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Sungrazer
STEREO IMAGES COURTESY NASA / NAVAL RESEARCH LABORATORY
ing the comet’s tail projecting up from the brightly lit
dawn horizon.
Until this point, Comet Lovejoy’s coma displayed a
normal, condensed appearance. But this abruptly changed
on December 20th when the comet’s head exhibited a long,
bright tailward-pointing ray instead of a compact central
condensation. This ray grew longer each day as the comet’s
head became weaker and more diff use. Comet expert
Zdenek Sekanina at JPL proposed that Lovejoy’s nucleus
completely disrupted about December 17.6 UT, and that
the stream of resulting debris was moving rapidly outward
into the tail, forming the ray-like feature. This debris
ranged from boulder-sized pieces nearest the position of
the comet’s former nucleus to micron-sized dust stretching
well out into the tail. We may well have been witnessing
the creation of innumerable future pygmy sungrazers.
With the comet moving away from the Sun and the
tailing growing longer each day, Comet Lovejoy looked
like a searchlight beam projecting up from the horizon
before it was washed out by increasing twilight. On the
morning of December 22nd, observers were describing
the comet’s dust and ion tails as each about 16° long and
similar in brightness to the Large Magellanic Cloud.
That same day the crew of the International Space Station
watched the comet as it rose over Earth’s curving limb.
While some observers thought that the fading of the
comet’s head heralded a rapid demise of the tail, the
performance of past sungrazers suggested that the show
was just beginning and that Comet Lovejoy might unfurl
a truly enormous tail in coming days. And it did just that.
By Christmas morning the tail had grown to at least 28°!
As the New Year opened, there were reports of the
comet’s dust tail stretching across 40° of sky, but the
head had faded so much that it could hardly be seen with
the unaided eye. Like the Great Southern Comet of 1887,
Comet Lovejoy had essentially become only a tail.
The searchlight appearance of the tail was maintained
Dec. 16, 2011
Dec. 18, 2011
40 May 2012 sky & telescope
Dec. 17, 2011
Dec. 19, 2011
In the days following perihelion, the bright central condensation
at the comet’s head evolved into a slender ray-like feature
that is thought to have formed as debris from the disrupted
nucleus spread outward into the tail. This view was obtained at
17:49 UT on December 24th with the Uppsala Schmidt Camera
at Australia’s Siding Spring Observatory.
as it faded and grew ever more ghostly and transparent
with the passing days. Although the end of the tail faded
into the Milky Way in Crux and Centuarus, images showed
that its length was an astounding 45° when it peaked in
mid-January. This corresponded to a truly enormous 1¼
astronomical units! But because of its faintness, the tail
was noticeably shorter to the unaided eye. By January 20th,
the comet could only be seen visually from the darkest,
pristine observing sites. Photographs in February captured
only hints of the grand visitor as the comet disappeared
from view, but Comet Lovejoy’s legacy will surely be
remembered for a long time to come. ✦
Amateur astronomer John Bortle has a long history of
observing and writing about comets, variable stars, and lunar
eclipses from his home in Stormville, New York.
The changing appearances of Comet Lovejoy’s brilliant, sweeping
dust tail and slender, straight gas tail are evident in this daily
sequence obtained by the Heliospheric Imager on the STEREO A
spacecraft between December 16th and 19th as the comet moved
away from the Sun. Streaks emanating from bright objects are
artifacts due to overexposure. The notably bright “stars” in the
fi eld are the planets Mercury (fainter of the pair) and Jupiter, which
appeared relatively close together from the spacecraft’s vantage
about 45° ahead of Earth on our orbit around the Sun.
ANU / UNIVERSITY OF ARIZONA / NASA / ROBERT H. MCNAUGHT