Year In Review 2017 (Part 4): One year to New Horizons’ flyby of MU69

by Chris Gebhardt

As science teams and researchers spent 2017 pouring through the wealth of data returned by the New Horizons spacecraft during its historic flyby of the dwarf planet Pluto in July 2015, the spacecraft’s controllers spent the year busily preparing the craft for its anticipated flyby of Kuiper Belt Object 2014 MU69 – a flyby event scheduled for exactly one year from today on 1 January 2019.

While New Horizons spent five months of 2017 in hibernation, its teams did anything but.  The year saw a dedicated ground-based campaign on Earth to observe flyby target MU69 as it occulted a distant star to determine a better estimate for its size, mass, shape, and near space environment – an observational campaign designed to help the New Horizons team gather better information on the object to help plan precisely how and at what distance the craft will fly by the distant object next year.

The campaign, previously reported by, revealed MU69 to be much weirder than first thought – with the Kuiper Belt Object now believed to be either two distinct objects orbiting each other or a close/contact binary – meaning the object is two separate hunks of rock that orbit each other extremely close or actually touch each other.  “This new finding is simply spectacular.  The shape of MU69 is truly provocative and could mean another first for New Horizons going to a binary object in the Kuiper Belt,” said Alan Stern.  “I could not be happier with the … results, which promise a scientific bonanza for the flyby.”

Now you see it, now you don’t. Kuiper Belt Object 2014 MU69 – 4.1 billion miles from Earth – briefly blocks the light from a background star as seen from a mobile telescope in Argentina on 17 July 2017. Credits: NASA/JHUAPL/SwRI

Based on the ground-based observations in 2017, MU69 now appears to be no more than 20 miles (30 kilometers) long, or, if it’s a binary, each pair is about 9-12 miles (15-20 kilometers) in diameter.  “These exciting and puzzling results have already been key for our mission planning,” said Marc Buie, a New Horizons science team member at the Southwest Research Institute, “but also add to the mysteries surrounding this target leading [up to] the New Horizons encounter.”

The information gathered from a fleet of telescopes deployed on the ground in South Africa and Argentina and airborne over the Pacific with NASA’s SOFIA telescope gave New Horizons’ mission planners the data needed to create a primary science objective flight path for the MU69 flyby.  That milestone will mark the farthest celestial object encounter in history – more than four billion miles (6.5 billion km) from Earth and more than one billion miles (1.5 billion km) beyond Pluto.  If all goes as planned, New Horizons will pass just 2,175 miles (3,500 km) over MU69, peering down on it from celestial north. 

If this close approach is executed, the highest-resolution camera on New Horizons, the telescopic Long Range Reconnaissance Imager should be able to spot details as small as 230 feet (70 m) across, compared to nearly 600 feet (183 m) on Pluto.  An alternate plan, to be employed in certain contingency situations such as the discovery of debris near MU69, would take New Horizons within 6,000 miles (10,000 km) of the object – still closer than the 7,800 mile (12,500 km) flyby distance at Pluto.

Artist’s impression of MU69 with its suspected moon as seen during New Horizons’ planned 1 January 2019 flyby event. Credits: NASA/JHUAPL/SwRI

Moreover, as the year came to a close, scientists were intrigued by data not seen in the first review of information gathered from the Earth-based observation campaign of MU69 during its occultation of a distant star this year.  In pouring through the data in detail, scientists were surprised and excited to see indications that MU69 might have an orbital companion: a small moon.  “We really won’t know what MU69 looks like until we fly past it, or even gain a full understanding of it until after the encounter,” said Buie.  “But even from afar, the more we examine it, the more interesting and amazing this little world becomes.”

The prospect that MU69 might have a moon comes from data collected by NASA’s airborne Stratospheric Observatory for Infrared Astronomy (SOFIA) on 10 July 2017 during the second of three occultation events observed this summer.  Focused on MU69’s expected location, SOFIA detected what appeared to be a very short drop-out in the star’s light.  Buie said further analysis of the data, including syncing it with MU69 orbit calculations provided by the European Space Agency’s Gaia mission, opens the possibility that the “blip” SOFIA detected could be another object around MU69.

“A binary with a smaller moon might also help explain the shifts we see in the position of MU69 during these various occultations,” Buie added.  “It’s all very suggestive, but [it is] another step in our work to get a clear picture of MU69 before New Horizons flies by.”   Alan Stern added, “The occultation effort that Marc Buie and his team led for New Horizons has been invaluable in opening our eyes to the very real possibilities that MU69 is both a lot more complex than anyone suspected and that it holds many surprises for us at flyby on New Year’s Eve and New Year’s Day, 2019.  The allure of its exploration is becoming stronger and stronger as we learn more and more about it.  It’s just fantastic!”

