Sky & Telescope - November 2020 - 15

Beautiful! Finally! It's about time!

That's the consensus amongst observers after Comet NEOWISE blazed forth from perihelion in the predawn skies of
early July. Not since Hale-Bopp departed for the outer limits of
the Oort Cloud has there been such a widely observed Northern Hemisphere comet.
Comet NEOWISE (C/2020 F3) was discovered on March
27, 2020, during the near-Earth observation extended mission of the Wide-ﬁeld Infrared Survey Explorer (WISE) space
telescope. When ﬁrst swept up by the spacecraft, the comet
was roughly 2 astronomical units (a.u.) from the Sun and just
a faint 18th-magnitude puff passing through the telescope's
ﬁeld. Preliminary orbital calculations determined it to be a
non-periodic comet, resulting in the designation C/2020 F3.
Observers throughout the Northern Hemisphere awaited
news of the comet's solar encounter with bated breath, fearing yet another disappointing breakup. After all, both comets
ATLAS (C/2019 Y4) and SWAN (C/2020 F8) promised good
shows in late May, only to break apart and fade before reaching perihelion.
Comet aﬁcionados anxiously watched NEOWISE as it
passed through the ﬁeld of the LASCO C3 coronagraph on the
SOHO spacecraft days before perihelion, awaiting conﬁrmation that the comet had survived its 44-million-kilometer
(27.3-million-mile) brush by the Sun. Fortunately, the third
time was the charm. NEOWISE's 5-kilometer nucleus survived
its July 3rd pass just 0.29 a.u. from the Sun, an event that
altered the comet's orbit to a 6,700-year-long ellipse. Conﬁrmation of the comet's fate came quickly, as several observers
picked up its bright magnitude -2 nucleus in morning twilight.

PHILIPP SA L ZGEBER

A Brilliant Display
Within days, NEOWISE slowly climbed higher from the horizon, displaying a bright, curved dust tail several degrees long
and easy to spot in the brightening twilight.
By the 6th, a faint, bluish ion tail became visible as the
comet crept higher by the day. Urban observers were able to
spot the comet's 1st-magnitude nucleus wherever a low northeastern horizon permitted. But the best show was yet to come.
The comet's orbit kept it low in the northeast for several
more days as it slowly skirted dawn's twilight horizon. Around
July 10th, NEOWISE began to be visible in the evening for
observers at northerly latitudes and was even circumpolar for
comet watchers north of 42° latitude.
At this point, the comet displayed synchronic bands
within its dust tail and fast-moving knots and kinks in its
growing ion tail. NEOWISE's position in the evening sky permitted long-exposure photographs, showing its bluish ion tail
streaming some 20° from the nucleus, as well as a third, reddish sodium tail. This was only the third time such a feature
has been seen in a comet.
By mid-July, dust production began to slow, and the comet
faded to roughly 2nd magnitude as it crossed the legs of Ursa
Major. However, both its bright tails continued to lengthen
due to its changing viewing geometry. Photographs of NEO-

p RED TAIL Comet NEOWISE briefly displayed a reddish tail made up
of sodium atoms, as seen in this deep image taken from Vorarlberg, Austria, on the 10th of July.

WISE are reminiscent of the V-shape last seen in Comet
Hale-Bopp (see the July issue, page 30). Most observers could
trace the comet's dust tail to about 5° from moderately dark
locations, and even longer when viewed with binoculars.
Telescopically, the inner coma of NEOWISE displayed
interesting detail. Due to its low altitude in the mornings at
the start of July, the false nucleus (thick atmosphere surrounding the nucleus) appeared distinctly orange, and mid-size
instruments revealed several "hoods" of material in the inner
coma, expanding outward before being swept into the trailing dust tail. Binoculars and small telescopes showed the dark
lane separating the two tails. Over the course of two weeks,
the coma turned strikingly greenish and expanded. Bob King
observed, "On July 14th through my 10-inch telescope it was
a tiny, bright light bulb at the head of the comet's tail, which
looked for all the world like streamlined fog."
The Moon entered the evening sky as July came to a close,
challenging the fading comet for dominance. August came
with the comet still just visible to the naked eye, though
greatly diminished.
By the time you read this, the comet will be a faint telescopic object well on its way back to the outer solar system.
But its wonderful show reminds us of the surprises the night
sky occasionally serves up. We don't know when the next
bright icy visitor from the outer reaches of the solar system
will blaze into view, but until then, we hope you enjoy this
collection of wonderful mementos from NEOWISE's surprise
summer appearance.

¢ Associate Editor SEAN WALKER will drop everything in pursuit of a good view of a bright comet.
sk yandtelescope.org * NOVEMBER 2 02 0

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Sky & Telescope - November 2020

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