Way out in the cold dark edges of the solar system — behind rocky inner planets, behind gas giants, a billion miles farther from Pluto — a tiny frozen world drifts, so mysterious that scientists are still not quite sure that this is one world or two.
Astronomers call this Ultima Thule, an old cartographic term meaning "outside the known world." Its name indicates its location in the Kuiper belt, an unexplored "third zone" of our solar system, inhabited by millions of small ice bodies.
Although they are numerous, not a single Kuiper belt object has ever been seen near. Two NASA Voyager probes, which passed through the third zone a decade ago, could catch a glimpse of one of them, if they were equipped with the right instruments, except that the Kuiper belt was not even detected yet. On New Year's Eve, NASA for the first time will get a chance at one of these mysterious space rocks.
At 9:33 pm Pacific time, 33 minutes after midnight on the east coast, the probe of the agency New Horizons will make a close passage along Ultima-Thule, making it the most distant object ever visited by a spacecraft.
Astronomers hardly know what awaits them. "How will it look like? Nobody knows. What will it be made of? Nobody knows. Does he have rings? Satellites? Does he have an atmosphere? Nobody knows. But in a few days we will open this gift, look in the box and find out, ”says Alan Stern, the chief investigator of the mission.
The new horizons passed 13 years and overcame 4 billion miles to reach this point, and the probe seems to be in good shape: mission planners confirmed earlier this month that it will pass within 2,200 miles of Ultima Thule after identifying such large objects as satellites and smaller ones, such as dust, were unlikely to pose a threat to the spacecraft, as it flew past at a speed of over 31,000 miles per hour. (“When you travel so fast, hitting something even the size of a rice grain can destroy a spacecraft,” says Hal Weaver, a project mission researcher.)
The trajectory of the New Horizons will bring it three times closer to Ultima Thule than to Pluto, which he shot in the summer of 2015. The photographs of New Horizons taken at that time were the most detailed ever taken, not only from the former planet, but also from the outer solar system. Because of its proximity, the images that the probe collects from the Ultima Thule will be even more detailed, and from a billion miles deeper in space. “Pluto blew our doors,” says Stern, “but now we are moving toward something much wilder and more obscure.”
Stern and his team discovered the object in 2014 using the Hubble Space Telescope, while they were looking for places in the sky that New Horizons could visit after their brief acquaintance with Pluto. In these first images, Ultima was just a pixel that shifted every few minutes against the background of the fixed stars.
In later images captured with New Horizons' Long Range Intelligence Camera, the object still looks like a speck in a sea of much brighter spots. “When you search for it, it looks like the stars are growing on all the images,” says planet scientist Amanda Zangari, who spent most of December collecting measurements of the position and brightness of Ultima Thule. "To even see the damn thing, you need to lay down a few images, take into account the distortion between them and subtract the stars." Ultima Thule, having 1/100 the diameter of Pluto and 1/10000 of its brightness, makes it more elusive a quarry than the former planet.
From their observations, the team determined that Thule (the official designation is MU69 2014) is either two separate objects orbiting around each other at close range, or a pair of bodies that were attracted to each other until they merged, forming two then astronomers call contact binary. In any case, the data shows that Ultima has a diameter of no more than 20 miles, dark as reddish dirt, and within reach of the New Horizons fuel reserve.
It is also, in all likelihood, very, very old. That is why astronomers are so excited to learn it closely.
It is believed that Kuiper belt objects, such as Ultima Thule, are the remnants of the formation of the solar system – space debris, which remained after the planets appeared about 4.6 billion years ago. This makes them an attractive place for astronomers: many of these objects are not just ancient, they are also, as astronomers believe, perfectly preserved at temperatures approaching absolute zero. (Ultima Tula is so far from the heating rays of the Sun that our parent star on its surface may seem to an observer the size of Jupiter from here on Earth). NASA's plan to visit one, map its features, study its composition, detect its atmosphere (if one exists), and search for it on satellites and rings is more than just a flight. This is an archaeological expedition of cosmic scale and consequences.
New Horizons explores Ultima using the same toolbox that was used to explore Pluto in 2015. A trio of optical devices will receive black-and-white and black-and-white images of the object, display its composition and topography, and search for gases emanating from its surface. Two spectrometers will also search for charged particles in the vicinity of Ultima Thule; a radio science device will measure its surface temperature; and the dust counter will detect the interplanetary debris spots. A fully charged probe the size of a piano weighs more than 1000 pounds of hair and requires less energy than a pair of 100 watt light bulbs to operate its equipment.
After its flight on the eve of the New Year, the New Horizons will continue their journey from the Kuiper belt. But the third zone is huge. Even traveling at a speed of almost nine miles per second, a spacecraft would take a decade to go through and enter interstellar space. Stern and his colleagues will use this time to search for another goal – even more distant from the Sun than Ultima Thule, and shrouded in, perhaps, even more mystery. This is a teasing prospect for the New Horizons team. “To visit a place you know nothing about,” says Weaver. "This is intelligence in all its glory."
Learn more about the New Horizons mission.
- In 2015, New Horizons flew past Pluto, giving astronomers the closest look at the former planet and its satellites.
- NASA's probe has traveled about 3 billion miles to reach Pluto. However, he achieved another billion to reach Ultima Thule.
- How does New Horizons transmit all of its observations back to Earth when it’s so far away? So slow.