NASA’s New Horizons spacecraft is set to fly by a distant «worldlet» 4 billion miles from the Sun in just six days, on New Year’s Day 2019. The target, officially designated 2014 MU69, was nicknamed «Ultima Thule,» a Latin phrase meaning «a place beyond the known world,» after a public call for name recommendations. No spacecraft has ever explored such a distant world.
The Kuiper Belt lies in the so-called «third zone» of our solar system, beyond the terrestrial planets (inner zone) and gas giants (middle zone). This vast region contains billions of objects, including comets, dwarf planets like Pluto and «planetesimals» like Ultima Thule. The objects in this region are believed to be frozen in time — relics left over from the formation of the solar system.
Ultima, as the flyby target is affectionately called by the New Horizons team, is orbiting in the heart of our solar system’s Kuiper Belt, far beyond Neptune. The Kuiper Belt — a collection of icy bodies ranging in size from dwarf planets like Pluto to smaller planetesimals like Ultima Thule (pronounced «ultima toolee») and even smaller bodies like comets — are believed to be the building blocks of planets.
Ultima’s nearly circular orbit indicates it originated at its current distance from the Sun. Scientists find its birthplace important for two reasons. First, because that means Ultima is an ancient sample of this distant portion of the solar system. Second, because temperatures this far from the Sun are barely above absolute zero — mummifying temperatures that preserves Kuiper Belt objects — they are essentially time capsules of the ancient past.
Marc Buie, New Horizons co-investigator from the Southwest Research Institute in Boulder, Colorado, and members of the New Horizons science team discovered Ultima using the Hubble Space Telescope in 2014. The object is so far and faint in all telescopes, little is known about the world beyond its location and orbit. In 2016, researchers determined it had a red color. In 2017, a NASA campaign using ground-based telescopes traced out its size — just about 20 miles (30 kilometers) across — and irregular shape when it passed in front of a star, an event called a «stellar occultation.»
From its brightness and size, New Horizons team members have calculated Ultima’s reflectivity, which is only about 10 percent, or about as dark as garden dirt. Beyond that, nothing else is known about it — basic facts like its rotational period and whether or not it has moons are unknown.
«All that is about to dramatically change on New Year’s Eve and New Year’s Day,» said New Horizons Principal Investigator Alan Stern, also of SwRI. «New Horizons will map Ultima, map its surface composition, determine how many moons it has and find out if it has rings or even an atmosphere. It will make other studies, too, such as measuring Ultima’s temperature and perhaps even its mass. In the space of one 72-hour period, Ultima will be transformed from a pinpoint of light — a dot in the distance — to a fully explored world. It should be breathtaking!»
«New Horizons is performing observations at the frontier of planetary science,» said Project Scientist Hal Weaver, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, «and the entire team looks forward to unveiling the most distant and pristine object ever explored during a spacecraft flyby.»
«From Ultima’s orbit, we know that it is the most primordial object ever explored. I’m excited to see the surface features of this small world, particularly the craters on the surface,» said Deputy Project Scientist Cathy Olkin, of SwRI. «Young craters could provide a window to see the composition of the subsurface of Ultima. Also by counting the number and impactors that have hit Ultima, we can learn about the number of small objects in the outer solar system.»
The New Horizons spacecraft is on course to fly by Ultima on New Year’s Day, Jan. 1, at 12:33 a.m. EST. For a listing of flyby programming and where to watch it online, visit the New Horizons website at http://pluto.jhuapl.edu/News-Center/Where-to-Watch.php.