Ultima Thule is one of the solar system’s most hermitlike homebodies. In the 4.6 billion years since the small icy world formed, astronomers think that it has never moved from its original orbit about 6.5 billion kilometers from the sun. And no other large object has ever come calling.
That’s about to change.
Just after midnight eastern time on January 1, NASA’s New Horizons spacecraft will zip past Ultima Thule at a distance of about 3,500 kilometers — likely the space rock’s closest visitor ever.
“It just screams out at you, ‘I’ve just been sitting here unperturbed for all of this time,’” New Horizons project scientist Hal Weaver says of Ultima Thule, whose official name is 2014 MU69. “‘Come and examine me.’”
Ultima Thule makes its home in the Kuiper Belt, a loose confederation of thousands of space rocks of various sizes that orbit the sun at greater distances than the solar system’s planets. New Horizons is the first spacecraft deliberately sent to explore the area; the probe flew past its first Kuiper Belt object, dwarf planet Pluto, in 2015 (SN Online: 6/5/18).
Two other missions, Pioneer and Voyager, also sent probes into the Kuiper Belt, and measured charged particles and magnetic fields there. But since those craft reached the region before the Kuiper Belt was discovered in the 1990s, they flew right past the zone’s rocky, loosely spaced residents unawares.
New Horizons is the first spacecraft deliberately sent to explore the Kuiper Belt, a region populated with space rocks big and small that orbit the sun in a disk starting just beyond Neptune’s orbit. Dwarf planet Pluto is considered a Kuiper Belt object.
The New Horizons team has been eager to get a close look at one of the smaller Kuiper Belt objects. But choosing the right rock was tricky. It had to be along the spacecraft’s path and it had to be antisocial, with an orderly orbit that planetary scientists think has never been disturbed. These loners, called cold classical Kuiper Belt objects, are the best-preserved remnants of the first materials present in the disk that formed the planets.
“The cold classicals have always been there, we think,” says planetary geologist Jeffrey Moore of NASA’s Ames Research Center in Moffett Field, Calif., the head of New Horizons’ imaging team. “They represent the oldest, coldest stuff that formed when the solar system formed. It’s just been sitting out there in deep storage since the beginning of solar system history.”
Not all of the Kuiper Belt’s denizens are so withdrawn. Some have made trips to closer-in parts of the solar system, where the rocks’ surfaces were heated by sunlight. Some have highly elliptical orbits, or orbits that are tilted against the plane of the rest of the solar system like Pluto’s, suggesting that the objects suffered a gravitational run-in with another body sometime in their past.
Ultima Thule was one of just two suitable objects found using the Hubble Space Telescope in 2014, and was chosen because the rock could be reached using the least amount of fuel. “This is the one we can get to, but it’s representative of a much larger population of objects,” says planetary astronomer Susan Benecchi of the Planetary Science Institute, based in Herndon, Va., who worked on the Hubble search.
Not much else is yet known about Ultima Thule. Observations made in 2017 as the object passed in front of a distant star suggest that the rock is about 30 kilometers across, and might even be two rocks orbiting each other (SN Online: 12/12/17). More Hubble observations suggest that Ultima Thule is slightly reddish, which could mean it doesn’t have a lot of exposed ice on its surface, Benecchi says.
New Horizons’ cameras finally caught sight of Ultima Thule in August, when it appeared as a tiny dot of light (SN Online: 8/29/18). “We’ve been intensively observing MU69, trying to tease out as much as we possibly can,” says Weaver, of Johns Hopkins Applied Physics Laboratory in Laurel, Md. “It’s not revealing its secrets very easily.”
Weaver and the hazard avoidance team — called the “hard hats” — scoured the region of space around Ultima Thule for any moons, rings or clouds of dust that could damage New Horizons as it approaches (SN: 10/28/17, p. 15). The team plans to have New Horizons fly past Ultima Thule at a distance of 3,500 kilometers while traveling at about 14 kilometers per second. That would be twice as close as the craft got to Pluto.
The spacecraft will take high-resolution images of Ultima Thule’s surface, and scan it with a spectrometer to measure its chemical makeup. “Knowing something about the kinds of ices that are preserved on MU69 would tell us about the kind of chemistry that was happening in the outermost solar nebula when the planets formed,” Moore says.
And the object’s overall shape and its surface features, such as craters and cliffs, could also give clues to how the space rock formed: whether it started as a large object that broke apart, or if it’s built up from an agglomeration of smaller rocks. “We’re excited to see what the object actually looks like” when the craft passes the object on January 1, Benecchi says. “It’s like opening a Christmas package, six days late.”
It will take much longer than that to get the full picture, though. New Horizons will spend the next 20 months sending its Ultima Thule data back to Earth as the craft continues coasting farther out into space.
While in the outer solar system, New Horizons will also observe another 25 to 30 Kuiper Belt objects, although none as closely as Ultima Thule. “This is the initial reconnaissance of the Kuiper Belt,” Weaver says. “We want to do it right.”
Editor’s note: This story was updated on December 19, 2018, to correct Hal Weaver’s title and add his research institution.