Australian Sky & Telescope - 04.2019

DISTANT WORLDS 1 by Alan Stern

Initial results from the

Ultima Thule

AFTER MORE THAN THREE YEARS

of continuous preparations, it’s hard

for me to believe that NASA’s New

Horizons flyby of Kuiper Belt object

2014 MU69, nicknamed Ultima Thule,

is now history — but it is.

The flyby past Ultima Thule was

far more challenging than our 2015

New Horizons exploration of the Pluto

system. The event culminated on

January 1 in complete and utter success.

Every planned observation, by every one

of the scientific instruments aboard

New Horizons, performed according

to plan. The spacecraft did, too. Our

navigators delivered us to a flyby so on

target that we came within 1% of our

closest approach aim point — 3,500

kilometres from Ultima Thule — and we

missed our precise arrival time by just

19 seconds. All this, after a nearly 3.5-

year journey spanning a billion and a

half kilometres! The flyby simply could

not have gone better.

The flyby’s immediate results were

history making: the farthest exploration

of any world in history, the first

close-up examination of a Kuiper Belt

object, and a smashing hit for public

engagement in space exploration that

spanned more than a thousand articles,

five TV documentaries and hundreds of

front-page headlines, not to mention

a dedicated ode to exploration called

‘New Horizons’ by Queen front man,

Brian May.

Now, with new datasets arriving

at Earth every week, the scientific

returns are also beginning. We are

already working on a first paper, which

I am leading as the mission principal

investigator, for the journal Science. And

even before that, on January 11, just 10

days after the flyby, the New Horizons

team submitted 40 separate abstracts

for lectures chock full of observational

and modelling results to be given at the

Lunar and Planetary Science Conference

in Houston in March.

Ultima Thule is providing an amazing

glimpse into the primordial Solar System,

when planetesimals came together to

form the planets. As summarised in my

invited review abstract for the March

meeting, perhaps the most important

initial finding is that Ultima Thule is the

first contact binary object ever observed

‘in the wild,’ where it formed, and in a

largely unmodified state since its birth.

The object consists of two similarly red,

similarly reflective lobes measuring

approximately 14 and 19 kilometres

across, respectively. Their similar nature

points to their individual accretion in

a swarm of like objects, followed by a

gentle merger. Nothing like this has ever

been seen up close and personal before.

In addition to discovering that

Ultima Thule is a contact binary, we

also found that it apparently has no

satellite, rings or atmosphere. These

weren’t big surprises but pieces of the

larger puzzle of understanding this

ancient and primitive body. Other

puzzle pieces include distinct albedo (ie.

reflectivity) markings on the surfaces

of Ultima and Thule, including both

bright and dark markings, various kinds

of quasi-linear albedo features, and a

bright collar encircling the joint, or

‘neck,’ at the interface between the two

merged lobes. And strangely, the Ultima

Thule binary is rotating much more

slowly (with a period of about 16 hours)

NASA’s New Horizons made

history when it flew by

Ultima Thule on New Year’s

Day, but the science return

is only beginning.

COLOUR IMAGE: NASA / JHUAPL / SWRI / ROMAN TKACHENKO; ROTATION: NASA / JHUAPL / SWRI

The Long Range Reconnaissance Imager aboard New Horizons captured these images of

Ultima Thule spinning over a period of nine hours soon before closest approach.

36 AUSTRALIAN SKY & TELESCOPE April 2019

flyby