It was six years ago, in early March of 2012, that Ukrainian amateur astronomer Vladimir Bezugly noticed an obvious comet rapidly approaching the Sun in SOHO/SWAN images. Unlike any ordinary comet, derived orbital elements from the SWAN images revealed the comet to be a Kreutz-group sungrazer! This was interesting because no such sungrazer had ever been detected in these images, not even comet C/2011 W3 (Lovejoy), aka the Great Christmas comet of 2011! This lead to speculation that this newly discovered comet (C/2012 E2 (SWAN)) would potentially brighten into an exceptionally bright naked-eye comet!
SOHO/LASCO C2 image extract of Comet SWAN taken on March 14th, 2012. Image credit: ESA/NASA/NRL SOHO/LASCO C2.
On March 8th, 2012, Vladimir Bezugly posted a message on the SOHOHunters forum indicating that a bright object could be seen close to the Sun in two SOHO/SWAN images, with the speculation that it might be Kreutz-group comet, if real. Other amateurs astronomers, including Michal Kusiak (Poland) and Rob Matson (USA) quickly intervened that same day and confirmed his sightning!
With the available SWAN images at the time of discovery, Michal Kusiak derived a possible preliminary orbit in favor of a Kreutz-group sungrazer! As quoted from the SOHOhunter message, Michal’s exact words were “ I do not want to [express] excessive enthusiasm, but it looks really interesting“! As of that day, amateur and professional astronomers were following the comet closely during its inbound journey towards perihelion!
Comet SWAN as seen in the SOHO/SWAN Comet tracker images, including some of the discovery frames! Notice how the comet appears rather stellar in these images, which is due to the resolution of these images. Also, one can see the comet dissapear behind the occulted region of the images on March 10th. Image credit: ESA/NASA/LATMOS SOHO/SWAN.
Despite the excitement, astronomers were cautious about estimating a peak brightness of this object, as comets can have a very unpredictable behavior. Indeed, comets can quickly flare up into beautiful and bright objects, as they may suddenly disentigrate and fade out of view! One example more recent than comet SWAN was comet C/2012 S1 (ISON), which gained lots of attention from the media, as it was speculated that it could be a “Great comet” in late-2013! Contrary to these speculations, comet ISON brightened slower than predicted, and didn’t reach more than mag -2 at perihelion, before fully disentigrating!
Enhanced STEREO/SECCHI HI1-A image exctract showing the dusty remnant of comet ISON taken three days after its perihelion. Image credit: NASA/NARL STEREO/SECCHI HI1-A.
Unfortunately, no definitve ground based observations could be obtained of this comet during its inbound journey, as the comet was unfortunately too close to the Sun. In other words, as seen from Earth, the comet was located in daylight, masked by the intense light of the Sun! I say “definitive” because amateur astronomer Terry Lovejoy (Australia) was later able locate a “stellar-looking” object in an image he took on March 10th, 2012 at about the expected location of the comet!
Only SOHO/LASCO and STEREO/SECCHI images could follow up on this object. With only the several SWAN images available, it was difficult to estimate a precise time when the comet would enter the FOV of either of those telescopes.
Despite the unpredictability of the comet’s brightness, most assumed that the comet would at least be a fainter rival comet comet Lovejoy, hence it was expected that it would be clearly detectable in the real time (low quality) STEREO/SECCHI HI1-B beacon images. Instead, it was hardly detectable in those images! In fact, these images showed the comet to be uncomparable to the brightness of comet Lovejoy. Moreover, it seemed like it would comparable to an “ordinary”, relatively bright SOHO/STEREO comet, such as SOHO-3184 or STEREO-23! This was later supported by SOHO/LASCO C3 observations when it entered the FOV on March 13th!
Comet SWAN as seen in the low quality STEREO/SECCHI HI1-B beacon images. Notice how the comet is poorly resolved and very faint in this image. Image credit: NASA/NRL Karl Battams STEREO/SECCHI HI1-B.
In SOHO/LASCO C3 images, there too, like observed in the real time HI1-B images, comet SWAN was uncomparable in brightness to comet Lovejoy. It seemed no different than the rather typical bright comets occasionally observed by SOHO (on an almost-yearly basis)! Notice in the image comparison below the similarity in brightness between SOHO-3184 (a typically bright Kreutz sungrazer) and comet SWAN! Unfortunately, it didn’t seem like it would become an exceptionally bright object, or even less survive perihelion…
Due to these observations, there were speculations that the comet might have been a direct fragment from a much larger comet that fragmented while in the SWAN FOV, suggesting that this object might be followed by a storm of Kreutz-group comets! However, this was not observed to be the case. Another suggestion: Knowing that the SWAN observes in the Halpha band (unlike LASCO), it was thought that the brightness of the comet in SWAN might be due to strong Halpha emissions. However, while studying the comet in different filters in SOHO/LASCO, this was proven not to be the case either…
Comparison between comet SWAN (left) and SOHO-3184 (right), the latter being a relatively bright Sungrazing comet, alot fainter than comet lovejoy (and definitely not detectable in SWAN!). Notice how they are rather comparable to each other, in brightness. Image credit: NASA/NRL SOHO/LASCO C3.
Animation of SOHO/LASCO C3 image extracts showing Comet SWAN rapidly approaching perihelion. Notice how the comet brightens until the last image, where the head seems fainter and more elongated (likely the result of severe disentigration). Image credit: ESAN/NASA/NRL SOHO/LASCO C3.
On March, 14th, the comet entered the SOHO/LASCO C2 FOV (see the first image, above), and as predicted, after passing behind the SOHO/LASCO occulter, it never re-emerged… The comet was already fading before having entered SOHO/LASCO C2 images, as can be seen in the animation above.
The final moments were also observed in the STEREO/SECCHI COR images, where the comet also appeared no different from any of the “ordinary” bright Kreutz comets occasionally observed in these images. Below is a comparison between comet Lovejoy and SWAN in COR2-B, notice the very obvious contrast in brightness!
Comparison between comet SWAN (left) and comet Lovejoy (right). Notice the very large difference in brightness between the two, comet SWAN being alot fainter! Image credit: NASA/NRL STERE/SECCHI HI1-B.
Animation of (slightly compressed) COR1-A images showing the final moments of comet SWAN. These are some of the last images ever taken of this comet. Image credit: NASA STEREO/SECCHI COR1-A.
The high-resolution HI1-B images were released just after the comet vanished. In these frames, one can clearly see the effect of the Solar wind on the tail of the comet, as can be seen in the animation below!
Animation of comet SWAN as seen in HI1-B images. Notice the effect of the Solar wind on the tail of this comet, causing the comet to temporarily sport an apparent “forked” tail! Image credit: NASA/NRL STERE/SECCHI HI1-B.
In the end, despite comet SWAN being alot fainter than expected, it was still a rather spectacular comet! It’s not every day that the SOHO/LASCO and STEREO/SECCHI telescopes are treated to a comet of this brightness! 🙂
The reason for the comet’s unusual brightness however still remains unknown. Some speculate that the comet suffered an outburst while it was in the SOHO/SWAN FOV, or that it was a larger comet that disentigrated a few days after discovery… The answer will most likely never be known!