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New Horizons makes farthest solar system flyby in history

Scott Sutherland
Meteorologist/Science Writer

Tuesday, January 1, 2019, 12:41 AM -

NASA's New Horizons spacecraft makes history again, as it has just flown past 2014 MU69, in an encounter with the farthest-explored object in our solar system!

This story has been updated.

In July of 2015, after travelling for over nine years in space, the New Horizons probe made history, by giving us our very first close-up look at Pluto and its moons.

Now, over three years later, the spacecraft has made another historic flyby, this time of an object another 1.6 billion kilometres farther than Pluto (over 6.6 billion km away from Earth), which was only discovered by the Hubble Space Telescope roughly a year before New Horizons sped past Pluto.

The New Horizons flyby of 2014 MU69 happened at 12:33 a.m. ET, January 1, 2019.

Watch below to see content from John Hopkins University's Applied Physics Laboratory, as they returned to present the newest findings from the New Horizons team, at 10 a.m. ET, Tuesday, January 1, 2019.

2014 MU69 is estimated at being somewhere around 30 km wide. Based on observations taken since its discovery, watching as the object passed in front of (or 'occulted') known background stars, astronomers believe that it has an elongated shape, and is very likely what's known as a 'contact binary'.

A contact binary is when two objects - asteroids or comets - orbit each other so closely that they touch. Dust and other material build up between the two, turning them into peanut shape.

This artist’s impression of 2014 MU69 depicts it as a contact binary, two smaller objects that orbit each other and are so close that they touch. Data from watching this KBO pass in front of known background stars suggests that this is the likely shape of 2014 MU69. Credit: NASA/JHUAPL/SwRI/Alex Parker


At mission control in John Hopkins University's Applied Physics Laboratory, the New Horizons team has been using the spacecraft's cameras to snap new pictures of 2014 MU69 on approach. Even so, this distant object appears very reluctant to give up any secrets.

Although scientists have modeled the shape of MU69, as the time of flyby draws closer, the team had hoped the object's "light curve" would have revealed more about it. This "light curve" is the changes in brightness over time that New Horizons should pick up from MU69, as it rotates in space and the different features on its surface reflect back different amounts of light from the Sun (even at its far distance).

Strangely, however, according to the mission team, the spacecraft's cameras have not been able to pick up any light curve from MU69. The amount of light it gives off is completely flat, and mission team doesn't know why! It could be that the object rotates with its polar axis pointing towards Earth and New Horizons, so we don't see any noticeable change. It could be surrounded by a cloud of dust or gas that scatters its light, preventing New Horizons from seeing a discernible light curve. Or, it could be due to something completely unexpected!

On the left, 2014 MU69 is imaged by New Horizons at the start of December 2018, from a distance of 38.7 million kilometres. On the right, a zoomed-in image of the region outlined in yellow on the left, and processed to remove the background stars from the view, helps this distant object to stand out better. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

This is truly unlike any object we've explored, elsewhere in our solar system!

"Even less than a day away, Ultima Thule remains an enigma to us, but the final countdown has begun," said New Horizons principal investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado. "What we’ll very soon learn about this primordial building block of our solar system will exponentially expand our knowledge of this relatively unknown third region of space."

See the update on this, further down the article!


The name '2014 MU69' is a temporary designation given to minor planets (asteroids, comets and trans-Neptunian objects), which notes when this object was discovered, and the order in which it was discovered.

These temporary designations start with the year of discovery. After that is a single letter, denoting the half-month when it was found (skipping I and Z, to give 24 half-month periods). Then, there is a letter, or letter-number combo, to note where it was found in the sequence of discoveries during that half month.

So, 2014 MU69 was discovered in 2014, in the second half of June ('M' is used for June 16-30), and 'U69' means that it was the 1745th object discovered in that time. The first object spotted in the second half of June was designated '2014 MA', the second was 2014 MB, and so on, skipping the letter I, until the 25th object is named 2014 MZ. The 26th object then becomes 2014 MA1, the 27th is 2014 MB1, and so on, going through the alphabet again and again, and adding one to the number on each run through, until we reach the 1745th object, which is 2014 MU69.

'Ultima Thule' is the unofficial nickname that the New Horizons team has given to 2014 MU69. It means "beyond the limits of the known world", and is a term that has seen use all the way back to the time of the Roman Empire. Unfortunately, there is also a darker use of the name, as was adopted into the mythology of the Nazi Party, in Germany, leading up to and into World War II, as the name of the mythical Aryan homeland. That's not to say that NASA or the New Horizons team are endorsing Nazi mythology. From a Newsweek article from early 2018, they carefully weighed the meanings and uses of the term, and chose to proceed with it, emphasizing its ancient origins, rather than what some have done with term in more recent times. Still, it's use here, given that modern supporters of Nazi ideology continue to use it, is still seen as problematic by many.

In that same article, science journalist Meghan Bartels says that 'Ultima Thule' does not satisfy the naming convention used by the International Astronomical Union, who will choose a final, official name for 2014 MU69 sometime before the end of 2019.

According to the IAU, minor planets with stable orbits (like 2014 MU69) are given mythological names associated with creation.


2014 MU69 is what's known as a "cold classical Kuiper belt object". 

What does that mean?

Just like there is an asteroid belt between Mars and Jupiter, where there are bands of rocky/icy objects orbiting the Sun, there is a similar (but far wider) band of (generally) larger objects out beyond Neptune. This is known as the Kuiper belt, and Pluto is considered to be the largest member, followed by Eris, Haumea, 2007 OR10, Makemake and Charon (Pluto's binary planet pair-bond, or its largest moon, whichever you prefer).


As for the "cold, classical" part, MU69's orbit has a very low 'inclination', meaning that it travels around the Sun in roughly the same plane as all the planets (except Pluto), and its orbit is nearly circular (unlike Pluto). Based on these two factors, this object could be a pristine piece of the early solar system, which formed at its current location, and thus was not tossed around by the gravity of the other planets (looking at YOU, Neptune), and has possibly gone completely (or nearly completely) untouched for over 4.5 billion years.

With 2014 MU69 keeping its secrets right up until the last moment, it seems, there's really no telling exactly what New Horizons will see as it whizzed past on New Year's Eve. 

The pre-encounter image of 2014 MU69 from New Horizons, taken on December 31, 2018, when the spacecraft was at a distance of 1.9 million kilometres. The processed image on the right confirms the object's elongated shape. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Partial rotation of 2014 MU69.

Update: The first mystery has been solved (apparently)!

MU69 doesn't show a light curve because it appears to have an axis of rotation pointed roughly at New Horizon's cameras!

The three image shown in sequence to the right, taken on the night of Dec 31, 2018 by the spacecraft, show that its elongated shape maintains roughly the same brightness, all the time, as it rotates relative to New Horizons.

Below is a composite image (on the left) taken by the spacecraft's LORRI camera (Long-Range Reconnaissance Imager) gives a slightly clearer idea of its shape, with an inset diagram (on the right), showing a close approximation to its form, and the location of its axis of rotation.

The latest imagery of 2014 MU69 shows that it is roughly 32 kilometers by 16 kilometers in size. The artist's impression illustrates one possible appearance of of the object. The direction of Ultima's spin axis is indicated by the arrows. Credits: NASA/JHUAPL/SwRI; sketch courtesy of James Tuttle Keane

It will take months for all the data from this flyby to come in from New Horizons, but we should start to see some great imagery arrive in the days ahead. So, come back for updates, as the New Horizons team shares more of what they discovered during this distant encounter!

Sources: JHUAPL | NASA | The Planetary Society


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