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| Space on Dec 25, 2011

Lovejoy survives encounter with sun and becomes Christmas comet

As it swept near the sun in early December 2011, Lovejoy’s dust tail was completely severed, and afterwards gently floated towards the sun while its head raced on without it.

Comets are often described as dirty snowballs, and we all know what happens when ice meets extreme heat. The week of December 12, 2011 began with great anticipation for a few planetary scientists and amateur comet hunters as they prepared to witness the death plunge of the recently discovered Comet Lovejoy into the sun. But, by week’s end, this comet had defied expectations so spectacularly that it garnered the attention of the press all around the world. And now the comet has gone on to become a Christmas comet for 2011, spectacular from Earth’s southern hemisphere, beloved by all.

Images of Comet Lovejoy, now visible from Southern Hemisphere

Comet Lovejoy, known formally as C/2011 W3 (Lovejoy) was named for Terry Lovejoy, an Australian astronomer who discovered the comet on November 27, 2011. It belongs to a class known as sungrazers, comets with highly elliptical orbits that bring them dangerously close to the sun. Most end up vaporized by the sun’s corona[5] where the temperature reaches over a million degrees Fahrenheit. The few sungrazing comets that survive emerge smaller in size due to significant vaporization of its icy nucleus. Those that are badly battered eventually disintegrate as they speed away from the sun. Remember Comet Elenin earlier this year? It was one of those.

Colin Legg shared a time-lapse video of Comet Lovejoy, taken on December 22, 2011, with EarthSky.

Comet Lovejoy (2011 W3) Reflections from Colin Legg on Vimeo.

Sungrazing comets were thought to be rare, that is, until space-based solar observatories monitoring the sun started seeing many more that were too faint to be detected from the ground.

Even though Comet Lovejoy was large for a sungrazer, many scientists thought it would not survive its encounter with the sun. Still, observing the comet’s last days was a remarkable opportunity to study its interaction with the sun, and this generated great interest in capturing the comet’s final moments using space-based solar telescopes. But those final moments turned out not to be final, after all.

As the comet got very close to the sun, it became impossible to observe it from the ground. Scientists and amateur astronomers turned their attention to data from spacecraft monitoring the sun. Among them were the Solar and Heliospheric Observatory (SOHO), a pair of spacecraft known as the Solar TErrestrial RElations Observatory (STEREO), the Solar Dynamics Observatory (SDO) and the European Space Agency’s Proba2.

The animated image below, created from STEREO data obtained between December 11 to 13, 2011, shows Comet Lovejoy speeding towards the sun. As the comet gets closer, notice how its tail starts to wiggle due to interaction with solar wind emanating from the sun. (The line seen left of the comet is a detector artifact caused by saturated pixels from the planet Mercury that was cropped out of this image sequence.) Image credit: NASA and Karl Battams/USNO.

As interest in this comet began to increase, many more professional and amateur astronomers started following updates posted by Karl Battams at the sungrazing comets web page, maintained by the US Naval Observatory’s Solar Physics Department. Much of what transpired, from December 13 to 17, 2011, appears below in excerpts from Battams’s engrossing updates that captured the initial anticipation and subsequent amazement of this event.

On December 13, 2011, Battams mused about Comet Lovejoy in his web page.

It’s almost a little sad when you think about it: originally as part of a much larger object, Comet Lovejoy has existed for billions of years, since the formation of the solar system. It has outlived countless species on Earth. Indeed, it existed before life on Earth! And now it will almost certainly be completely destroyed within 72 hours. …. For ground-based observers, the comet is now all but lost in the blinding glare of the Sun. But for Sun-watching spacecraft such as SOHO and STEREO, it is an increasingly easy target to view, and that is exactly what we are doing.

A day later, December 14, 2011, Battams reported an interesting observation while processing images taken on December 12, 2011 by one of the STEREO spacecraft.

This is too cute: Comet Lovejoy has a friend! Look in the upper-half of the animation, starting at center and moving diagonally up and to the left, perfectly in step with Lovejoy. It’s another Kreutz-group[1] comet! They are obviously closely related though, and the smaller one must have fragmented from Lovejoy some significant time ago, and with some slight (non-gravitational) force between them to “push” them apart like this. …

Look carefully at the looping animated image below. Can you spot the tiny companion comet above Comet Lovejoy, towards the end of the clip? Image Credit: NASA/STEREO and Karl Battams/USNO.

