itself. 2014 MU69 is what is known as a classical Kuiper Belt object, which are icy and rocky bodies beyond the orbit of Neptune that have relatively circular orbits, meaning that unlike Pluto, they never cross Neptune’s orbit. At this great distance, between about 40 and 50 Astronomical Units, or about 3.5 to 4.5 billion miles from the sun, classical Kuiper Belt objects constitute an incredibly primitive population, virtually unchanged since the dawn of the solar system.
“Because of [2014 MU69’s] current orbit, we think it’s been in that position for 4.6 billion years, in which case it’s been kept in a deep freeze since the time of its formation,” Weaver says.
The fact that the object is a contact binary allows scientists to further study how material aggregates into objects like 2014 MU69 and continues to grow and form full planets. “It’s actually gratifying to see these almost perfectly formed contact binaries in their native habitat,” says Jeff Moore, geology and geophysics team lead for New Horizons. “People have speculated for a long time the processes… [of] how the initial primordial clumps come together to form what’s called planetesimals, which are the things which in turn go on to the make the planets. But to actually see the things that are consistent with the explanations that we have and theories we’ve had for how these things form is extremely gratifying.”
More than 4 billion miles from the sun, 2014 MU69 serves as something of a remnant of the original material the solar system formed from. Stern called 2014 MU69, “probably the best time capsule we’ve ever had for understanding our solar system.”
It was not known until the 1990s that the region beyond Neptune is not empty, but rather full of hundreds of thousands of objects in a distinct zone of the solar system now called the Kuiper Belt, named after Dutch-American astronomer Gerard Kuiper, who predicted the region’s existence decades earlier. The discovery of Eris in 2003, a dwarf planet in the Kuiper Belt about the same size as Pluto, further revealed the significance of this third region and its influence on the formation and evolution of all that orbits the sun.
While it can be easy to think of the planets in the solar system forming in the orbits they are in today, astronomers now know that this was not the case. The giant planets migrated inward and back outward as the solar system was taking shape, affecting the orbits of everything else and even ejecting some objects from the solar system entirely.
“During the first tens of millions of years of the solar system’s history, Jupiter and Saturn get into this weird dance that caused a lot of chaos in the orbits of all the giant planets,” Weaver says. “Jupiter may have come in almost as close as Mars’ orbit and then went back out again. We think that Neptune and Uranus actually flipped places. … And that stirred the pot in the solar system, finally ending up with what we have today.”
While the pot was stirring, astronomers believe that some objects in the Kuiper Belt like 2014 MU69 were thrown inward on elliptic trajectories that pass close to the sun before flying back out to distant realms. Today we call these objects comets, and when one gets close to the sun, the ices near the surface are heated and sublimate into gas, forming a “coma” or ball of gas that surrounds the comet's rocky core, called the nucleus.
“Anytime we see comets we need to remember that they’re post-toasties,” Moore says. “They’ve been fried and crackled and crunched by the sun, and they’re badly damaged examples of former Kuiper Belt objects. And so being able to go out and see a pristine Kuiper Belt object tells us now that indeed contact binaries really do form, and maybe when we see comets we’re seeing smaller versions of very badly damaged contact binaries.”
At the moment, only about one percent of the data stored on New Horizons has been received by the science team on the ground. The spacecraft will transmit data to Earth for the next 20 months, revealing more about 2014 MU69’s topography and composition. Meanwhile, New Horizons will continue its flight toward the edge of the solar system at some 30,000 mph—but its days of exploration are not over yet.
“The spacecraft is in peak health,” Stern says, adding that New Horizons has enough power in its radioisotope thermoelectric generator (RTG) to operate for 15 to 20 more years. The craft can continue science operations to about 2.5 times its current distance from the sun, and it has enough fuel left to fire its thrusters to change course toward another object. (The New Horizons team had to conduct multiple trajectory corrections leading up to the flyby with 2014 MU69.) As it continues to fly toward the edge of the solar system, New Horizons will keep an eye out for additional planetary bodies to study, either by observing them through its telescopic cameras or, if we are lucky, by flying near another object.
“The key to the science that we do is whether it’s studying objects remotely with our telescopes, or whether that science in the Kuiper Belt also will include one more flyby,” Stern says. “And I can’t give you the answer today, because we don’t know.”
For now, the team looks forward to receiving the remaining data on the spacecraft to learn more about 2014 MU69, the most distant and ancient world ever explored.