28 ASTRONOMY • JANUARY 2020
Astronomy kicked off 2019 in high gear: At
12:33 A.M. EST on January 1, New Horizons made its
closest approach to 2014 MU 69 , nicknamed Ultima
Thule. It is the most pristine distant world ever
explored — albeit remotely — by humans.
The craft zipped by the Kuiper Belt object (KBO)
at a distance of about 2,175 miles (3,500 km). From
its location about 4 billion miles (6.4 billion km)
from Earth and 1 billion miles (1.6 billion km) past
Pluto, it took more than six hours for the first
images, traveling at light-speed, to reach Earth.
“It always is really exciting to see a world for the
first time, just revealed before your eyes,” says Anne
Verbiscer, a research professor at the University of
Virginia and assistant project scientist for the New
Horizons Kuiper Belt extended mission. “Just over-
night, going from this tiny little dot, a speck in a
telescope image or a spacecraft image, into a
resolved world — that experience is rare, unique,
and takes your breath away every time it happens.
It’s amazing.”
The images confirmed Earth-based observations3
New Horizons
flies by
Ultima Thule
that MU 69 is a bi-lobed object called a contact
binary, which occurs when two separate bodies
slowly spiral closer until they touch and form a sin-
gle body. It spins once every 15.92 hours around a
point where the two lobes connect.
Almost exactly 22 miles (35 km) at its longest,
Ultima Thule had appeared on approach like a
snowman, with two spherical lobes. Images snapped
by New Horizons, however, allowed researchers to
watch background stars disappear behind the object,
revealing a different shape. The larger lobe (nick-
named Ultima) is f lat and shaped more like a pan-
cake, while the smaller lobe (Thule) has greater
girth, roughly the shape of a football or walnut.
“[The two pieces] originated separately, but prob-
ably not that far from each other because they’re so
similar,” Verbiscer says. “They have the same ref lec-
tivities on their surface, same composition, not a lot
is different about them other than size.”
The surface of Ultima Thule is red; the color
likely comes from tholins, which are compounds
created by interactions between sunlight and mol-
ecules containing carbon, such as methane or eth-
ane. Occasional craters pockmark the surface, likely
the result of past impacts, but not as many as
expected. “We should be seeing more of those and
we don’t. But when you start thinking about how
porous it is and how low-density it is, would an
impact crater even look the same as it does on, say,
something bigger like Pluto or the Moon? Probably
not,” Verbiscer says.
The New Horizons team published their first
official results in the May 17 issue of Science, a mere
four months after the f lyby. By September, much of
the data had been downloaded, although images
taken before and after the encounter, which do
not include Ultima Thule but still provide clues
about its environment, are still on the spacecraft,
Verbiscer says. Still aboard as well, she says, are
images New Horizons has taken of other KBOs as
it f lies through the Kuiper Belt. It will continue to
do so for the rest of its extended mission, giving
astronomers a closer look at these objects than they
can attain from Earth.
And because Ultima Thule “is not extraordinary
in any way,” Verbiscer says, researchers can compare
their findings with the less detailed observations of
other KBOs to draw conclusions about the objects in
the distant reaches of our solar system, which repre-
sent pristine pieces of the solar nebula from which
the planets were born.
“This is the first time we’ve ever seen an object
from this part of the solar system,” Verbiscer says,
“and it gives us a window into what the early solar
system looked like. It’s just an incredible thrill and a
privilege, really, to be in one of the front-row seats as
this unfolds. We’re writing the textbooks here on
how the solar system was put together.”New Horizons
flew past 2014
MU 69 , nicknamed
Ultima Thule, at
a distance of
about 2,175 miles
(3,500 km) on
January 1. This
image is a composite
of several high-
resolution images
from the spacecraft,
with enhanced color
to show roughly how
it would appear to
the human eye from
up close. NASA/JHUAPL/
SWRI/ROMAN TKACHENKO