Way out on the cold and dark edge of the solar system – past rocky planets inside, outside the gas giant, a billion miles farther than Pluto – hovering in a small frozen world that is so mysterious, scientists are still not entirely sure whether it one world or two.
Astronomers call it Ultima Thule, an old cartographic term meaning "outside the known world." Its name is a reference to its location in the Kuiper Belt, the unexplored "third zone" of our solar system inhabited by millions of small ice objects.
Although there are many, no Kuiper Belt object has ever been seen up close. NASA's two Voyager vehicles – which crossed the third zone decades ago – might catch a glimpse of one if they were equipped with the right instruments, except that the Kuiper Belt had not even been detected. On New Year's Eve, for the first time, NASA will get a chance at some time with one of these enigmatic space rocks.
At 21:33 PST, past midnight 33 minutes on the East Coast, a New Horizons investigation into the body will make a close pass from Ultima Thule, making it the most distant object the spacecraft has ever visited.
Astronomers hardly know what awaits them. "What does it look like? Nobody knows. What is it made of? Nobody knows. Is there a ring? Moon? Is there an atmosphere? Nobody knows. But in a few days we will open the gift, look in the box, and find out, "said Alan Stern, mission lead investigator.
New Horizons has traveled for 13 years and crossed 4 billion miles to reach this point, and the vehicle looks in good shape: The mission planner confirmed earlier this month that he would pass a distance of 2,200 miles from Ultima Thule after determining the large object, such as months, and smaller ones, such as dust, are unlikely to pose a threat to spacecraft because they flare more than 31,000 miles per hour. ("When you travel that fast, hitting something even the size of a grain of rice can destroy a spaceship," said Hal Weaver, mission project scientist.)
The New Horizons track will take it three times closer to Ultima Thule than Pluto, which passed it in the summer of 2015. Radiant New Horizons photographs at that time were the most detailed ever captured not only from the previous planet, but the outer solar system. Because of its proximity, the images collected by the probe from Ultima Thule will be even more detailed, and from a billion miles deeper in space. "Pluto opened our door," said Stern, "but now we are headed for something far more wild and hairy."
Stern and his team discovered objects in 2014 using the Hubble Space Telescope, while searching for the sky for places that New Horizons can visit after a brief meeting with Pluto. In the first pictures, Ultima is just a lump of pixels that shifts every few minutes against the background of a stationary star.
In a more recent image, captured by the New Horizons Long Range Reconnaissance Imager, the object still looks a little more than a speck in a brighter speck of ocean. "When you look for it, it looks like the stars are vomiting in all imagery," said planetary scientist Amanda Zangari, who spent most of December collecting Ultima Thule's position and brightness measurements. "To see the damn thing, you need to stack a lot of pictures, calculate the distortion between them, and reduce the stars." At 1/100 of Pluto's diameter, and the brightness of 1/10/10, Ultima Thule makes the quarry more difficult to understand than the previous planet.
Through their observations, the team determined that Thule (whose official designation was MU69 2014) is two separate objects that orbit each other at close range, or a pair of objects leaning toward each other until they join, forming two lobes something astronomers call binary contact. Either way, the data shows Ultima is no more than 20 miles in diameter, dark like reddish soil, and also within New Horizons' fuel supply range.
Most likely too, very, very old. That is why astronomers are very eager to study it up close.
Kuiper Belt objects such as Ultima Thule are considered the remnants of the solar system formation – the cosmic rejection that remained after the planet formed around 4.6 billion years ago. That made them an attractive destination for astronomers: Many of these objects were not only ancient, they also, thought astronomers, were guarded perfectly by temperatures close to absolute zero. (So far this is Ultima Thule from the warm sun, that our parent star will appear to the observer on its surface about the size of Jupiter from here on Earth). NASA's plan to visit one, map its features, study its makeup, detect its atmosphere (if any), and search for satellites and rings is more than a flying mission. This is an archaeological expedition of scale and cosmic consequences.
New Horizons will investigate Ultima with the same set of instruments used to study the Pluto system in 2015. Three optical devices will capture images of objects in color and black and white, map their composition and topography, and look for gases originating from the surface. Two spectrometers will also look for charged particles in the Ultima Thule environment; radio science instruments will measure the surface temperature; and the dust counter will detect interplanetary debris spots. Fully loaded, this piano-sized probe weighs more than 1,000 pounds and requires less power than a pair of 100-watt light bulbs to operate the equipment.
After flying New Year's Eve, New Horizons will continue its path out of the Kuiper Belt. But the third zone is very wide. Even traveling at nearly nine miles per second, it will require a decade of spacecraft to pass through and enter interstellar space. Stern and his colleagues will use that time to look for other targets – one even farther away from the sun than Ultima Thule, and envelop, perhaps, in even more mysteries. This is a lucrative prospect for the New Horizons team. "To visit a place you don't know," Weaver said. "That's the best exploration."
Learn More About the New Horizon Mission
- In 2015, New Horizons passed Pluto, giving astronomers their closest view of the planet and its moons.
- The NASA probe traveled about 3 billion miles to reach Pluto. Another billion is still needed, to reach Ultima Thule.
- How did New Horizons transmit all of its observations back to Earth, even though it was so far away? Very slow.