Images and other observations taken by the New Horizons spacecraft early on New Year's Day reveal a remarkable (but not unexpected) two-lobed object.
Ever since NASA's New Horizons made its triumphant sweep through the Pluto system 3½ years ago, mission scientists have eagerly awaited the craft's arrival at its next destination: the Kuiper Belt object designated 2014 MU69 but better known as "Ultima Thule," the informal nickname (meaning "beyond the known world") adopted by mission scientists.
That encounter occurred in the first hours of 2019, and in the four days since the spacecraft has raced another 5 million kilometers (3½ million miles) from Earth along its never-to-return trajectory. In that time the slow but steady trickle of observations beamed to Earth by a 15-watt transmitter have both stunned and elated the mission's scientists. Those results have morphed 2014 MU69 from a 26th-magnitude blip barely observable by the Hubble Space Telescope into a tiny, colorful, intriguingly detailed world.
Actually, it's two roundish worlds nestled against one another, termed a contact binary, with one lobe somewhat larger than the other and a combined length of 33 km (21 miles). Images released by the mission team so far, which show details as small as 140 meters (450 feet) across, show that the lobes are both dark, slightly reddish, and mottled with brighter and darker markings.
This two-lobed "snowman" appearance wasn't unexpected — that shape had been inferred from a challenging but successful ground-based effort to record the object's passage in front of a star in July 2017. Moreover, astronomers now suspect that roughly a third of 100-km-size objects in the "classical" Kuiper Belt are also binaries. These objects are dynamically "cold" — their roughly circular, low-inclination orbits are 40 to 50 astronomical units from the Sun and oriented much like the planets'. Some of these binaries are likely two halves joined together, while others are separate, similar-sized spheres closely orbiting one another.
Still, "suspecting" isn't "knowing," and the spacecraft's flyby of 2014 MU69 offers planetary scientists a game-changing view of what happened when our solar system formed 4½ billion years ago. Dynamicists suspect that this a body that formed directly in the frigid fringe of the solar nebula and has remained largely unchanged ever since.
Remarkably, this object became New Horizons' post-Pluto target simply because it could be reached by the spacecraft. But as principal investigator Alan Stern notes, "I'm surprised that by picking just one Kuiper Belt object out the hat, we were able to get such a winner as this."
The larger globe of 2014 MU69 has roughly three times the volume of its companion. (The mission team has dubbed them Ultima and Thule, respectively, to remember which is which.) They probably consist primarily of ice, but their surfaces are actually quite dark, reflecting between 6% and 13% of the weak sunlight striking them. The reddish hue, thought to arise from complex organic compounds irradiated by space radiation for eons, matches that of other low-inclination Kuiper Belt objects.
But the narrow "neck" joining the two globes is telling a different story. It's both the brightest and the least red of the surface seen so far. According to Jeff Moore (NASA-Ames), this could mean that the neck ha a different composition. Or perhaps it's where small particles have "rolled" down steep slopes toward the object's center of mass.
Although initially uncertain, two lobes rotate around in roughly 15 hours. This relatively slow spin doesn't create nearly enough centripetal force to fling them apart. They're "soundly bound" in a structural sense, notes Moore, though they're essentially "resting on each other."
Meanwhile, the spacecraft's ultraviolet spectrometer has been searching for any wisps of gas nearby, and its cameras have been searching for small moons or hints of rings. No one expects such a small object to have an atmosphere, but the lack of moons is something of a disappointment. As co-investigator Mark Showalter (SETI Institute) notes, "Any moon at all, on any orbit at all, will tell us the mass and density" of 2014 MU69.
Beginning today, through January 7th, New Horizons' line-of-sight direction will be too close to the Sun for reliable radio transmissions, but they'll resume later next week. Still to come are observations taken when the spacecraft passed closest, at a distance of just 3,535 km (2,195 miles).
Buried among those are a series of images taken by the Long Range Reconnaissance Imager (LORRI), essentially a 20-cm f/13 telescope not unlike what you might use for stargazing in your backyard. Stern is "guardedly optimistic" that, when those views reach the ground late next month, they'll reveal details on the two lobes' surfaces down to about 17 m (55 feet) across.