The Shadow of 2014 MU69 Tells a Tantalizing Tale

Is MU69 A Double-lobed World?

In just under a year and a half, the New Horizons spacecraft will pass by a distant Kuiper Belt Object called 2014 MU69.

No one was quite sure what shape this tiny world is, and the mission scientists needed to know. So, they put together an ingenious plan.
Earth-based observations gave the New Horizons scientists a tantalizing look by watching what’s called a “stellar occultation”. They watched as MU69 passed in front of a star on July 17, 2017,  watching a little eclipse-type event more accurately called an “occultation”. This was after a first occultation observation mission on June 3, 2017, was set up for observers in South Africa and Argentina, and a crew aboard the airborne Strategic Observatory for Infrared Astronomy (SOFIA) watched the event on July 10. The observations, plus high-resolution scans using Hubble Space Telescope (which looked for debris in the area right around MU69) gave clues to this little world’s shape.

Unlike eclipses, which can last for minutes, the MU69 occultations lasted only a few seconds. That was precious little time to gather information about the size and shape of the object. Scattering observers along pre-selected sites from where the event could be seen (and imaged) gave the New Horizons team much more data. That gave a better feel for what MU69’s shape must be.

What Does 2014 MU69 Look Like?

MU69
The Kuiper Belt object 2014 MU69 may be a double-lobed object or possibly a more spherical one with a chunk missing. The New Horizons spacecraft will fly by this object in January 2019. Courtesy JHU/APL/SWRI/Alex Parker

It turns out 2014 MU69 is pretty weird-looking, at least from our great distance from it. The measurements reveal what looks like a double-lobed object or maybe a tiny world with a big bite taken out of it. It’s not a perfectly round place. It’s also possible that 2014 MU69 could be two objects orbiting very close to (or even touching) one another — too close to be distinguished from each other. Right now, MU69 is more than 6.5 billion kilometers from Earth and its tiny size (about 30 km) makes it difficult to get a final answer about that oddball shape. Still, it’s a pretty amazing feat to use occultations and high-resolution measurements to get this far 17 months ahead of the flyby. It will be interesting to see how it all pans out as the spacecraft gets closer and returns better images. That won’t happen until later in 2018.

Threading the Needle at MU69

Not only is this information pretty cool to know for its own sake, but the observations made this summer also help the New Horizons mission planners do a more accurate job of targeting the flyby. That’s set for January 1, 2019. The closer they can get for a safe flyby, the better the science will be. However, it’s like threading an incredibly tiny needle. Obviously, it’s good to know if there’s a debris field orbiting along with 2014 MU69. We don’t want to see New Horizons whack into something that could be avoided.

Mission Science

The science to be gained at MU69 itself is incredibly important. However, the upcoming flyby will also tell us a lot about its environment. The Kuiper Belt, the third “regime” of the solar system, contains many objects that range in size from fairly small to some larger than Pluto. This one is among the smaller worldlets out there. The short-period comets also come from this region, and Pluto is among its more famous planetary bodies. MU69 lies on the way “out” of the solar system along the New Horizons trajectory from Pluto (which it explored on a quick flyby in July 2015). The mission is essentially sampling KBOs as it goes. From its data, we may see more than just this oddball world; if there’s debris, then that will also give clues to the population of objects in the neighborhood.

Questions about 2014 MU69

I can imagine all kinds of questions to be answered by the next flyby, all aimed at figuring out just how MU69 (and any companions) formed. Is nearby debris the result of an ancient collision? Or, could it be left over from MU69’s formation (which itself could be the result of a collision)? What’s it all made of? Are they mostly rocks? Mostly ice? A mix? If so, which ices are predominant out there? What do the surfaces look like? These are all questions to be answered with images and data from the flyby. New Horizons is our “forward scout”, exploring the territory ahead and giving ever-more-detailed answers about the outer frontiers of the solar system.

Addendum:  Check out the comments below, where artist Adolph Schaller shares his interpretation of the occultation data for MU69. He does a remarkable job of extrapolating the possible shape of this object.

7 Comments

  1. Adolf Schaller

    Excellent post! This occultation observation was certainly revealing. It showed pretty unambiguously that the object is elongated. But it could well supply potential confirmation that relatively low-energy interactions between small bodies – especially in the outer Solar System – over long spans of time can result in a significant population of contact binary objects. I’ve taken the New Horizons graphic and removed the ‘best-fit’ circles to show the occultation profile:

    With best-fit circles:

    https://www.facebook.com/photo.php?fbid=2016611681904595&set=a.1674269302805503.1073741829.100006673473552&type=3&theater

    With best-fit circles removed:

    https://www.facebook.com/photo.php?fbid=2016614125237684&set=p.2016614125237684&type=3&theater

    Given the uncertainties involved with the timings, the raw presentation the traces present is also obviously open to a variety of interpretations. It could be an elongate object (say, like Eros or 25143 Itokawa). Or, it could very well be a contact binary consisting of two ‘lobes’ in the manner of comet 67P/Churyumov–Gerasimenko that was so spectacularly visited by the Rosetta spacecraft. Imagination comes into play: What ‘shape’ do you see?

  2. C.C. Petersen

    Thanks for the links and the comments! This whole occultation study was pretty amazing. There was also an artist’s concept in the original release showing a cigar-shaped object. I guess this one won’t be over til the Fast Lady (New Horizons) sings!

    C

  3. Adolf Schaller

    You’re welcome! I thought it was rather peculiar that they would feature the occultation traces with superimposed round circles like that. It gives a rather impartial impression and reminds me of the many times an astronomer has advised me in terms of conceptual modeling based on such geometric regularity that turned out to be almost worthless when it confronts the actuality better gleaned by improved observation.

    It’s also rather odd that they presented the circles as to place the ‘big lobe’ at lower left so as to place it in front of the smaller one. I don’t see just from these traces how they can arrive at such a deduction. (Note the large circle color is brighter and implies being nearer than the smaller circle). The situation with the orientation could as well be the reverse! The smaller ‘lobe’ at upper right could as well be nearer. Or the traces may just show the object’s profile broadside. We do not know!

    While it may be a mathematically useful assumption in terms of modeling, the idea of placing best-fit round circles like that gives a false impression to the public of the intrinsically clumpy nature of small objects. At that size range, gravitation wouldn’t round collected debris into spherical form like that. Of course, the ultimate arbiter of the actual shape of MU69 must await the flyby. In the interim, that shouldn’t preclude reasonable speculation based on theory as well as observational experience. That’s part of science too.

  4. C.C. Petersen

    Hi, I just assumed the circles were really just the best approximation of error bars they could do, given the constraints. It will be interesting to see how well this plays out over the next year as the spacecraft gets closer. I should find out if they have more occultation runs planned (if there are any possible).

  5. Adolf Schaller

    That’s what I assumed at first too – just as you say. I haven’t seen any error bars with the timings, but if you look at those circles (particularly the big one) they do not seem to obey any rules of approximation based on those traces. It just looks like somebody drew circles that fit most of the end-points of the traces. Unless we are seriously under-informed on any background data that justifies it, that they featured it in their announcement doesn’t exactly elevate my estimation of it. That’s why I sought immediately to remove the circles to see the trace data that they saw WITHOUT the circles.

    As far as I am aware, there were three opportunities for occultation observations they were counting on. The first one with a deployment over South Africa apparently missed it. I’m not sure whether they consider to have completed their basic goal of pinpointing the exact position and orbit of 2014 MU69 with this observation.

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