New Horizons’ 2019 Target: A Binary Body?
By: Kelly Beatty | August 4, 2017
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 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.
