Sky & Telescope - September 2019 - 8

Orbit of
Jupiter
Sun
Earth
Comet
2P/Encke

SOLAR SYSTEM

Astronomers Stalk the "Taurid Swarm"
IF THEORISTS ARE RIGHT, observers
could spot potentially hazardous asteroids - part of the hypothesized Taurid
resonant swarm (TRS) - as they cruise
past Earth this summer.
In 1984 Victor Clube (then at Oxford
University, UK) and Bill Napier (then at
Royal Observatory, UK) proposed that
Comet 2P/Encke, the Taurid meteors,
and a number of near-Earth asteroids
with similar orbits are actually fragments of a giant comet that broke apart
some 20,000 years ago. If so, it's possible that huge chunks could still lurk
unseen along and near Encke's orbit.
In support of their theory, Clube and
Napier pointed out that the Tunguska
impact in 1908 involved a roughly 100meter (300-foot) object with a Tauridlike orbit (S&T: Dec. 1978, p. 497).
Later work by dynamicist David J.
Asher (then at the Anglo-Australian
Observatory, Australia) showed that a
concentrated bunch of these fragments
should orbit in resonance with Jupiter,
completing seven highly elongated loops
around the Sun for every two by Jupiter.
In 1975, seismometers left on the Moon
by Apollo astronauts recorded a spike in
tremors due to meteorite impacts just as
the swarm would have neared Earth.
In June and July, the core of this
hypothesized swarm makes its closest post-perihelion pass to Earth since

S E P TE M B E R 2 019 * SK Y & TELESCOPE

p This illustration shows Comet 2P/Encke
sharing its orbit with asteroids, smaller fragments that create the annual Taurid meteors,
and a concentration of objects known as the
Taurid resonant swarm (TRS, lower right). The
TRS approached Earth in June.

1975. Dynamicists aren't expecting
Tunguska-like encounters, but the close
passage does offer observers the chance
to discover sizable near-Earth objects,
with advance knowledge of where and
when to look.
David Clark, Paul Wiegert, and Peter
Brown (University of Western Ontario,
Canada) simulated the swarm's arrival
in the June Monthly Notices of the Royal

Astronomical Society: Letters. In the
study, the researchers identiﬁed two
main observing windows when astronomers will have the best chance of discovering TRS objects more than roughly
100 meters across.
The ﬁrst of these, between June
25th and July 11th, gives observers the
opportunity to see relatively bright
but fast-moving objects. Some will be
around magnitude 21 or brighter, but
they'll move rapidly across the sky, up to
½° per hour. They'll be moving through
declinations of -40° to -60°, with the
brightest objects the farthest south.
Within this window, from July 5th
to 11th, there's a second opportunity to see slower but fainter objects.
These individual objects would still be
relatively close by but moving across
the southern sky at less than 2¼° per
day. The brightest bodies will be around
magnitude 22 - a tall order for amateur-scale telescopes but easy pickings
for professional instruments.
A ﬁnal opportunity falls in the darksky window between full Moons, from
July 21st to August 10th. During this
time TRS objects will appear fainter by 3
or 4 magnitudes, but their sky motions
will be slower still and the target area is
not quite so far south.
The June-August encounter provides
a unique opportunity to identify large
bodies in the swarm, if they exist. Stay
tuned for the results!
■ J. KELLY BEATTY

SOLAR SYSTEM

Jupiter's Great Red Spot Unfurls
JUPITER'S SOUTH EQUATORIAL
BELT (SEB) has started peeling

material from the Great Red Spot
(GRS) in an event that has been
visible in small telescopes.
The GRS currently sports a
rich, orange-red color, made more
distinct by the white "hollow" that
often surrounds it. In May, a dark
swirl began adding to the distinction: Large ﬁ laments of GRS mate-

rial, some spanning more than 10,000
km (6,000 miles), started peeling
away from the west end of the famous
spot roughly once a week and dissipating in a churning bridge connecting
the GRS to the SEB. JunoCam (aboard
NASA's Juno spacecraft) and amateurs
alike observed the phenomenon.
The "ﬂakes" appeared to contain a
substantial amount of GRS material,
as they were seen most prominently at

G REGG DINDER M A N / S&T; SOURCE: DAVID CL A RK /
UNIV. OF WESTER N ON TA RIO

NEWS NOTES

Sky & Telescope - September 2019

Table of Contents for the Digital Edition of Sky & Telescope - September 2019