First flyby images reveal Ultima Thule is a contact binary

Laurel Kornfeld

New Horizons‘ first flyby images of Ultima Thule, released publicly at a press conference Jan. 2, show the Kuiper Belt object (KBO) to be a contact binary composed of two lobes joined together. Mission scientists have nicknamed the larger lobe Ultima and the smaller lobe Thule.

Taken on approach from a distance of 17,398 miles (28,000 kilometers), the image confirms the suspicions of mission scientists who, following several occultations of stars by Ultima Thule, predicted the KBO to be either a double-lobed object or two objects very closely orbiting one another.

The dataset that revealed Ultima’s shape arrived on Earth in the late afternoon hours of New Year’s Day. Members of the New Horizons team rushed back to work to interpret and discuss the image in a meeting that lasted until 4:30 a.m. EST (09:30 GMT) Jan. 2.

Panelists at the press conference included New Horizons principal investigator Alan Stern of the Southwest Research Institute (SwRI) in Boulder, Colorado; deputy project scientist Cathy Olkin of SwRI; co-investigator Carly Howitt of SwRI, and geology and geophysics team leader Jeff Moore of the NASA Ames Research Center.

Thomas Zurbuchen, associate administrator for the NASA science missions director, was also in attendance.

“What this spacecraft and this team accomplished is unprecedented,” Stern said. “The object that we rendezvoused with at midnight on New Year’s Day wasn’t even known until the summer of 2014, and we had to use advanced tools, like the ESA Gaia Observatory and the Hubble Space Telescope to get its orbit well enough to intercept it at a precise place and moment in time, at a speed of 32,000 miles an hour.”

Sunlight at Ultima is 1,900 times fainter than it is on Earth, Stern said, noting the KBO is approximately the size of Washington, D.C.

Less than one percent of the data collected and stored on New Horizons’ solid state recorders has been returned to date. Return of all the data will take approximately 20 months.

According to NASA, Ultima Thule is the first primordial contact binary ever explored by a spacecraft. It is neither an asteroid nor a comet but a new class of object known as a planetesimal.

Asteroids, most of which are located in the region between Mars and Jupiter, are heated by the Sun, causing them to evolve, while comets, which have elliptical orbits, have all been imaged by spacecraft only when they came close to the Sun.

In a statement to Spaceflight Insider, Stern acknowledged scientists have not found evidence for Ultima Thule having one or more moons but said he assigned a team to comb the latest data to search for possible satellites.

Like most solid objects in the outer Solar System, Ultima Thule likely has a significant amount of water ice. The high reflectivity of some regions of the KBO suggests it might have water ice exposed on its surface. Data from the composition mapping spectrometer, which will be returned early this year, will either confirm or refute this.

“Composition mapping is one of our highest priority objectives,” Stern said.

Olkin revealed Ultima’s rotation period to be approximately 15 hours plus or minus one hour and noted its reflectivity or brightness varies on different parts of its surface. Ultima’s brightest areas reflect 13 percent of the sunlight the KBO receives while its darkest areas reflect just six percent of that light, she stated.

“This is a very dark object, but it shows significant variation by a factor of two across its surface,” Olkin said.

One of Ultima’s brightest regions is the “neck,” which connects the two attached lobes. Olkin said that is consistent with having fine grain materials that have settled there due to gravity and slope.

The KBO is a reddish color similar to other cold classical KBOs in the region, Howitt said. Its color is similar to that of the north pole of Charon, Pluto’s large moon, and to some regions on Pluto.

Mission scientists overlayed enhanced color images taken by New Horizons’ Ralph/MVIC instrument on black-and-white images captured by its Long Range Reconnaissance Imager (LORRI) to create an approximation of the KBO’s appearance, Howitt said.

One of the newest high-resolution images of Ultima shows it to have a mottled appearance, brightness variations, a pronounced bright “neck” feature, possible hills and ridges, and no obvious impact craters, Moore stated. Lighter regions have a circular shape while darker ones appear elongated.

“We think what we’re looking at is perhaps the most primitive object that has yet been seen by any spacecraft and may represent a class of objects which are the oldest and most primitive objects that can be seen anywhere in the present solar system,” Moore said.

Ultima Thule likely formed in the very early days of the solar system when a rotating cloud of small, icy bodies started to coalesce. Low velocity collisions and/or gravitational interactions among these bodies flung away some of the tiny particles, decreasing the angular momentum or rate at which the objects were spinning.

The two nodes that make up Ultima Thule formed separately and eventually spiraled closer and closer to one another until they merged into one object, Moore said.

The KBO’s shape strongly suggests it formed via accretion rather than through other processes, such as collisions, More said. In the inner Solar System, such planetestimals came together to form the rocky planets. That leaves objects like Ultima Thule as the only remaining original Solar System building blocks.

“We should think of New Horizons as a time machine that has brought us back to the very beginning of the Solar System, to a place where we can observe the most primordial building blocks of the planets,” Moore said.

Video courtesy of SciNews