The team behind the Boulder-born New Horizons mission on Wednesday released the clearest image to date from its New Year's Eve flyby of Ultima Thule, the Kuiper Belt Object on which the spacecraft set its sights after a historic brush with Pluto more than three years and a billion miles ago.
The initial images reveal two conjoined spherical lobes, what planetary scientists call a contact binary, that is already being likened to a dusky, celestial snowman.
Alan Stern, principal investigator on the mission and associate vice president of the Space and Engineering Division of Southwest Research Institute in Boulder, called the flyby an historic achievement.
"Never before has any spacecraft team tracked down such a small body at such high rate of speed so far away in the abyss of space," he said, referring to the target originally known by the less poetic name 2014 MU69, which dwells in the icy outer region of the solar system beyond Neptune, some 4 billion miles from the sun. "New Horizons has set a new bar for state-of-the-art spacecraft navigation."
New Horizons' achievements are getting wide notice. Brian May, the lead guitarist of Queen, who also is an astrophysicist, released a new single commemorating the event — it is called, fittingly, "New Horizons" — just after midnight Monday.
The latest images of Ultima Thule were taken from about 17,000 miles out, showing a formation that end to end measures about 19 miles in length, and has a rotation period of 15 hours, plus or minus an hour.
Project scientists have named the larger sphere, judged to be about 12 miles across, "Ultima," and the smaller lobe, about 9miles across, "Thule." The ratio of the larger lobe to the smaller is estimated to be about 3-to-1.
Stern and other team members discussed early findings on Wednesday from the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., where the spacecraft was designed and built and also is operated. Southwest Research Institute leads the science team, payload operations and encounter science planning.
"This is just the tip of the iceberg," Stern said. "We have far less than 1 percent of the data" that is stored on the spacecraft from the flyby, which at its closest — about 2,200 miles — hurtled past Ultima Thule at a speed of 32,000 mph at 10:33 p.m. MST on Monday.
The balance of the massive amount of data collected by New Horizons' suite of seven instruments will be streamed back to Earth — there is about a six-hour delay in receiving the radio signal from the spacecraft — over the next 20 months.
Jeff Moore, the mission's team lead for geology and geophysics, likened New Horizons to a time machine, taking scientists back to the birth of the solar system.
"We are seeing a physical representation of the beginnings of planetary formation, frozen in time," Moore said. "Studying Ultima Thule is helping us understand how planets form, both those in our own solar system and those orbiting other stars in our galaxy."
The scientists believe its two spheres probably came together as far as 99 percent of the way back to the time of the solar system's formation, coming together at about the speed one might pull into a parking space.
To the extent that the earliest imagery can show it, the surface of Ultima Thule, is mostly reddish in color — less so, at the neck, or, where the two lobes join — with a relatively low reflectivity, ranging from about 13 percent of the incident sunlight down to about 6 percent in some areas.
The early images do not show obvious signs of craters caused by impacts. But the scientists cautioned that could be because of the high angle of the available sunlight at the time initial images were recorded by New Horizons' cameras, and that the topography, possibly including hills or ridges, will be better revealed by future images.
"We aren't saying there are craters on it or not," Stern said, noting that greater shadowing in subsequent photos from other angles will better tell the tale of its surface features. "Unfortunately the approach images that came down first just aren't conducive to determining whether there are craters on the surface or not."
Moore called Ultima Thule "the most primitive object that has yet been seen by any spacecraft, and represents a class of objects that are the oldest and most primitive objects that can be observed," and will help unspool the story of how "innumerable small pebbles" could form "nodes," then planetesimals, and how planetesimals could come together under gravitation pull to form planets.
The selection of Ultima Thule as a next target after New Horizons' July 14, 2015 flyby of Pluto was largely determined by its being one of the more promising candidates that could be reached on its trajectory without a significant — and fuel-consuming — change of course.
Stern expressed surprise, and elation, that after picking the mission target "more or less" out of the hat, "that we were able to get as big a winner as this, that is going to revolutionize our knowledge of planetary science."
Clearer images and more confident assessments of Ultima Thule's surface composition and topography are expected in the immediate days ahead.