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Starts With A Bang

New Horizons’ Big Reveal On MU69: Ultima Thule Is A Typical ‘Future Comet’

We’d never flown past or imaged a small, isolated Kuiper belt object before. Here’s what we know so far.

As 2018 ended and 2019 began, NASA’s New Horizons flew past its first target after Pluto: 2014 MU69.

Ultima Thule is barely a blip in images (left) from the New Horizons spacecraft. The remote world stands out more when the stars have been removed (right); the dark blobs are artifacts from imperfect star subtraction. Yellow crosshairs mark Ultima’s position. Until just a few days before its arrival, 2014 MU69 (Ultima Thule) was no more than a single pixel in New Horizons’ detectors. (NASA/JHUAPL/SWRI)

Nicknamed Ultima Thule, it’s transformed from a single pixel in our detectors to a red-hued, mottled snowman.

The first color image constructed (via a composite from New Horizons data) of 2014 MU69: Ultima Thule. The reddish color is likely due to tholins: the same reddish color visibly present on the surface of Charon. (NASA/JHUAPL/SWRI)

The first three weeks of data have revealed spectacular details concerning this distant world.

Multiple images of Ultima Thule (2014 MU69) as New Horizons approached it reveal a body that’s rotating and tumbling, but also reveal additional details about the object, as the distance from the camera decreased from 500,000 km down to 28,000 km: a decrease of 94%. (NASA/JHUAPL)

Aside from its inactivity, it conforms perfectly to our expectations of cometary nuclei.

Many comets have had their nuclei imaged by a variety of spacecraft, revealing two main classes of cometary nuclei: a single-object nucleus and a contact binary nucleus. 2014 MU69 appears to be of the contact binary type, and marks the first time we’ve ever imaged such an object before it’s ever developed a tail or lost some of its volatiles. (THE PLANETARY SOCIETY / VARIOUS (SEE IMAGE FOR FULL CREDITS))

In 1986, Halley’s comet was imaged by the ESA’s Giotto mission, revealing a two-lobed core.

This view of Comet Halley’s nucleus was obtained by the Halley Multicolour Camera (HMC) on board the Giotto spacecraft, as it passed within 600 km of the comet nucleus on 13 March 1986. The comet was clearly quite active at the time. (ESA/MPAE LINDAU)

Similarly, Deep Impact’s 2010 pictures of comet Hartley 2 revealed volatile-laden lobes connected by a smooth neck.

NASA’s Deep Impact probe took these images of comet Hartley 2, revealing outgassing from the edges of one of its lobes and vast differences in surface reflectivity from region to region. The smooth neck is likely not a flaw, but a feature common to many contact binaries that originate in the Kuiper belt, as the accumulation of icy material leads to this configuration. Scientists are still gathering the data from New Horizons’ flyby of 2014 MU69, which could shed additional light on the details of smooth-neck formation. (NASA / JPL / UMD)

But the ESA’s Rosetta mission set a new standard in cometary imaging.

A high-resolution image of comet 67P/Churyumov-Gerasimenko reveals a large body consisting of two lobes connected by a thinner neck. Similar to Halley’s comet or 2014 MU69, comet Hartley 2 shows a ‘contact binary’ configuration. We now believe this is common among Kuiper belt objects. (ESA/ROSETTA/NAVCAM)

Its now-legendary snapshots and movies of comet 67P/Churyumov-Gerasimenko show offgassing, plumes, and even snow.

The sun-facing sides of comets heat up first, with the presence of easily-sublimated ices leading to offgassing, the release of pressure, and the loss of material. The longer comets spend in close proximity to the Sun, the faster they evaporate. For objects still in the Kuiper belt, evaporation should be negligible. (ESA/ROSETTA/NAVCAM)

Volatile, icy materials are abundant on these comets, and change phase rapidly when they’re exposed to sunlight.

The most spectacular movie from ESA’s Rosetta mission shows what the surface of comet 67P/Churyumov-Gerasimenko looks like, including the volatile ices that sublimate and re-freeze when they’re in sunlight or shadow, respectively, causing this snow-like behavior.(ESA/ROSETTA/NAVCAM)

Ultima Thule is currently rotating and tumbling in a similar fashion to these known, close-in comets.

This movie shows the tumbling, propeller-like rotation of Ultima Thule over the span of nine hours between 20:00 UT (3 p.m. ET) on Dec. 31, 2018, and 05:01 UT (12:01 a.m. ET) on Jan. 1, 2019, as seen by the Long Range Reconnaissance Imager (LORRI) aboard NASA’s New Horizons.(NASA/JHUAPL)

The only difference? It’s still incredibly distant from the Sun, causing its ices to remain intact.

Based on the data that has come back so far from the New Horizons mission and its images of 2014 MU69 (Ultima Thule), we’ve been able to construct a 3D model of what this object looks like. Its two-lobes appearance, with a smooth, reflective neck, reveals a comet-like nature that is still frozen completely, having never had its volatiles sufficiently heated by the Sun. (GETTY)

Ultima Thule looks just like a typical cometary nucleus, marking the first time we’ve imaged one in its place of origin: the Kuiper belt.

This Tuesday, Jan. 1, 2019 image made available by NASA shows the Kuiper belt object Ultima Thule, about 1 billion miles beyond Pluto, encountered by the New Horizons spacecraft. The brightness differences correspond to differences in surface reflectivity. It will take approximately 20 months, given New Horizons’ current distance and trajectory, to download all the data taken during the 2019 New Year’s flyby. (ASSOCIATED PRESS)

Mostly Mute Monday tells the scientific story of an astronomical phenomenon or object in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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