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.

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.

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.

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.

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.