NASA’s New Horizons spacecraft made a New Year’s flyby of this snowman-shaped asteroid, which the team nicknamed “Ultima Thule,” a Latin phrase that refers to a distant place beyond known borders. (The International Astronomical Union will formally determine an official name at a later date.)
MU69 is located a vast 44 astronomical units (Earth-sun distances) away from Earth — about two-thirds farther away than Pluto, which the spacecraft flew by 3.5 years ago. The new data from New Horizons is fresh, low-resolution, and only a fraction of the information that is stored on board the spacecraft’s hard drive.
It will take 20 months to receive all of the data.
“Those of us on the science team can’t wait to begin to start digging into that treasure trove,” principal investigator Alan Stern of the Southwest Research Institute remarked at a press briefing on Thursday. He cautioned journalists not to read too much into what we see so far — or what we don’t yet see, for that matter.
Moons would be little surprise in the Kuiper Belt, a string of icy objects that lies far beyond Pluto, because so many dwarf planets and asteroids are known to have them. So the team did a large-scale search for moons before and during the flyby, and several more observations are planned in the coming weeks and months. The team hasn’t yet spotted any object within 100 miles of MU69 or as far out as 500 miles.
The team will keep looking, because finding a moon would be advantageous. Satellites are a common way for scientists to learn about the mass and density of the worlds they orbit, which can reveal important details about their composition and interior.
Moons would also give credence to the favored origin story for MU69. It’s a two-lobed, dumbbell-shaped world with two circular-shaped objects just barely touching each other – a formation that astronomers call a “contact binary.” As the story goes, MU69 formed in a cloud of debris. Over time, a mass of dust came together and grew into pebbles, then rocks. The largest two objects eventually spiraled in toward one another and collided. But where the rest of the debris lies remains a mystery.
Team members are also looking hard for craters, but the geometry is against them right now. New Horizons imaged MU69 in full sunlight, making it hard to see anything but vague shapes of ridges. Fresh stereo pictures downloaded to Earth in the past day do show divots on the surface of MU69, but Stern pointed out that there aren’t enough shadows to see if there are craters for sure.
“I’m going to trust in my stereo experts that say that at this phase angle, you cannot trust the topography,” he remarked at the press conference.
New Horizons will disappear behind the sun (from Earth’s perspective) for much of the coming week, so data downloading will need to pause for five days. Better images will start filtering down to Earth before mid-month, allowing scientists to look for not only moons and craters, but also an atmosphere.
While the flyby is over, the work for New Horizons in the Kuiper Belt continues. The spacecraft will scan other objects from a distance with its telescope to learn more about this vast region of icy bodies. And team members plan a new mission proposal to NASA next year that could include another flyby, as long as funds allow for it. The spacecraft will be in good shape for at least a decade and probably much longer, Stern noted, and there’s ample fuel to spare.