The first science results from the pioneering New Horizons mission to visit an object in the outer Solar System have been released, providing new insights into this fascinating object at the far reaches of the Sun.
On January 1, 2019, NASA’s New Horizons spacecraft flew past the object 2014 MU69 (also informally known as Ultima Thule) at a distance of 3,500 kilometers, and 6.6 billion kilometers from the Sun. This is the most distant object ever visited by humanity.
This paper published today in the journal Science lays out some of the major findings from New Horizons so far. But only 10% of its data from the flyby has been returned, with the last not returning until September 2020, so there will be plenty more information to come in the future.
“The major findings are first that it’s a contact binary,” says Alan Stern from the Southwest Research Institute in Boulder, Colorado, principal investigator on New Horizons and lead author on the paper. “Secondly the flattened shape, which was completely unexpected. Third, the lack of craters on the surface. And fourth, the strange reflectivity markings across the surface.”
Shortly after the flyby, New Horizons returned images that revealed MU69 to have an odd two-lobed structure, earning it the nickname “the snowman”. It later transpired that the two lobes were actually flattened disks, believe to have stuck together early in the Solar System.
MU69 is itself thought to be a remnant of the primordial Solar System, giving us a glimpse at some of the material that would go on to form the planets and moons that now orbit the Sun. It orbits in the Kuiper Belt, a region of asteroids beyond Neptune, and is known as a Cold Classical Kuiper Belt Object – one that has been largely untouched since the formation of the Solar System.
“The New Horizons flyby of MU69 is humanity’s first look at one of the building blocks of our Solar System,” says Anne Verbiscer from the University of Virginia, one of the co-authors on the paper. “We knew very little about this object before the flyby.”
In the paper, the researchers note that they believe MU69 was formed by a relatively gentle collision of two bodies, known as a contact binary. It’s thought many similar such objects exist in the Solar System, but this is the first time we’ve been able to get up close and study one.
It’s unclear why each lobe has a flattened shape, with ideas ranging from the bodies forming from a thin sheet of pebbles, a head-on collision with similarly-sized bodies, or deformation caused by rapid rotation of the bodies. The flatness itself came as a surprise to researchers though when the first images were returned. “What was not anticipated was that it was so flat!” says Verbiscer. “It was a delight to see it in full detail.”
Noticeably present on the smaller lobe is a large impact crater, named Maryland. It’s thought that this must have formed prior to the two bodies merging together, or else they likely wouldn’t still be together now. “A collision that’s large enough to create that big a hole in the ground would probably dislodge the binary today,” says Stern.
On the larger lobe is a ring of bright material, which has been nicknamed “the road to nowhere”, owing to the fact the ring itself is not quite complete. This is thought to be a low-lying area where bright material has collected around a mound on the surface of the object.
A neck of bright material appears to connect the two lobes, which was initially thought to have resulted from the two bodies joining together. But that may not be the case. “The neck is a mystery,” says Stern. It’s unclear if the neck’s material appeared at the time of the merger, or sometime after.
One of the other surprising findings from the object was that it had no small moons or satellites orbiting it. Many other such objects in the Solar System have been observed to have satellites, so it’s somewhat odd that MU69 does not. One possibility is that it did, but they gradually merged into the object over time.
Despite releasing these first science results, there are still many unanswered questions about MU69. Further images to be released later should show us the side of the object, letting us look at its flatness edge-on. Scientists also want to try and work out how this object formed and evolved, and whether this process was rare or common in the Solar System.
“The big questions are to figure out how MU69 was formed, how those two lobes joined and what that process tells us about the formation of the Solar system and how other cold classical objects like MU69 were formed,” says Verbiscer.”
The New Horizons team are now also working on exploring another object in the Kuiper Belt with the remaining fuel on board the spacecraft. A decision on that isn’t expected until 2020, however, with that flyby occurring later in the 2020s. But visiting a second object would give us some fascinating extra detail on these distant Solar System objects.
For now, all eyes are on MU69, as New Horizons continues to transmit data from its flyby back to Earth. And there could be plenty more interesting discoveries along the way as researchers get a better handle on the origin of this object, and how it has changed over time.