Sky & Telescope - USA (2019-09)

NEWS NOTES

IF THEORISTS ARE RIGHT, observers

could spot potentially hazardous aster-

oids — 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 frag-

ments of a giant comet that broke apart

some 20,000 years ago. If so, it’s pos-

sible 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 100-

meter (300-foot) object with a Taurid-

like 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 clos-

est post-perihelion pass to Earth since

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 identifi ed two

main observing windows when astrono-

mers will have the best chance of dis-

covering TRS objects more than roughly

100 meters across.

The fi rst 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 opportu-

nity 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 ama-

teur-scale telescopes but easy pickings

for professional instruments.

A fi nal opportunity falls in the dark-

sky 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

Astronomers Stalk the “Taurid Swarm”

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 distinc-

tion: Large fi laments of GRS mate-

p This illustration shows Comet 2P/Encke

sharing its orbit with asteroids, smaller frag-

ments 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.

8 SEPTEMBER 2019 • SKY & TELESCOPE

rial, some spanning more than 10,

km (6,000 miles), started peeling

away from the west end of the famous

spot roughly once a week and dissipat-

ing in a churning bridge connecting

the GRS to the SEB. JunoCam (aboard

NASA’s Juno spacecraft) and amateurs

alike observed the phenomenon.

The “fl akes” appeared to contain a

substantial amount of GRS material,

as they were seen most prominently at

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