ASTRONEWS
14 ASTRONOMY • APRIL 2019
S
hortly after the stroke of midnight on
New Yea r’s Day, NASA’s New Hor i z ons
mission made a historic f lyby of the
far-f lung space rock Ultima Thule,
making it the most distant solar system
object ever explored by a spacecraft.
New Horizons’ closest approach occurred
at 12:33 A.M. EST on January 1, when the
spacecraft zipped within 2,200 miles
(3,540 kilometers) of the icy Kuiper Belt
object, located a staggering 4.1 billion
miles (6.6 billion km) from Earth. By
10:28 A.M. EST, the mission team had
received New Horizons’ pre-programmed
“phone home,” letting everyone know that
the craft had completed its f lyby unharmed.
Over the next few days, its treasure trove of
data trickled back to Earth.Double trouble
Among the most surprising early finds
researchers have made is that Ultima
Thule, formally known as 2014 MU 69 ,
is actually a contact binary: Its body is
formed by two lobes that are gently in
contact, giving it a somewhat snowmanlike
appearance. Planetary scientists have since
nicknamed the two lobes, fittingly call-
ing the larger one Ultima and the smaller
one Thule.
Researchers believe that Ultima Thule
(pronounced TOO-lee, a Latin phrase
meaning “a place beyond the known
world”) represents pristine, unchanged icy
material from the early days of the solar
system. It likely formed when small, frozen
bodies came together, accreting into larger
ones. Two of the largest bodies then stuck
together as much smaller ones were cleared
out. These remaining two lobes formed
Ultima Thule, and with its material, we now
have a window into the early solar system.The basics
In addition to Ultima Thule’s intriguing
shape and formation history, New Horizons
also taught us a great deal about the fun-
damental attributes of this strange object.
It’s about 21 miles (34 km) long by 10 miles
(16 km) wide. It has a rotation period of
about 15 hours. It’s very dark, reflect-
ing only about 10 percent of the light that
strikes it. And it’s reddish in color, probably
a result of cosmic rays and other radiation
hitting its icy surface.
Scientists also can infer some important
geological properties of Ultima Thule. The
body has a mottled appearance due to some
surface irregularities or differences in ele-
vation. It appears to lack any clear impact
craters. Some areas are lighter than others,
such as the tight, squeezed region of the belt
where the two lobes are in contact. This
connection point probably consists of fine-
grained material that has slid down due to
gravity, and possible hills and ridges.More to come
New Horizons was set to take a total of
900 high-resolution images throughoutthe f lyby; over time, the mission team will
discover how many of those images clearly
show Ultima Thule. However, it’s challeng-
ing to correctly frame every shot when your
camera takes over six hours to receive a
command and is traveling at about 9 miles
(15 km) per second.
New Horizons will return about 50 giga-
bits of data on Ultima Thule in all, com-
pared with 55 gigabits collected during the
spacecraft’s 2015 flyby of Pluto. Because the
download rate between New Horizons and
Earth is extremely slow, it will take roughly
20 months for researchers to receive the full
report. But even with only part of the data
in hand, eager planetary scientists are
already working on their first papers out-
lining the object’s properties.
One of the best summaries yet of why
this flyby is so exciting came from a team
slide during a January 2 press conference.
It read: “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 pri-
mordial building blocks of the planets.”
— David J. Eicher, C.G., J.P.ULTIMA THULE REVEALED AS CONTACT BINARY
To kick off the new year,
New Horizons explored the distant
Kuiper Belt object — and the science
is just starting to come in.COSMIC SNOWMAN. The first close-up images of Ultima Thule were released January 2, about a day after
New Horizons whizzed by the Kuiper Belt object. The images revealed a contact binary with two lobes that are
gently touching. NASA/JHUAPL/SWRISHORT SEASONS. Astronomers recently confirmed an additional 104 exoplanets spotted by the now-defunct
Kepler space telescope, including seven planets with “years” that are less than 24 hours long.10 milesRED ROCK. Ultima Thule
shows color variations over
its surface. Its overall color is
reddish, presumably due to
the interaction of radiation
with the object’s surface ice
organics. But some areas
are lighter than others,
such as the fine-grained belt
between the two lobes.
NASA/JHUAPL/SWRIBRIGHT BELT. Ultima
Thule has wide variations
in surface reflectivity, with
dark, less reflective areas
mottled on the two lobes
and brighter material
around the belt. Overall,
the space rock reflects
only about 10 percent
of the light that hits it.
NASA/JHUAPL/SWRI