Sky and Telescope - November 2017 - 9

MARS

Trojan Asteroids Might Come from Red Planet

p Artist's impression of an impact that excavated a piece of Mars's surface

TROJAN ASTEROIDS orbit ahead of and
behind Mars in the stable regions where
the gravitational pull of the Red Planet
balances that of the Sun. While previous
studies have suggested that these rocks

might come from the asteroid belt, a
study published in the July 17th Nature
Astronomy points to an alternative
source: Mars itself.
Of the nine Mars Trojans currently known, seven belong to a single
cluster named for its largest member,
5261 Eureka. At least 1 billion years
old and possibly the parent body of the
cluster's asteroids, Eureka is known
to be rich in olivine, a mineral rare in
asteroids but common in the mantle of
Mars as well as in Martian basins.
David Polishook (Weizmann Institute
of Science, Israel) and colleagues used
NASA's Infrared Telescope Facility to
look at spectra of two other Trojans in
the Eureka cluster: 2007 NS2 (311999)

and 2001 DH47 (385250). The light
reﬂected off these asteroids also shows
the characteristic absorption around
1 micron, implying olivine-rich surfaces.
Polishook's team then used numerical
simulations to show that the olivine-rich
Eureka cluster is more likely to have
originated from Mars than to have been
randomly captured from the asteroid
belt. While it's easy for an impact to
excavate and eject some of a planet's
surface, moving those ejecta into a
stable Trojan orbit requires a signiﬁcant
change in orbital energy. The simulations show that, to make this change,
Mars itself would have to "jump" before
settling into its current orbit, a possibility during the ﬁnal stages of our solar
system's planet formation.
■ DAVID DICKINSON

KUIPER BELT

M A RS: DAVID POLISHOOK / WEIZM A NN INSTIT U TE OF SCIENCE; K UIPER BELT: N ASA / JHU A PL / SWRI / A LE X PA RK ER

Is New Horizons' Next Target a Binary Body?
THE QUEST IS ON to learn everything
possible about the Kuiper Belt object
(KBO) designated 2014 MU69 before
New Horizons ﬂies by at close range on
January 1, 2019. And that effort got a
huge boost when the notoriously dim
(27th-magnitude) object passed in front
of a dim star in Sagittarius on July 17th.
The occultation campaign's ﬁrst
two events, on June 3rd and July 10th,
didn't see any blink-outs (S&T: Oct.
2017, p. 11). But on July 17th, ﬁve of
the 23 observing teams recorded brief
occultations of up to 1 second.
These teams, which included some
experienced amateur observers, were
arrayed about 4½ km (3 miles) apart
along a "picket fence" perpendicular
to the occultation's predicted path in
remote parts of south-central Argentina. Given the separations of the ﬁve
successful teams, 2014 MU69 has to
be 20 to 30 km (12 to 18 miles) long.
That's smaller than originally thought,
so the surface material must be somewhat more reﬂective than the surfaces
of the Kuiper Belt's usual denizens.
However, the occultation timings
don't match the silhouette expected for

p What's the real shape of 2014 MU69, a Kuiper Belt object that's 43.3 astronomical units
(6.5 billion km) from the Sun? New studies suggest it's either highly elongated and up to 30 km
long (left) or a binary body whose halves are each about 15 to 20 km across (right).

a spherical object. Instead, campaign
leader Marc Buie (Southwest Research
Institute) reports, it could be highly
elongated (like a skinny eggplant) or it
might be a pair of objects, either touching or twirling around each other at
very close range.
Corroborating observations come
from Hubble Space Telescope observations carried out by Susan Benecchi
(Planetary Science Institute) between
June 25th and July 4th. She reports that
2014 MU69 showed little variation in
brightness during the ﬁrst observing

run but that the changes became more
pronounced later on. Benecchi says this
is consistent with a double object; Buie
prefers that option, too.
Meanwhile, negative results from
the ﬁrst two occultations (especially
observations from NASA's SOFIA ﬂying
observatory on July 10th) build conﬁdence that New Horizons' forthcoming
target isn't ringed by impact debris,
which would have posed a threat to the
spacecraft as it whizzes by the rock at
13.7 km per second.
■ J. KELLY BEATTY
s k y a n d t e l e s c o p e . c o m * N O V E M B E R 2 0 17

http://www.skyandtelescope.com

Sky and Telescope - November 2017

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