Ultima Thule: Ancient Planetestimal in the Outer Solar System

Ultima Thule as seen in one of the first high-resolution images sent back by the New Horizons spacecraft.
Ultima Thule as seen in one of the first high-resolution images sent back by the New Horizons spacecraft.

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Early in the morning hours (eastern time) on January 1, 2019, the New Horizons spacecraft sped past the most distant explored object in the solar system. The tiny planetesimal it encountered is called 2014 MU69, nicknamed Ultima Thule. That term means "beyond the known world" and was chosen as a temporary name for the object during a public naming competition in 2018. 

Fast Facts: Ultima Thule

  • 2014 MU69 Ultima Thule is an ancient planetesimal orbiting in the Kuiper Belt, a region beyond Neptune. It is probably made largely of ice and its surface is reddish.
  • Ultima Thule is more than 44 astronomical units from Earth (an AU is 150 million kilometers, the distance between Earth and the Sun).
  • Two lobes, named Ultima and Thule, make up the body of this planetesimal. They attached early in solar system history in a gentle collision.
  • The New Horizons mission has been traveling to the outer solar system since its launch on January 19, 2006. It will continue through the solar system, through the Oort Cloud, and eventually to interstellar space. It has enough power to continue exploration through the 2020s.

What Is Ultima Thule? 

This tiny object orbits the Sun out in a region of space called the Kuiper Belt, well beyond the orbit of Neptune. Since Ultima Thule lies out in that region, it's sometimes referred to as a "trans-Neptunian object." As with many planetesimals there, Ultima Thule is a mainly icy object. Its orbit is 298 Earth-years long, and it gets only a tiny fraction of the sunlight that Earth receives. Planetary scientists have long been interested in little worldlets like this one because they date back to the formation of the solar system. Their distant orbits preserve them in very cold temperatures, and that also preserves scientific information about what conditions were like some 4.5 billion years ago, when the Sun and planets were forming. 

A schematic of the Kuiper Belt.
This perspective view shows the path of NASA’s New Horizons spacecraft (yellow) through the outer solar system and the Kuiper Belt. The orbits of the terrestrial and giant planets are shown in blue. The dots show the locations of representative asteroids, close to the Sun, and Kuiper Belt objects (KBOs), which are mostly beyond the orbit of Neptune, the outermost giant planet. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

Exploring Ultima Thule

Ultima Thule was the target of a hunt for another object to study by the New Horizons spacecraft after its successful flyby of Pluto in July 2015. It had been spotted in 2014 by the Hubble Space Telescope as part of a survey for distant objects beyond Pluto in the Kuiper Belt. The team decided to program the spacecraft's trajectory to Ultima Thule. To get an accurate idea of its size, New Horizons scientists programmed ground-based observations of this little world as it occulted (passed in front of) a more distant set of stars during its orbit. Those observations in 2017 and 2018 were successful and gave the New Horizons team a good idea of the size and shape of Ultima Thule.

Armed with that information, they programmed the spacecraft's path and science instruments to observe this dark distant planetesimal during the January 1, 2019 flyby. The spacecraft flew past at a distance of 3,500 kilometers at a speed of just over 14 kilometers per second. Data and images began streaming back to Earth and will continue until late 2020.

The scene in mission control at Johns Hopkins Applied physics lab when the first clear image of Ultima Thule arrived on January 1, 2019.
The scene in mission control at Johns Hopkins Applied physics lab when the first clear image of Ultima Thule arrived on January 1, 2019. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

For the flyby, the New Horizons team invited friends, family, and press. To celebrate the close flyby, which took place at 12:33 a.m. (EST) on January 1, 2019, the combined visitors and team held what one newspaper called "the geekiest New Year's party ever." One special part of the celebration was the performance of an anthem for New Horizons by Dr. Brian May, astrophysicist member of the New Horizons team and former lead guitarist for the rock group Queen.

To date, Ultima Thule is the most distant known body ever explored by a spacecraft. Once the Ultima Thule flyby was done, and the data transmissions began, the spacecraft swung its attention to more distant worlds in the Kuiper Belt, possibly for future flybys.

The Scoop on Ultima Thule

Based on data and images taken at Ultima Thule, planetary scientists have found and explored the first contact binary object in the Kuiper Belt. It's 31 kilometers long and has two "lobes" joined to form a "collar" around one part of the object. The lobes are named Ultima and Thule respectively for the small and large components. This ancient planetesimal is thought to be made largely of ice, with perhaps some rocky material mixed in. Its surface is very dark and may be covered with organic materials created as the icy surface was bombarded by ultraviolet radiation from the distant Sun. Ultima Thule lies 6,437,376,000 kilometers away from Earth and it took more than six hours to send a one-way message to or from the spacecraft. 

The first color images of 2014 MU69 Ultima Thule. The reddish material is likely a coating created by ultraviolet light interactions with the ices.
The first color images of 2014 MU69 Ultima Thule. The reddish material is likely a coating created by ultraviolet light interactions with the ices.  NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

What's Important About Ultima Thule?

Due to its distance from the Sun and its steady orbit in the plane of the solar system, Ultima Thule is thought to be what's called a "cold classical Kuiper Belt object." That means it has likely orbited in the same place throughout much of its history. Its shape is interesting because the two lobes indicate that Ultima Thule is made of two objects that gently drifted together and remained "stuck to each other" for most of the object's history. Its spin indicates motion that was imparted to Ultima Thule during the collision and it hasn't spun down yet. 

There appear to be craters on Ultima Thule, as well as other features on its red surface. It doesn't appear to have any satellites or a ring surrounding it and there's no discernible atmosphere. During the flyby, specialized instruments onboard New Horizons scanned its surface in various wavelengths of light to learn more about the chemical properties of the reddish surface. What those observations and others reveal will help planetary scientists understand more about conditions in the early solar system and out in the Kuiper Belt, which is already being called the "third regime of the solar system."

Sources

  • New Horizons, pluto.jhuapl.edu/Ultima/Ultima-Thule.php.
  • “New Horizons Successfully Explores Ultima Thule – Solar System Exploration: NASA Science.” NASA, NASA, 1 Jan. 2019, solarsystem.nasa.gov/news/807/new-horizons-successfully-explores-ultima-thule/.
  • Official, Queen. YouTube, YouTube, 31 Dec. 2018, www.youtube.com/watch?v=j3Jm5POCAj8.
  • Talbert, Tricia. “NASA's New Horizons Makes First Detection of Kuiper Belt.” NASA, NASA, 28 Aug. 2018, www.nasa.gov/feature/ultima-in-view-nasa-s-new-horizons-makes-first-detection-of-kuiper-belt-flyby-target.