Sky and Telescope - October 2017 - 11

SOLAR SYSTEM

Observers Track New Horizons' Next Target
DURING JUNE AND JULY, two dozen

teams of observers in South America
and South Africa learned crucial new
details about distant 2014 MU69, the
Kuiper Belt object that New Horizons
will ﬂy past on January 1, 2019. Knowing its diameter in advance would be a
huge help in planning the ﬂyby.
Based solely on its apparent brightness in Hubble Space Telescope images,
where it appears exceedingly faint at
27th magnitude, astronomers guessed
that this object is between 25 and 45 km
(16 to 28 miles) across. But the exact size
depends on the reﬂectivity of its surface
- and that's still unknown. The object
orbits some 6½ billion km (43.3 astronomical units) from the Sun, a third
farther out than Pluto is. "This object
has so far proven to be impossible to
detect from the ground," laments Marc
Buie (Southwest Research Institute).

NE W HORIZONS

June 3,
2017

To learn more and guess less, Buie
and the New Horizons team have turned
to an observing technique that can
be even more powerful than Hubble:
stellar occultations. It turns out that
2014 MU69, currently drifting among the
rich star ﬁelds of Sagittarius about 5°
northeast of the Teapot, passed directly
in front of three stars this year, on June
3rd, July 10th, and July 17th. Unfortunately, those stars are themselves faint,
and traveling to the predicted occultation
tracks presented plenty of challenges.
In South America, the predicted track
for the June 3rd event came very near
Santiago, Chile, missing all the worldclass observatories perched in the Andes
farther north. In South Africa, the track
skirted very close to the South African
Astronomical Observatory site in Sutherland. So the New Horizons team funded
a massive occultation campaign. Led by

July 10

July 17

p During mid-2017, the small Kuiper Belt object 2014 MU69 briefly hid three faint stars as seen
from Earth. Here are the predicted tracks, each only about 50 km wide, from which those disappearances would have been visible.

with a mass 12 times Jupiter's, OST44
lies right on the planet-brown dwarf
boundary. Yet the 2 million-year-old
object is growing from an accretion
disk, as stars do.
Bayo's team used the Atacama Large
Millimeter/submillimeter Array in
Chile to examine OST44's disk, ﬁnding
that its dust matches what's expected
based on observations of disks around
other stars and more massive brown
dwarfs. The question remains, though,
how such a low-mass object manages
to form like a star.
■ JOHN BOCHANSKI

IN BRIEF
Asteroid Mission to Shut Down
Program director Michele Gates announced on June 13th that NASA's plan
to retrieve a boulder from an asteroid
and place it in lunar orbit, known as the
Asteroid Redirect Mission (ARM), will be
shut down due to lack of Congressional
support in the proposed FY 2018 budget.
The focus will now turn to closing out the
program while salvaging key technologies
and lessons learned for future applications.
ARM was an ambitious plan. First
proposed in 2013, the project called for an
automated rendezvous with a near-Earth

Buie, the effort dispatched 22 two-person mobile observing teams to Argentina
and South Africa. Each team observed
through a Sky-Watcher 16-inch trusstube Dobsonian reﬂector paired with a
QHY174M-GPS CCD camera.
Buie deployed his teams in Argentina
10 to 25 km apart to create a "fence"
of observers perpendicular to the path.
Across the Atlantic, Anne Verbiscer
(University of Virginia) coordinated the
teams in South Africa.
June 3rd's effort couldn't have gone
better. Buie says that every team collected usable data, though some might
have been partially impacted by clouds.
"That is quite remarkable," he says,
"and it took some heroics on the part of
the South African teams" to avoid bad
weather. However, none of the observing
teams recorded an occultation. This suggests that 2014 MU69 is more reﬂective
and thus smaller than had been assumed
- or perhaps it's a binary object.
The effort fared better during the
occultation on July 17th, which 24 teams
tracked from remote locations in Argentia's Chubut and Santa Cruz provinces.
According to Buie, ﬁve teams recorded
brief coverups of the dim star. Once fully
analyzed, these data should yield a muchimproved diameter for 2014 MU69.
■ J. KELLY BEATTY

asteroid, where robotic devices would pick
up a boulder from its surface (S&T: July 2015,
p. 14). The spacecraft would then return the
boulder, placing it in orbit around the Moon.
Astronauts would rendezvous with the boulder in lunar orbit for further study.
Politically, the mission failed to find support
in Congress from the start, even though it was
touted as a stepping-stone between leaving
low-Earth orbit and heading to Mars.
But even as the ARM mission closes out,
research and development will continue in key
areas, including solar electric propulsion and
searches for near-Earth asteroids.
■ DAVID DICKINSON

* To get astronomy news as it breaks, visit skypub.com/newsblog.
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Sky and Telescope - October 2017

Table of Contents for the Digital Edition of Sky and Telescope - October 2017