On December 15, 2011, Battams wrote:

Welcome to the beginning of the end of Comet Lovejoy’s billions of years long journey through space. In less than 10 hours time, the comet will graze some 120,000 km above the solar surface, through the several million degree solar corona, and — in my opinion — completely evaporate. We have here an exceptionally rare opportunity to observe the complete vaporization of a relatively large comet, and we have approximately 18 instruments on five different satellites that are trying to do just that.

He – along with everyone else who expected the comet to disintegrate – was in for a big surprise, as evidenced by this update on December 16, 2011.

I don’t know where to begin. I simply don’t know. What an extraordinary 24 hours! I suppose the first thing to say is this: I was wrong. Wrong, wrong, wrong. And I have never been so happy to be wrong! For the past two weeks I have been saying that Comet Lovejoy would not survive perihelion[2] in “any appreciable form.” When I said this, I envisioned that we would see some very diffuse component [that would] maybe last a few hours after perihelion, but not much else. I was spectacularly incorrect!

Comet Lovejoy, essentially composed of dust and ice, initially estimated to have a diameter of about twice the width of a football field (320 feet or 98 meters), had spent almost an hour in the corona, in temperatures over a million degrees Fahrenheit, about 140,000 kilometers over the sun’s surface. Scientists were stunned to see it re-emerge from the sun, albeit without its tail. However, Battams commented, on his web page, that he felt confident that Comet Lovejoy would grow a new tail as it receded from the sun. This time he was right, as the images at the bottom of this post so beautifully testify.

Time-lapse images, below, taken by the European Space Agency’s Proba2 spacecraft, show a faint wisp of the comet, entering, then leaving the sun, as pointed out by the arrows. Image credit: European Space Agency. (A larger version is available here.)

Two animated image clips below show highly magnified views of the comet’s entry and exit, taken by another spacecraft, the Solar Dynamics Observatory (SDO). Image credits: NASA/SDO

The image below was taken by one of the STEREO spacecraft cameras on Deember 16, 2011, shortly after the comet’s close encounter with the sun. In it, a coronagraph blocks the sun so its intense brightness does not outshine Comet Lovejoy. On the right, Comet Lovejoy re-appears with only its head, but no tail. (The long horizontal line across its head is a detector artifact[3].) The left side of the obscured sun shows dust illuminated by reflected sunlight, left behind by the comet as it approached the sun — in other words, that’s the comet’s dust tail, still visible on the opposite side of the sun from the comet’s nucleus. Image credit: NASA/STEREO.

Reflecting on the past few days, Battams wrote on December 16, 2011,

Sungrazing comets, particularly those of the Kreutz-group, have fascinated astronomers for decades, and no doubt terrified civilizations of the past, as their orbits hurled [the comets] through the solar atmosphere, resulting in a brilliant daytime illumination of these enormous dirty snowballs. There is arguably no other object in the solar system that goes through such an intense experience as one of these comets. For days now we have been witness to such a beautiful object racing through the STEREO, SOHO, and now SDO and Proba2 images, blasting through the solar corona, and miraculously re-emerging, albeit with much less of a tail than it started with. And whereas sungrazers of the past have been lost at least temporarily, if not permanently, in the sun’s glare, thanks to an amazing fleet of sun-watching spacecraft we have now been enthralled by this entire passage without a single hour passing by unwitnessed. Purely for the spectacle of the event, and the way it has unfolded before our eyes over the internet, this comet has sealed its place in the history books.

Writing in his web page on December 19, 2011, Battams wondered about what Comet Lovejoy had in store for the days to come.

So what surprises could Lovejoy have for us? It’s hard to say as we’ve already been well off-the-mark in several respects. My prediction a couple of weeks ago of approximately -3 or -4 peak magnitude[4] was shockingly good (it was luck, I assure you), but I was totally wrong about the survival prospects of this object. We thought the comet was only a couple of hundred meters in diameter, based on its brightness prior to reaching the sun, giving it no chance of survival. Now we know it has survived, and therefore must be bigger than we thought. The rough guideline is that a comet would need to have a nucleus of about 500 meters to be able to survive as well as Lovejoy has, so my latest estimate of the (pre-perihelion …) nucleus size would be something on that order. It’s going to be much smaller now as the intense solar heating would have taken its toll.

