The remarkable case of comet lovejoy: despite the odds against it, comet lovejoy survived its swing by the sun and became one of the most spectacular comets in the past few decades.
Comet Lovejoy, C/2011 W3, was officially announced on December 2nd (see page 41). But the real rush of excitement came a few days later when its orbit revealed it to be a sungrazer that would rendezvous with the Sun on December 16th, passing within a scant 200,000 kilometers (125,000 miles) of its visible surface.
Among the most extraordinary of all comets, the family of sungrazers is thought to be the remnants of a huge comet that broke apart millennia ago, and it might even have been the one Aristotle wrote about in the year 371 BC that had a tail that spanned a third of the heavens. Progressive fragmentation at each subsequent swing by the Sun broke up the major fragments of this progenitor to spawn a host of ever smaller pieces.
When near perihelion, the largest of these surviving pieces become so brilliant that they are visible in the daytime and thereafter may unfurl tails as long as 60[degrees]. Some Sky & Telescope readers will undoubtedly recall the brilliant sungrazer Comet Ikeya-Seki in 1965, which was one of the most spectacular comets of the 20th century.
In addition to these showpiece comets, several solar-monitoring satellites have discovered more than a thousand pygmy sungrazers during the last three decades. They are the flotsam from the repeated break-ups experienced by the larger pieces at previous perihelion passages, and they now form an almost steady stream of debris spread along the parent comets' orbits. These fragile fragments are more than a thousand times fainter than sungrazers such as Comet Ikeya-Seki and are unable to withstand the Sun's furious heat and gravity. Not one of them has been observed to survive its fiery brush with the Sun, typically vaporizing completely in the final hours before perihelion.
At the outset it seemed that Comet Lovejoy's intrinsic faintness grouped it with the pygmy sungrazers, but the internet was still set abuzz by an early suggestion that the comet might reach magnitude -7 before its demise in the solar inferno. Nevertheless, the faintest sungrazer seen to survive its perihelion had an intrinsic brightness of about magnitude +7 or +8, or roughly a thousand times brighter than Comet Lovejoy. Thus, it seemed that the odds were all but nil that the comet would come through its rendezvous with the Sun unscathed.
Approaching the Sun
Terry Lovejoy was able to spot his comet visually late on December 3rd with his 30-cm (12-inch) telescope. It appeared as a small, modestly condensed mass, just 1 acrminute in diameter and glimmering at magnitude 11.6. As the comet slid deeper into the netherworld of morning twilight, visual sightings grew sparse, but astrophotographers in the Southern Hemisphere were able to keep the vigil against the ever-increasing dawn background. Participants in various internet comet forums could hardly contain their excitement as they awaited each morning's new images.
On December 8th, Comet Lovejoy, now with a gossamer tail that grew ever longer by the day, was estimated to be near 8th magnitude and perhaps as bright as magnitude 6 only three days later. Thereafter, with its distance from the Sun shrinking daily, Comet Lovejoy was lost to ground-based observers. It was, however, soon to be followed by solar-monitoring spacecraft.
A new rush of excitement met the first glimmers of Comet Lovejoy when it entered into the field of view of the STEREO/SECCHI A and B spacecraft on December 12th and then SOHO's C3 coronagraphic instrument on December 14th. At about 14:00 Universal Time on the 14th, the comet's brightness in SOHO images seemed close to magnitude +1, making it several magnitudes brighter than most of the larger pygmy sungrazers seen similar at solar distances. A scant two hours later people monitoring each newly posted SOHO image were stunned to see a tiny companion traveling slightly ahead of Comet Lovejoy. And not long afterwards another comet was spotted slightly trailing the pair.
But the main show was still Comet Lovejoy, which was now brightening dramatically hour by hour. Several experienced observers tried spotting the comet visually in the daytime sky, as had been done with Comet Ikeya-Seki. In New Zealand, John Drummond reported trying to see the comet when it wsa a mere 1.9[degrees] from the Sun. His efforts, like those of others observers, proved fruitless.
The SOHO images obtained around 4:00 UT on December 15th showed the comet was about magnitude -1 and sporting a long, bright, slightly curving dust tail. Also visible was a long but decidedly fainter tail of ionized gas, which was a bit of a surprise because an ion tail had never been positively seen on earlier pygmy sungrazers.
As the time of perihelion grew near, the astronomical fraternity remained appraised of the comet's developments in almost real-time with updates that Karl Battams posted on the SOHO Sungrazer Comet website. They included extraordinary images from spacecraft along with a running commentary on their implications. In the final hours before perihelion on December 16.01 UT, observers around the world watched the updated SOHO images in fascination, thinking they were witnessing Comet Love-joy's final death plunge.
When the comet's head disappeared behind the occulting disks of SOHO's coronagraphs, NASA's Solar Dynamics Observatory (SDO) team took up the challenge of following the comet with the extreme-ultraviolet camera onboard the orbiting SDO. Its images showed the comet's tail being violently distorted by the solar corona, looking like cigarette smoke in a turbulent breeze, just before the comet disappeared behind the solar limb. Incredibly, however, the same camera soon captured images of the comet emerging from behind the Sun's opposite limb. Comet Lovejoy had somehow survived its hellish passage through the solar atmosphere.
In the hours that followed, the SOHO cameras also obtained images of the now-outbound comet's extraordinary appearance. The old tail was visible southeast of the solar disk, still drifting sunward. To the southwest of the of the Sun, and no longer connected with the tail, stood the brilliant star-like "head" of Comet Lovejoy. At magnitude -2, or perhaps even -3, it was even brighter than it had been before perihelion! Furthermore, the comet began growing a brilliant new tail.
Once again, ground-based visual attempts to see the comet in daylight failed, but it was captured in photographs, especially those taken at near-infrared wavelengths. And beginning about December 19.5 UT, amateurs in the Southern Hemisphere were photographing 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 diffuse. 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[degrees] 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[degrees]!
As the New Year opened, there were reports of the comet's dust tail stretching across 40[degrees] 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 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[degrees] when it peaked in mid-January. This corresponded to a truly enormous 1 1/4 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.
"I trust that most here appreciate that we are witnessing one of the most extraordinary events in cometary history."
In the days leading up to Christmas 2011, Comet Lovejoy appeared increasingly spectacular as it rose higher in the Southern Hemisphere morning sky. This was the view on December 23rd.
Above: Rising tail first in the morning twilight after its swing around the Sun, Comet Lovejoy was described by many Southern Hemisphere observers as looking like a searchlight beam. It's easy to see why from this photograph taken by Robert McNaught on December 21st.
The extreme-ultraviolet camera onboard NASA's orbiting Solar Dynamics Observatory captured these extraordinary images of Comet Lovejoy (indicated by the tick marks) before (1) and after (2) its swing behind the Sun on December 16th.
Below: Observers around the world awaited each updated image from the coronagraphic instrument aboard the orbiting Solar and Heliospheric Observatory as Comet Lovejoy made what most people thought was its death plunge toward the Sun. But the comet survived its fiery encounter and emerged tailless (third image in sequence) early on December 16th, even while its pre-perihelion tail was still visible on the inbound side of the Sun. Within hours the comet grew a new tail that was far more spectacular than before.
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.
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 field are the planets Mercury (fainter of the pair) and Jupiter, which appeared relatively close together from the spacecraft's vantage about 45[degrees] ahead of Earth on our orbit around the Sun.
John Bortle writing on the Comets Mailing List on December 16th after Comet Lovejoy had rounded the Sun.
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.
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|Author:||Bortle, John E.|
|Publication:||Sky & Telescope|
|Date:||May 1, 2012|
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