New Horizons’ 2019 Target: A Binary Body?

A challenging and intensive campaign by telescope-toting teams yields big-time results on the body that the New Horizons spacecraft will fly past in 17 months.

The quest is on to learn everything possible about the Kuiper Belt object (KBO) designated 2014 MU69 before New Horizons flies by at close range on January 1, 2019 — now less than 17 months away. And that effort got a huge boost from an intensive campaign to capture the object passing in front of background stars during three occultations over the past two months.

Three occultations by object 2014 MU69

During mid-2017, the small Kuiper Belt object 2014 MU69 occulted (briefly hid) three faint stars as seen from Earth. Here are the predicted tracks, each only about 50 km wide, from which those disappearances were visible.

The campaign’s first two events, on June 3rd and July 10th, didn’t score a "direct hit” — catching the starlight blinking out momentarily as notoriously dim (27th-magnitude) 2014 MU69 passed in front. But ground-based observers fared much better on July 17th, when five of the 24 observing teams recorded brief cutoffs of light from a 12.6-magnitude star near the Teaspoon asterism in Sagittarius.

Marc Buie holds up five fingers

Researcher Marc Buie raises a handful of fingers to signify how many observing teams recorded the occultation of a star by Kuiper Belt object 2014 MU69 on July 17, 2017.
NASA / JHU-APL / SwRI / Adriana Ocampo

These teams, which included some experienced amateur observers, were arrayed about 4½ km (3 miles) apart along a “picket fence” perpendicular to the occultation’s predicted path. The field conditions, in remote parts of Argentina’s Chubut and Santa Cruz provinces, weren’t ideal. Strong winds buffeted the 16-inch-aperture Sky-Watcher reflectors, and some local citizens (including the mayor of Comodoro Rivadavia) helped out by positioning their trucks as windbreaks.

As seen from the ground, the unseen 2014 MU69 approached the star at about 24 km per second (due to Earth’s orbital motion, not the KBO’s), meaning that any cover-up would last at most 3 seconds. The teams used cameras that recorded five frames per second. Since five of them reported disappearances, and given their 4½-km spacing, the object has to be 20 to 30 km (12 to 18 miles) long.

2014 MU69 occultation plot

This plot shows the five chords created when a star disappeared behind 2014 MU69 on July 17, 2017. The circles represent how a binary body can be matched to the observations.
NASA / JHU-APL / SwRI / Alex Parker

However, the occultation timings don’t match the silhouette that you’d expect for a round object. Instead, reports campaign leader Marc Buie (Southwest Research Institute), it’s likely to be highly elongated (like a skinny eggplant). Or it might be generally spherical with a big chunk missing.

Or it might actually be two objects in a close binary pairing, either touching one another or twirling around each other at very close range. “The shape of MU69 is truly provocative,” says Alan Stern (also at SwRI), New Horizons’ principal investigator.

Corroborating observations could come from an extensive series of Hubble Space Telescope observations — four sets of six consecutive orbits — carried out by Susan Benecchi (Planetary Science Institute) between June 25th and July 4th. She reports that 2014 MU69 showed little variation in brightness during the first observing run but that the changes got more pronounced later on. Also, earlier HST observations revealed “drop-outs” in some images. All this is consistent with a double object, Benecchi says.

2014 MU69 comparison artwork

What's the real shape of 2014 MU69, a Kuiper Belt object that's 43.3 astronomical units (6.5 billion km) from the Sun? Data from ground-based stellar occultations and the Hubble Space Telescope suggest it's either highly elongated and up to 30 km long (as depicted at left) or a binary body whose halves are each about 15 to 20 km across.
NASA / JHU-APL / SwRI / Alex Parker

Meanwhile, negative results from the two earlier occultations (especially observations from NASA's SOFIA flying observatory on July 10th) build confidence among Stern and his science team that their forthcoming target isn’t ringed by impact debris — clearly a threat to any spacecraft whizzing by at high speed.

The occultation results imply that 2014 MU69 is smaller than originally thought, so it’s surface must be somewhat more reflective than those of the Kuiper Belt’s usually darkish denizens. Add to that the prospect of visiting a binary body for the first time (if you don’t count Comet 67P/Churymov-Gerasimenko), and you’ve got a forthcoming flyby that’s sure to amaze.

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