Sky & Telescope - January 2022 - 52

JANUARY 2022 OBSERVING
Exploring the Solar System by Thomas A. Dobbins
Monitor
Jupiter
for Planet
Strikes
Amateurs keep catching
objects hitting Jupiter, and
you can, too.
F
or eight days in July of 1994 the
fragments of Comet ShoemakerLevy
9 smashed into Jupiter, producing
huge clouds of sooty particulates that
encircled the planet and lingered for
months. Arguably the most impressive
spectacle ever witnessed by astronomers,
these events drove home the grave
danger to Earth posed by comet and
asteroid impacts (S&T: June 2018, p. 12)
and added a new urgency to determining
the frequency of cosmic collisions.
In recent years a series of serendipitous
discoveries by amateur astronomers has
provided vital clues.
Despite its lack of a solid surface,
Jupiter is the ideal place to look for
asteroid and comet impacts. With a
diameter over 11 times greater than
Earth's, Jupiter is a big target. And at
two and a half times the mass of all the
other planets combined, it is the solar
system's major attractor for impacting
bodies. Jupiter's powerful gravitational
pull also accelerates objects to
high speeds. Ignoring orbital velocities,
objects strike Jupiter with roughly five
times the velocity of those that strike
Earth, resulting in impact energies that
are 25 times higher.
On July 19, 2009 - almost 15
years to the day after the ShoemakerLevy
9 events - Australian amateur
52 JANUARY 2022 * SKY & TELESCOPE
The Hubble Space Telescope recorded
this image of the dark debris field produced
by fragments G and D of Comet
Shoemaker-Levy 9 on July 18, 1994.
Anthony Wesley captured images of a
fresh impact " bruise " as it rotated into
view over Jupiter's morning limb. (The
impact itself was unobserved because
it occurred on Jupiter's averted night
hemisphere.) Less than two days later,
NASA's Infrared Telescope Facility
obtained the first images of this feature
through an 890-nanometer methane
band filter in the near-infrared region
of the spectrum. Methane filters effectively
block the sunlight reflected off
clouds deeper in the Jovian atmosphere,
so the presence of material high above
the main cloud deck stands out. A
dark appearance in visible light combined
with a bright appearance in the
near infrared is the hallmark spectral
signature of impact scars. The methane
pictures showed the spot to be much
brighter than its surroundings.
In June of the following year, Wesley
and others recorded the flash of a bright
bolide above Jupiter's clouds that left
no detectable mark on the face of the
planet. A similar event followed only
10 weeks later, and a third occurred in
September 2012. Four more have been
recorded in recent years. All seven of
these events were confirmed in videos
acquired by at least two geographically
separated observers, all amateurs.
In the past, small impacts producing
flashes lasting for a second or two were
widely regarded as undetectable with all
but the largest ground-based telescopes.
But the modest instruments that
today's amateurs wield have proven to
be remarkably powerful tools. In recent
years, the sensitivity, frame rates, and
signal-to-noise ratio of high-speed video
cameras employed by amateurs have
improved dramatically. Higher sensitivity
makes it possible to detect impact
flashes using smaller telescopes, while
faster frame rates (>30 frames per second)
provide better temporal resolution
of the light curves of the events, permitting
astronomers to infer the nature of
the impacting object (stony, metallic,
icy compact, or icy porous) through its
fragmentation behavior.
With these impacts in mind, French
amateur Marc Delcroix, Spanish physiH.
HAMMEL / MIT / NASA, PROCESSING: JUDY SCHMIDT

Sky & Telescope - January 2022

Table of Contents for the Digital Edition of Sky & Telescope - January 2022

Contents
Sky & Telescope - January 2022 - Cover1
Sky & Telescope - January 2022 - Cover2
Sky & Telescope - January 2022 - 1
Sky & Telescope - January 2022 - Contents
Sky & Telescope - January 2022 - 3
Sky & Telescope - January 2022 - 4
Sky & Telescope - January 2022 - 5
Sky & Telescope - January 2022 - 6
Sky & Telescope - January 2022 - 7
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Sky & Telescope - January 2022 - Cover3
Sky & Telescope - January 2022 - Cover4
Sky & Telescope - January 2022 - SA1
Sky & Telescope - January 2022 - SA2
Sky & Telescope - January 2022 - SA3
Sky & Telescope - January 2022 - SA4
Sky & Telescope - January 2022 - SA5
Sky & Telescope - January 2022 - SA6
Sky & Telescope - January 2022 - SA7
Sky & Telescope - January 2022 - SA8
Sky & Telescope - January 2022 - SA9
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