Ultima Thule is without doubt one of the photo voltaic system’s most hermitlike homebodies. Within the four.6 billion years because the small icy world fashioned, astronomers suppose that it has by no means moved from its unique orbit about 6.5 billion kilometers from the solar. And no different massive object has ever come calling.
That’s about to alter.
Simply after midnight jap time on January 1, NASA’s New Horizons spacecraft will zip previous Ultima Thule at a distance of about three,500 kilometers — probably the area rock’s closest customer ever.
“It simply screams out at you, ‘I’ve simply been sitting right here unperturbed for all of this time,’” New Horizons mission supervisor Hal Weaver says of Ultima Thule, whose official identify is 2014 MU69. “‘Come and study me.’”
Ultima Thule makes its residence within the Kuiper Belt, a free confederation of 1000’s of area rocks of assorted sizes that orbit the solar at higher distances than the photo voltaic system’s planets. New Horizons is the primary spacecraft intentionally despatched to discover the realm; the probe flew past its first Kuiper Belt object, dwarf planet Pluto, in 2015 (SN On-line: 6/5/18).
Two different missions, Pioneer and Voyager, additionally despatched probes into the Kuiper Belt, and measured charged particles and magnetic fields there. However since these craft reached the area earlier than the Kuiper Belt was found within the 1990s, they flew proper previous the zone’s rocky, loosely spaced residents unawares.
New Horizons is the primary spacecraft intentionally despatched to discover the Kuiper Belt, a area populated with area rocks large and small that orbit the solar in a disk beginning simply past Neptune’s orbit. Dwarf planet Pluto is taken into account a Kuiper Belt object.
The New Horizons staff has been desirous to get an in depth have a look at one of many smaller Kuiper Belt objects. However selecting the best rock was tough. It needed to be alongside the spacecraft’s path and it needed to be delinquent, with an orderly orbit that planetary scientists suppose has by no means been disturbed. These loners, known as chilly classical Kuiper Belt objects, are the best-preserved remnants of the primary supplies current within the disk that fashioned the planets.
“The chilly classicals have at all times been there, we expect,” says planetary geologist Jeffrey Moore of NASA’s Ames Analysis Middle in Moffett Discipline, Calif., the top of New Horizons’ imaging staff. “They characterize the oldest, coldest stuff that fashioned when the photo voltaic system fashioned. It’s simply been sitting on the market in deep storage because the starting of photo voltaic system historical past.”
Not all the Kuiper Belt’s denizens are so withdrawn. Some have made journeys to closer-in elements of the photo voltaic system, the place the rocks’ surfaces had been heated by daylight. Some have extremely elliptical orbits, or orbits which might be tilted towards the airplane of the remainder of the photo voltaic system like Pluto’s, suggesting that the objects suffered a gravitational run-in with one other physique someday of their previous.
Ultima Thule was considered one of simply two appropriate objects discovered utilizing the Hubble Area Telescope in 2014, and was chosen as a result of the rock might be reached utilizing the least quantity of gas. “That is the one we will get to, but it surely’s consultant of a a lot bigger inhabitants of objects,” says planetary astronomer Susan Benecchi of the Planetary Science Institute, based mostly in Herndon, Va., who labored on the Hubble search.
Not a lot else is but identified about Ultima Thule. Observations made in 2017 as the item handed in entrance of a distant star counsel that the rock is about 30 kilometers throughout, and may even be two rocks orbiting each other (SN On-line: 12/12/17). Extra Hubble observations counsel that Ultima Thule is barely reddish, which might imply it doesn’t have quite a lot of uncovered ice on its floor, Benecchi says.
New Horizons’ cameras finally caught sight of Ultima Thule in August, when it appeared as a tiny dot of sunshine (SN On-line: eight/29/18). “We’ve been intensively observing MU69, attempting to tease out as a lot as we presumably can,” Weaver says. “It’s not revealing its secrets and techniques very simply.”
Weaver and the hazard avoidance staff — known as the “onerous hats” — scoured the area of area round Ultima Thule for any moons, rings or clouds of mud that could damage New Horizons because it approaches (SN: 10/28/17, p. 15). The staff plans to have New Horizons fly previous Ultima Thule at a distance of three,500 kilometers whereas touring at about 14 kilometers per second. That might be twice as shut because the craft received to Pluto.
The spacecraft will take high-resolution photographs of Ultima Thule’s floor, and scan it with a spectrometer to measure its chemical make-up. “Understanding one thing concerning the sorts of ices which might be preserved on MU69 would inform us concerning the sort of chemistry that was occurring within the outermost photo voltaic nebula when the planets fashioned,” Moore says.
And the item’s total form and its floor options, akin to craters and cliffs, might additionally give clues to how the area rock fashioned: whether or not it began as a big object that broke aside, or if it’s constructed up from an agglomeration of smaller rocks. “We’re excited to see what the item really seems to be like” when the craft passes the item on January 1, Benecchi says. “It’s like opening a Christmas package deal, six days late.”
It’ll take for much longer than that to get the total image, although. New Horizons will spend the following 20 months sending its Ultima Thule information again to Earth because the craft continues coasting farther out into area.
Whereas within the outer photo voltaic system, New Horizons may also observe one other 25 to 30 Kuiper Belt objects, though none as intently as Ultima Thule. “That is the preliminary reconnaissance of the Kuiper Belt,” Weaver says. “We wish to do it proper.”