than would be expected from a simple
merger event.
At this writing, only about 1% of all
the data that New Horizons collected
on Ultima Thule and its environment
have been radioed back to Earth. The
entire downlink of these many precious
observations will take until August or
September of 2020. But some of the
most important datasets, including
the very highest resolution images and
the first comprehensive compositional
spectroscopy were due to reach Earth
before the end of February.
As a result, many more scientific
results will no doubt soon be
forthcoming. As for New Horizons
itself, even as it downlinks its treasure
trove of data about Ultima Thule, it is
already collecting new observations of
the radiation and dust environment
farther and farther out in the Kuiper
Belt. It’s also taking new telescopic
imagery to explore other objects, a
project that will continue until the
spring of 2021.
Next year, our mission team
will propose a second, even deeper
exploration of the Kuiper Belt to NASA.
The prospects for that future mission
of New Horizons became brighter in
recent weeks with the success of the
Ultima Thule flyby, the amazing results
being obtained, and the news from our
engineering team that the flyby left
more propellant in the tank for future
exploration than they had predicted!■ Alan Stern is the New Horizon’s
NASA / JHUAPL / SWRImission principal investigator.
http://www.skyandtelescope.com.au 37In the weeks following New Horizons’ flyby of
2014 MU 69 , better known by the nickname ‘Ultima
Thule’ (pronounced UL-ti-muh TOO-lee, meaning
‘beyond the known world’), the slow trickle of
observations have morphed 2014 MU 69 from a
26th-magnitude blip barely observable by the
Hubble Space Telescope into a tiny, colourful
and intriguing two-lobed object. It’s made of two
roundish worlds nestled against each other, with
one lobe somewhat larger than the other, and a
combined length of 33 km.
This ‘snowman’ appearance was expected —
the shape had been inferred from a challenging
but successful ground-based effort to record the
object’s passage in front of a star in July 2017.
Moreover, astronomers now realise that many
objects in the ‘classical’ Kuiper Belt are also
binaries. These objects, like 2014 MU 69 , lie 40
to 50 astronomical units from the Sun and have
roughly circular, low-inclination orbits.
Ultima Thule’s two lobes rotate as a single
structure in roughly 16 hours, a relatively slow
spin that doesn’t create nearly enough centripetal
force to fling them apart. They’re “soundly bound”
in a structural sense, notes investigator Jeffrey
Moore (NASA Ames), though they’re essentially
“resting on each other”.
The larger globe of 2014 MU 69 (dubbed‘Ultima’) has roughly three times the volume of
its companion (‘Thule’). They probably consist
primarily of ice, but their surfaces are actually quite
dark, reflecting between 6% and 13% of the weak
sunlight striking them. The reddish hue, thought to
arise from complex organic compounds pounded
for eons by space radiation, matches that of other
low-inclination Kuiper Belt objects. But the narrow
‘neck’ joining the two globes is both the brightest
and the least red of the surface seen so far. This
could mean that it has a different composition, or
perhaps it’s where small particles have ‘rolled’ down
steep slopes toward the object’s centre of mass.
Observations taken when the spacecraft
passed closest, at a distance of just 3,535 km,
will tell us more. Specifically, a series of images
taken by the Long Range Reconnaissance Imager
(LORRI), essentially a 20.8-cm f/13 telescope,
could reveal details on the two lobes’ surfaces
down to about 17 metres across.
■ J. KELLY BEATTYThe spinning snowman revealed
SLeft: This view of 2014 MU 69 combines a
detailed black-and-white image (140 metre
per pixel) with a lower-resolution, enhanced-
colour image using blue, red and near-IR
frames. Right: Overall, the lobes are roughly
as dark as the Moon’s maria, though the
‘neck’ is brighter and less red than elsewhere.0.120.100.080.060.040.020.00Albedo