Sky and Telescope - November 2017 - 12

COSMIC RELIEF by David Grinspoon

Chasing the
Elusive 2014 MU69
New Horizons' next target was hard to ﬁnd and remains
a bear to pin down, but persistence has paid off.
tion, they requested discretionary time
on the Hubble Space Telescope. After an
initial rejection and some tense backand-forth, they gained a couple weeks'
worth of Hubble observing time.
In June 2014, this investment paid
off with the discovery of two targetable
objects. One of those, now designated
2014 MU69, became the spacecraft's
next destination. On New Year's Day,

Finally, on July 17th, just over two years after the Pluto flyby,
five observing teams in Argentina nailed it.
2019, this tiny world will become the
most distant place ever visited by a
robotic craft.
But identifying that distant blip
was just the start of planning for this
challenging encounter. The team needs
to know the exact locations of both the
spacecraft and the target, as well as
something about the object's properties
- size, shape, and surface reﬂectivity -
to take all the planned images and spec-

p 2014 MU69, seen here with the New Horizons spacecraft in an artist's concept that may
now need revision, has seriously challenged astronomers seeking to get a better fix on it.

N O V E M B E R 2 0 17 * S K Y & T E L E S C O P E

They tried again on July 10th with a
dedicated ﬂight over a remote stretch
of the Paciﬁc by NASA's SOFIA airborne
infrared observatory. Again, no luck.
These negative results at least told us
where the object wasn't, and they suggested that MU69 has no encompassing
cloud of spacecraft-killing debris. But
the thought that it might not be exactly
on the predicted orbit, or might have
properties very different from what we
expected, was unsettling to those tasked
with preparing for its one ﬂeeting ﬂyby.
Finally, on July 17th, just over two
years after the Pluto ﬂyby, ﬁve observing
teams in Argentina nailed it. The results
did more than reﬁne MU69's orbit; they
also hint that it's a supremely strange
object, perhaps a double-lobed one (see
page 9). It's another victory for the New
Horizons team, and it promises the rest
of us more frozen delights from the
Kuiper Belt.
■ DAVID GRINSPOON, an astrobiologist
at the Planetary Science Institute, is author of Earth in Human Hands: Shaping
Our Planet's Future.

N ASA / JHUA PL / SWRI / A LE X PA RK ER

AS THE WORLD KNOWS, the New
Horizons spacecraft spectacularly
achieved its primary mission - to
explore Pluto and its moons - in July
2015 (S&T: Oct. 2016, p. 14). But all
along NASA managers had counted
on visiting another object farther out
in the Kuiper Belt. At launch in 2006,
there was no known body it could
reach, yet astronomers had sound statistical arguments for why they would
ﬁnd such an object during the decadelong trip to Pluto.
Except it almost didn't happen.
Despite searching for years using the
best telescopes on Earth, New Horizons
team members failed to turn up a suitable second destination. One problem:
The spacecraft was headed toward a
region of sky packed with stars, and this
made a dim Kuiper Belt object much
harder to discover.
By 2014, New Horizons scientists
realized they might not ﬁnd their
post-Pluto target by late summer, after
which it would be too late. In despera-

tra. And it'd be helpful to ensure that
the target doesn't lie amidst a swarm of
debris that might doom the spacecraft.
Fortuitously, the New Horizons team
recently received some help from three
faint stars that MU69 was predicted to
pass directly in front of in early June
to mid-July of this year. If the orbit
for MU69 determined from the Hubble
observations was correct, then wellpositioned observers could watch each
star's light blocked for a few seconds.
Those cover-ups would, in turn, reveal
the target's size and overall shape.
So the New Horizons team dispatched astronomers to observe the
occultations (S&T: Oct. 2017, p. 11).
The ﬁrst event - visible on June 3rd
from Argentina and South Africa -
failed to catch the star's disappearance.

Sky and Telescope - November 2017

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