Early Tuesday morning (Jan. 1), NASA's New Horizons probe cruised past Ultima Thule, a small body that lies about 4 billion miles (6.4 billion kilometers) from Earth. At 12:33 a.m. EST (0533 GMT), the spacecraft zoomed within a mere 2,200 miles (3,500 km) of Ultima — more than three times closer than New Horizons got to Pluto during its epic flyby of the dwarf planet in July 2015.
The best images from the flyby won't start coming down to Earth until today (Jan. 2), but the mission team already has a pretty good idea of Ultima's shape. Photos taken by the probe yesterday (Dec. 31) during its approach reveal that the object is highly elongated, with two distinct lobes — or that it's a system of two close-orbiting bodies. [New Horizons at Ultima Thule: Full Coverage]
"If it's two separate objects, this would be an unprecedented situation in terms of how closely they're orbiting to one another," New Horizons principal investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado, said during a news conference yesterday.
So, he added, "my bet would be that it's probably a single object — it's bilobate. And if I'm wrong, I'll tell you tomorrow."
New Horizons project scientist Hal Weaver, of the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, is also putting his money on the single-body option. Small, bilobate objects are common throughout the solar system, Weaver noted during the news conference. He pointed to Comet 67P/Churyumov-Gerasimenko, which was explored by Europe's Rosetta mission, as an example.
The mission team already had inklings of Ultima's shape. "Occultation" observations made from the ground, when mission scientists watched Ultima pass in front of background stars, had suggested the object is elongated, as had photos taken by New Horizons over the past week or so.
The mission team also now has a better idea of Ultima's size — about 21.7 miles long by 9.3 miles wide (35 by 15 kilometers), Stern said. And the newly released imagery has helped scientists solve a mystery — why New Horizons hasn't seen any substantial brightness variation from Ultima over time, as would be expected with an elongated body. From New Horizons' perspective, Ultima is spinning like a propeller, with its axis of rotation pointed at the probe, mission team members said.
The shape of MU69. It is about 35 by 15 kilometers. Still don't know if it's 2 separate objects or one merged one. pic.twitter.com/WL6NmisajA— Emily Lakdawalla (@elakdawalla) January 1, 2019
New Horizons will continue beaming Ultima Thule flyby imagery and measurements for a long time to come; all the data won't be in hand until about 20 months from now, Stern said.
And it's possible there could be another deep-space encounter in New Horizons' future. The probe has enough fuel to potentially pull off another flyby, if NASA grants an additional mission extension, Stern has said.
Mike Wall's book about the search for alien life, "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate) is out now. Follow him on Twitter @michaeldwall. Follow us @Spacedotcom or Facebook. Originally published on Space.com.