Another surprise was how Lovejoy has both regained and sustained its intense pre-perihelion brightness. As it raced through the solar corona, Lovejoy’s extensive dust tail was completely severed, and thereafter gently floated towards the sun while its head raced on without it. As it re-emerged from perihelion, all that remained was an intense, condensed nucleus that seemed a shadow of its former self. But within just three-and-a-half hours, it underwent a spectacular resurgence to return to its former glory! I really did not see that coming, nor did I envision that it would become as bright as when it plunged in [towards the sun], that its tail would re-grow so strongly, that its ion tail (the thin one you see here) would strengthen more than ever, and that it would begin to grace the Southern Hemisphere skies as it is now beginning to do.

http://www.youtube.com/watch?v=xKb3WswVI9Q

Image Credits: NASA/STEREO and Karl Battams/USNO.

What could be left for it now? What does fate hold for Comet Lovejoy? Well, some component of it will survive now to return into its several-hundred year orbit, and indeed I am comforted by the thought that sun-watchers in a few hundred years will recall these images and this story, and remember all who have played a part in it. But it’s not all smooth-sailing for Lovejoy. It has been through the most hostile environment that our solar system offers, comets are low-density objects, and scars don’t heal. If Lovejoy has suffered any serious fractures then at any time it could fragment into one or more pieces, and it’s still close enough to the sun to evaporate significantly. It is, however, headed out of the “bottom” of the solar system, and does not encounter any planets or major bodies out there (the Kreutz orbit takes it well inside Mercury’s orbit) so it has a really good chance to survive and return to our future descendents.

The clip below, spanning 40 hours on December 15 to 16, 2011, is a composite animated image taken by different instruments in one of the STEREO spacecraft, showing Comet Lovejoy plunging towards the sun, and re-emerging. Image Credits: NASA/STEREO and Karl Battams/USNO.

There’s more information, including animated images, at the SDO You-Tube web page and at Battams’s web site.

By December 18, 2011, southern hemisphere comet observers were able to see Comet Lovejoy in the predawn sky. Stunning images of the comet have been posted on-line. There is no doubt that astrophotographers, like Grahame Kelaher, who provided EarthSky with the stunning images below, will continue to photograph the comet till it’s no longer visible. Meanwhile, scientists are continuing to collect a wealth of data on the comet, from ground-based telescopes and spacecraft observatories, that will keep them busy for a long time to come.

Grahame Kelaher shot this sequence of Comet Lovejoy, near Perth, Australia, rising before the Sun on December 22, 2011. Image credits: Grahame Kelaher.

FOOTNOTES

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[1] Comet Lovejoy belongs to a family of sungrazers known as the Kreutz group,’ comets that follow a very similar path that’s thought to take about 800 years to orbit the sun. The Kreutz sungrazers likely originated from a large object that broke apart sometime in the history of our solar system, and continued to fragment due to gravitational interactions with other solar system bodies. Those fragments, the Kreutz comets, continue to follow the orbit of their progenitor.

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[2] Perihelion refers to a point in an elliptical orbit when a solar system object, like a planet or comet, is closest to the sun.

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[3] The line across the comet’s head is a detector artifact called saturation. When light from a bright object overwhelms some detector pixels, the resulting accumulated charge spills into adjacent pixel columns.

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[4] In astronomy, ‘magnitude’ or ‘apparent magnitude’ is a measurement of an object’s brightness as seen by a person, or measured by a detector. Magnitudes have a logarithmic scale; in other words, an object with magnitude 1 is ten times brighter than an object of magnitude 2, and 100 times brighter than an object of magnitude 3. The larger the magnitude number, the fainter the object. On a dark moonless night, away from the glare of city lights, the human eye can detect stars as faint as magnitude 6. As another reference, the planet Venus, which appears as a bright evening star during certain times of the year, can be as bright as magnitude -4.6.

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[5] The corona is a region around the sun composed of plasma — that’s gases, mostly hydrogen, stripped of their electrons. It extends about a million kilometers from the sun’s surface and can reach temperatures of more than a million degrees Fahrenheit. The corona is visible during solar eclipses, as a ghostly glow around the obscured disk of the sun. In space-based instruments, a coronagraph can be used to block the sun’s disk to collect information about the corona.

Bottom line: In early December 2011, planetary scientists and amateur comet hunters prepared to witness the death plunge of the Comet Lovejoy into the sun. But – to everyone’s surprise – the comet survived its pass near the sun went on to become a Christmas comet for 2011, spectacular from Earth’s southern hemisphere, beloved by all.

Images of Comet Lovejoy, now visible from Southern Hemisphere

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