New Horizons’ journey through the solar system to its encounter with 2014 MU69 – formerly known a Potential Target 1 (PT-1). Credit: NASA

But 2014 MU69 is not the only mission target New Horizons’ scientists learned more about this year.  Two and a half years ago, when New Horizons flew past the dwarf planet Pluto, the craft began returning a stream of data to Earth that took more than a year to fully transmit.  With all the data from the flyby in hand for more than a year now, scientists have been busy examining everything observed by the craft, including information regarding strange blade-like terrain seen on the dwarf planet’s surface.

Among the many discoveries made by New Horizons on Pluto were strange formations resembling giant knife blades of ice.  The series of bladed ridges on Pluto’s surface, known as “penitentes,” are bowl-shaped depressions with blade-like spires around the edge that rise several hundreds of feet above the local terrain.  At the beginning of the year, research by John Moores of York University, Toronto, in collaboration with scientists at the Johns Hopkins University Applied Physics Laboratory and NASA Goddard Space Flight Center, showed that these icy features may also exist on other planets where environmental conditions are similar.

The identification of these ridges in Pluto’s Tartarus Dorsa area suggests that the presence of an atmosphere is necessary for the formation of penitentes – which Moores says would explain why they have not previously been seen on other airless icy satellites or dwarf planets.  “Exotic differences in the environment give rise to features with very different scales,” he adds.  “This test of our terrestrial models for penitentes suggests that we may find these features elsewhere in the solar system and in other solar systems where the conditions are right.”

Though Pluto’s environment is very different from Earth’s – it is much colder, the air much thinner, the sun much dimmer and the snow and ice on the surface are made from methane and nitrogen instead of water – the same laws of nature apply.  Pluto’s ridges stand more than 1,600 feet (about 500 m) tall and are separated by 2-3 miles (about 3-5 km).  

“This gargantuan size is predicted by the same theory that explains the formation of these features on Earth,” says Moores.  “In fact, we were able to match the size and separation, the direction of the ridges, as well as their age: three pieces of evidence that support our identification of these ridges as penitentes.”

As the year progressed, and more data from New Horizons was reviewed, scientists discovered a fascinating explanation for the composition of Pluto’s bladed terrain: the penitente structures are made almost entirely of methane ice and likely formed as a specific kind of erosion wore away their surfaces, leaving dramatic crests and sharp divides.  The jagged geological ridges appear to be related to Pluto’s complex climate and geological history.  Jeffrey Moore, a research scientist at NASA’s Ames Research Center, and his team have determined that formation of the bladed terrain begins with methane freezing out of the atmosphere at extreme altitudes on Pluto, in the same way frost freezes on the ground on Earth.

An example of the penitentes from the southern end of the Chajnantor plain in Chile. Though these ice formations only reach a few feet in height, while Pluto’s bladed terrain reaches hundreds of feet, they both have similar sharp ridges. Credits: Wikimedia Commons/ESO

“When we realized that the bladed terrain consists of tall deposits of methane ice, we asked ourselves why it forms all of these ridges, as opposed to just being big blobs of ice on the ground,” said Moore.  “It turns out that Pluto undergoes climate variation and sometimes, when Pluto is a little warmer, the methane ice begins to basically evaporate away” in a process known as sublimation – when ice transforms directly into gas, skipping the intermediate liquid form.  

This erosion of Pluto’s bladed terrain indicates that its climate has undergone changes over long periods of time – on a scale of millions of years – that cause this ongoing geological activity.  Early climatic conditions allowed methane to freeze out onto high elevation surfaces, but, as time progressed, these conditions changed, causing the ice to “burn off” into a gas.  As a result of this discovery, we now know that the surface and air of Pluto are apparently far more dynamic than previously thought.

Unlocking the nature of this bladed terrain on Pluto’s surface also brought scientists closer to understanding the global topography of the dwarf planet.  Since methane has now been linked to high elevations, researchers can use data that indicates where methane is present around Pluto to infer which locations are at higher altitudes, providing an opportunity to map out altitudes of parts of Pluto’s surface not captured in high resolution, where bladed terrains also appear to exist.

Global highest resolution surface map of Pluto, compiled from New Horizons data. Credit: NASA

Because New Horizons did not stop as it flew by Pluto, portions of the planet were captured in higher resolution than others.  Therefore, surface features captured in low resolution must be inferred from what is seen elsewhere on the dwarf planet, where high-resolution images were captured by New Horizons.  This is true of the bladed territory, where detailed coverage was only obtained in a small area.  Nevertheless, NASA researchers and their collaborators have been able to conclude from several types of data that these sharp, bladed ridges may be a widespread feature on Pluto’s so-called “far side”, helping to develop a working understanding of Pluto’s global geography, its present, and its past.

“The complexity of the Pluto system – from its geology to its satellite system to its atmosphere – has been beyond our wildest imagination,” said Alan Stern.  “Everywhere we turn are new mysteries.”  All of the information returned by New Horizons is expected to takes years to sift through and evaluate.  While 2017 was a quiet year compared to 2015 and 2016 for scientific revelations of Pluto, the coming years promise to reveal much more about the complexities and dynamics of the largest known dwarf planet in our solar system.

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