After the New Horizons mission zoomed by the then-unknown Ultima Thule object in the outer Solar System early on New Year's Day, the spacecraft began returning data to Earth via a deep-space network. Although only about 1 percent of that data is now on the ground, scientists were able to share some top-line findings on Wednesday.
What they have found at a distance of 44 astronomical units (AU) from Earth, or 44 times the distance between the Earth and the Sun, is something of a wonderland. The Ultima Thule object, formally named 2014 MU69, is a 33km-long contact binary that somewhat resembles a snowman. Each of the two components is nearly spherical, coming together at a "neck," and this has proved important to confirming scientific ideas for how the planets and moons of the Solar System formed. Overall, the object has a reddish hue, similar to the north pole of Pluto's moon Charon.
With Ultima Thule, the astronomers behind the New Horizons mission say they have uncovered a pristine planetesimal—a small object that served as a building block for larger objects in the Solar System. In the early years of the Solar System, some 4.5 billion years ago, a large, congested disk of these planetesimals came together to form the planets and moons we now recognize in the inner Solar System.
But in the outer Solar System (and particularly the Kuiper Belt that lies beyond the orbit of Neptune), there were fewer of these planetesimals to spread around a much larger area. So some of them, like Ultima Thule, are relics of the Solar System's first few million years. "This may be the most primitive object seen by any spacecraft, and oldest that can be seen anywhere in the Solar System," said Jeff Moore, who leads the geology team for the New Horizons spacecraft, during a news conference on Wednesday.
Based on the shape and appearance of Ultima Thule, scientists believe it formed from the very slow accretion of two bodies. Over the millennia, the bodies spiraled glacially inward, perhaps at a rate of just 2-3km per hour with respect to one another. They are bound by gravity.
The data revealed Wednesday is pretty remarkable considering that Ultima Thule was not even discovered until the summer of 2014 and that scientists had to photograph the object while traveling by it at a rate of 51,500km/hour. It is only about the size of Washington, DC, illuminated by sunshine that is 1,900 times fainter than on the surface of Earth, and has the reflectivity of potting soil.
"What this spacecraft and team have done is unprecedented," said Alan Stern, the mission's principal investigator.
And New Horizons is not finished. More data is now coming down that will detail Ultima Thule's composition, and more and better images are on the way. The best photographs released so far have had a resolution of about 140 meters per pixel and were taken from 50,000km away. Astronomers hope to get images with a resolution of up to 35m per pixel, Stern said.
He is also hopeful that the spacecraft will win additional funding for a third flyby—it has already captured extraordinary data about Pluto and, now, Ultima Thule. New Horizons has power and fuel for another 15 to 20 years, he said. The Kuiper Belt extends out to about 70 AU, and at its present rate, New Horizons will spend about another decade in it.
This will offer plenty of time to discover an additional object of interest along or near New Horizons' path, Stern said. The mission team will submit a bid to to NASA next year, and if the space agency approves, the astronomers will work with telescope observing teams to find another Kuiper Belt Object of note. The Johns Hopkins Applied Physics Laboratory designed, built, and operates the New Horizons spacecraft, and it manages the mission for NASA's Science Mission Directorate.
Listing image by NASA