At just 1 minute past midnight on New Year’s Day, a spacecraft flew by a tiny chunk of rock 6.4 billion kilometers from the Sun.
The object, called 2014 MU69, orbits within the Kuiper Belt, a region beyond Neptune’s orbit that is populated by small, icy remnants from the solar system’s formation. NASA’s New Horizons spacecraft zipped just 3,500 kilometers from the surface of MU69 at a speed faster than 14 kilometers per second. Roughly 10 hours after the craft’s closest approach, the mission team received confirmation that the flyby was successful. The team glimpsed the first images from the approach shortly after.
Scientists are now poring over the few crumbs of data that have come in from New Horizons in the days since the flyby. The first data packages contained high-resolution images, multispectral images, and reflectivity data. Here are 10 things that the team has already learned about MU69 from the first downlink and closest-encounter data.
1. A Contact Binary
Images taken before the flyby showed that MU69 is oblong. Whether it was one object, two separate objects, or two objects in contact remained unclear. With the new images, “we believe that this object is a contact binary that formed close by each other in the protoplanetary disk,” Silvia Protopapa, a mission coinvestigator from the University of Maryland in College Park, said during a 3 January press conference. This means that two ellipsoidal objects formed separately, came into contact, and stuck together without being destroyed.
2. Rusty Red
“We had a rough color from Hubble, but now we can definitively say that [MU69] is red,” Carly Howett, New Horizons coinvestigator from the Southwest Research Institute (SwRI) in Boulder, Colo., said in a 2 January press conference. The lobes of MU69 are similar shades of red, are mottled across their surfaces, and are significantly redder than the neck region and other bright spots, she explained. The color is slightly lighter than the dark north pole region of Pluto’s moon Charon.
3. Rolling Hills
The two lobes of MU69 are not perfect ellipsoids, according to Jeff Moore, the mission’s lead investigator for geology and geophysics at NASA Ames Research Center in Moffett Field Calif. Compared to a smooth ellipse, there are elevation changes greater than 1 kilometer on the larger lobe. There are also noticeable divots that flatten out portions of the rim of the smaller lobe.
4. No Moons Yet
“We’ve seen bodies one and two, so the question now is, What about bodies three, four, five, and so on?” asked Mark Showalter, New Horizons coinvestigator from the SETI Institute in Mountain View, Calif. The object’s likely formation scenario suggests that there may be smaller objects in orbit around MU69. Finding satellites would also help reveal the body’s total mass and average density, which are key for pinpointing its interior structure and composition, Moore said. “We have not found any other bodies in the system,” he said, “but we’re still looking.”
5. Slowly Rotating
The object “rotates with a period of about 15 hours,” said mission coinvestigator Cathy Olkin from SwRI. What’s more, MU69 rotates in such a way that it keeps roughly the same surface area facing Earth at all times, she said, similar to an airplane propeller. An impact with a smaller object may have slowed down MU69’s initial rotation from about 4 hours to its current 15-hour period, the team said.
Zoom with a view: Made from two images taken 38 minutes apart, here is #UltimaThule! The "Thule" lobe is closest to the @NASANewHorizons spacecraft. As Ultima Thule is seen to rotate, hints of the topography can be perceived. @NASAhttps://t.co/fKsyLeW9pY pic.twitter.com/5S9zZ5d3Eg
— Johns Hopkins APL (@JHUAPL) January 3, 2019
6. Old and Primitive
“What we’re looking at is perhaps the most primitive object that has yet been seen by any spacecraft,” said Moore. MU69 is likely a cold classical Kuiper Belt object, a class of small bodies that are thought to be relatively untouched since the formation of the solar system, Protopapa said. “The term ‘cold’ does not refer to the objects’ temperatures, although they are very cold,” she said, “but refers instead to the primitive aspect of this population.” MU69 falls well within the average color and orbital tilt of this class.
7. Mostly Dark with a Shiny Necklace
“The light that we see on [the surface] is reflected from the Sun,” Olkin explained, “and we can see variations depending on how bright the surface is.” MU69’s reflectivity, or albedo, ranges from 13% at its brightest to 6% at its darkest. “This is like potting soil,” she said. The narrow neck contains some of the most reflective material on the object, Olkin explained, which may be a clue to the composition or grain size in that region.
My new sketch of @NewHorizons2015’s small Kuiper Belt object, #MU69! It’s a glimpse into the discussions going on behind the scenes: what do the shapes and colors mean and what geologic processes are at play? We will soon find out! @NASANewHorizons @JHUAPL #UltimaFlyby pic.twitter.com/4gyXY4forD
— James Tuttle Keane (@jtuttlekeane) January 3, 2019
8. No Atmosphere
Mission scientists do not expect MU69 to have an atmosphere or alter the flow of the solar wind in any measurable way, said Leslie Young, who is part of New Horizons’ atmospheric research team at SwRI. After the craft’s closest approach, it turned back toward MU69 to capture a backlit image of the object, which is how it measured the many layers of Pluto’s atmosphere in 2015. Initial data from the atmospheric dust, plasma, and ultraviolet instruments are in scientists’ hands and should be processed soon, Young said.
9. Ices Maybe?
“Our current theory as to why [MU69] is red is the irradiation of exotic ices,” Howett said. Alteration of methane, nitrogen, and carbon monoxide ices made much of the surface of Pluto and Charon’s north pole red. This theory, Protopapa warned, may not hold up on such a small object, which usually have surfaces covered with ammonia or water ice. The team does not expect to find exotic ices on MU69, Protopapa said, “but we will look for it and have the spectral resolution to identify these ices,” she said.
10. Craters and Ridges
The first close-up of MU69 sent back was not at the right angle to show small-scale elevation changes from craters or ridges. Neither was the second. When the team combined the two, however, they were able to create a low-resolution stereographic image that shows some high and low reliefs.
“You see how narrow the neck is, and we also get a sense that bright spots may be associated with topographic lows,” said Paul Schenk, a New Horizons coinvestigator from the Lunar and Planetary Institute in Houston, Texas. “This may indicate downslope movement of material from topographic highs like ridges. This may be an indication that the dark striping materials might be topographic ridges.” More images will refine these stereographic views and test these theories, he said.
20 Months of Data Ahead
Astronomers still have a long list of questions about MU69 that they are looking to New Horizons to answer. Data from the encounter will take about 20 months to fully downlink from the spacecraft. Forthcoming higher-resolution data should answer the lingering questions about this distant object, the team expects.
Just days after the flyby, New Horizons is already more than 3 million kilometers past MU69. “The first exploration of a small Kuiper Belt object and the most distant exploration of any world in history is now history, but almost all of the data analysis lies in the future,” Alan Stern, principal investigator for New Horizons from SwRI, said during a press conference. “Those of us on the science team can’t wait to begin to start digging into that treasure trove,” he said.
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer