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Fact-bites, stories, general information, and points of interest relating to near-sun comets, comet images, SOHO and STEREO data...
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October 4th, 2011 -- Sungrazer Project Status update... we're back! (Almost...)

My last message here on May 2, 2011, was not a happy one, as once again funding for this tremendously popular and important project somehow once again became scarce. Nonetheless, another year of funding has now been secured and I soon plan to get everything back up-to-date once again. This will take time, however, but I will try to get the flow of information going again.

The SOHO Comet Count now stands at 2,143 with the most recent being a rather spectacular Kreutz-group comet seen in LASCO C3 and C2, and both STEREO spacecraft. Shortly after this comet's demise, a large CME blasted out from the Sun, leading to the usual speculation about comets causing CMEs. In honor of this event, and the few others like it that have similarly proven completely unrelated, I have created a permanent page discussing comets and CMEs. I'm sure I'll be pointing to this a lot in the future...

So stay tuned to the site over the coming weeks as things slowly start to get back to normal...

May 2nd, 2011 -- Sungrazer Project Status

Below is a copy of a message that I just posted to the "Yahoo! SOHOHunter" group page regarding the funding status of the Sungrazer project.

"Once again I'm sad to report that the Sungrazer project is currently unfunded. Much of the problem this year has been due to the great uncertainty in (or complete lack of...) the budget for the US government. That particular issue has now finally been resolved but it is going to take a while before it can be determined if/how that might trickle down to my level. Again I have tried the peer-reviewed proposal process but the Sungrazer project is a tricky one for a number of reasons that I won't go into right now. I will continue to pursue this avenue but that's a very slow process.

So, what does it mean for the Sungrazer work and comet hunting? Well of course the website will remain active, and you should all continue to hunt for comets. I will not be able to perform the astrometric measurements for the comets, or employ a student to do that. If a particularly noteworthy object comes along then I will volunteer my time to make sure it gets the necessary attention. I will seek assistance from one or two of you to help me keep on top of newly reported comets, and again I will try to volunteer some of my time to keep you informed of the newest comet discoveries.

I will try and keep an eye on the web site in general, just to make sure things are running OK. However, I might only check it every week or so, so please do not hesitate to email me if there's something that you think I need to be aware of, or take care of. Tweeting is quick-and-easy for me, so I will continue to do that (again, on my own time) for the almost-500 folks that now follow @SungrazerComets (http://twitter.com/sungrazerComets)

The ultimate aim here is to do the absolute bare minimum possible so that it is not a huge headache to get the project running again if/when money is found once more. This procedure worked pretty well last time -- and funding did eventually turn up -- so I do have hope that it will happen again. The Sungrazer project has to count as one of the most successful NASA-data-based "Citizen Science" projects ever launched, and I think that there is an appreciation of that fact that will hopefully translate into funding once again.

Many thanks for your understanding and continued support of the Sungrazer project during both the good and bad times. It is tremendously appreciated. I will, of course, keep you all updated."


December 28, 2010 -- SOHO's 2000th comet!

I am honored and absolutely thrilled to be able to announce that the ESA/NASA SOHO satellite has discovered its 2,000th comet!

This incredible milestone was reached late in the day on December 26th, when amateur astronomer Michal Kusiak from Poland (who recently found his 100th SOHO comet) reported a possible small Kreutz-group comet in the SOHO/LASCO C2 images. This comet was hot on the heels of another one he had discovered earlier in the day, and together they became SOHO's 1,999th and 2,000th discoveries!

As you can see in the thumbnail opposite, which you can click on to get a big picture, SOHO-2000 was not a whole lot to look at. It was not big (maybe the size of a medium-sized house house) and did not have any of the "classic" comet features that people associate with the term, such a the dusty coma and long flowing tail. But those familiar with SOHO's comets know that we rarely see objects like that. So I think it's quite fitting that SOHO's 2000th comet looks so much like the majority of the other 1,999 comets it has discovered!

SOHO-2000 is a member of the 'Kreutz' comet group -- a (very) large population of comets that all share a common orbital path in space. Kreutz comets originate back to a single "progenitor" (parent) comet that was believed to have broken up near the Sun centuries (or maybe millennia) ago. Approximately 85% of the comets SOHO discovers are tiny fragments of this original object.

But regardless of what it looks like, this comet represents an extraordinary achievement. SOHO was not designed to look for comets -- it is purely a solar observatory. But, albeit unintentionally, SOHO has discovered well over half of all known comets in its 15-years of operation. And even more remarkable to me is that the overwhelming majority of these discoveries have been made by volunteer amateur "armchair" astronomers from all over the world, dedicating their free time to look for them. Because, in all honesty, there is no way that a fraction of these comets would have been recognized if not for this community effort. The combined man-hours would just be extraordinarily and unfeasibly expensive. So this is a remarkable project -- one that I'm truly honored to be running -- and one that I'm sure has at least a few hundred more comets left in it!

So moving on to a slightly different note here, I really did not think that I would be making the SOHO-2000 announcement in 2010. It was only back on August 11th, 2010, that we were celebrating our 1,900th comet discovery. Based on typical comet discovery rates, this gave a projected date of February or even March, 2011. But the SOHO comets had other ideas, and something very unique happened...

A comet storm!

September and November were reasonably busy for comet discoveries, and I felt that the 2000th might push into January. Then December came, and the recent cold weather that has plagued much of the United States and western Europe apparently brought with it an ice-storm at the Sun, too! Between December 3rd and 9th, SOHO discovered ten Kreutz group comets! And then it got really busy... In an unprecedented ten-day period, SOHO observed an icy storm in space that resulted in another twenty-five comet discoveries! This is a record for SOHO... by a looooong way. I can recall once or twice seeing eight or ten comets in a period of a few days but such a sustained 'comet storm' as this one is quite incredible.

A deeper meaning here?

So is there a deeper meaning to all these SOHO comets? Obviously it's not related to Earth weather! But why would this happen? Well there are a couple of possibilities. The first is the most fun, and says that all these small comets are a precursor to a much larger object. It has been over 40yrs since we saw a "Great" Kreutz-group comet, so perhaps we're due one. That would be fabulous! However, I have to present possibility number two, which while far less fun, is also far more likely.

So the second possibility is that this is just a statistical anomaly. SOHO averages one comet discovery about every three days, although this average has trended upwards somewhat in the past few years. But back in August and September of 2006, SOHO went 38-days without a comet discovery -- the longest dry spell on record, discounting times when the instrument(s) were not observing. While that 38-day dry spell does not fully cancel out our 25-comets-in-10-days, it is nonetheless an almost equivalent anomaly. And that really is most likely what we've seen -- an unusually dense cluster of small comets that have no greater significance than their statistical value. I hope I'm wrong though!

August 27, 2010 -- The return of comet 2P/Encke

Flashback to 2007: The world watches in awe as comet Encke streaks through the STEREO/SECCHI HI1-A images. We gasp as a coronal mass ejection (CME) erupts out towards the comet. We hold our breath in shock as the CME blasts over the comet, brutally ripping away its tail. Finally, we sigh with relief as the tail grows back, as long and strong as ever before! The emotional roller-coaster can be relived right here as an animated gif or here as a processed (enhanced) huge 134Mb .mov file (right-click and save that last one -- don't try and view it in your browser...).

But that was three-and-a-half years ago. We've all moved on, as did Encke. But now it's back... back in the STEREO/SECCHI HI-1A images, facing the Sun's fury again, and now the Sun is even more active than it was in 2007! What can we expect to see this time? Keep reading and you'll find out...

OK, so perhaps I have over-dramatized things a little. But it was a very cool event back in 2007 and we have been waiting impatiently for Encke to come back (which we knew it would on its 3.3-year lap through the solar system). So it was exciting news when veteran STEREO comet-spotter Alan Watson (Australia) reported that it was visible in the SECCHI HI-1A images from August 23rd. Here's an image of it:

Comet Encke
(click to enlarge)

It the little thing on the right, between the two white markers I drew on there. Yeah, it's not much to look at, is it? It does look a little better if you view it in action:
The images in that movie have been processed in such a way that it enhances the solar outflow (the material flowing out from the right), and reduces the brightness of the background stars. As a result, Encke shows up quite nicely, and you can even see some tail movement. Right now we only have data for Aug 23 and 24. In a few days time I'll update the movies to add in the new images we get -- hopefully by which time something dramatic will have happened!

So you can tell from these images that Encke in 2010 is not as bright, and not as impressive, as Encke in 2007. Why is this? Did it lose too much material when the CME hit it? Is it running out of volatile elements to give off the dust and gas? No, of course not! (At least, it's extremely unlikely.) No, the problem is here quite fortuitously related to the last article I wrote on here (see below -- "Peaceful summer days"), and is a consequence of what we call the "viewing geometry".

The STEREO spacecraft is in a completely different location in its orbit than it was 3.3 years ago. So even though Encke is following the same path, we are seeing it from a very different view-point. The geometry in 2007 was very good -- the comet was somewhat between the spacecraft and the Sun, and we got to watch it from a nice side-on angle with the tail pointing out to the left. In 2010 however, Encke is sweeping around from "behind' the Sun, and so it's tail is pointed (approximately, again) away from the spacecraft and so it looks much shorter*. We are also not getting any "forward scattering" (again, see my other article) of light, and so Encke just looks smaller and fainter. Unfortunately, this situation is not really going to improve much for the duration of it's pass through the HI-1 camera.

So we will perhaps have to wait until 2013/2014 to see if we can get another show like we did in 2007. (And by then, the Sun will be at the maximum of the solar cycle... which means lots of CMEs flying in all directions!) But the Sun is waking up now, and we could still see a CME slam into Encke again -- we just wouldn't get quite as good a view as we did last time. It's still well-worth keeping an eye on though, and I'll certainly post any exciting movies as soon as we have them.

---
* Need convincing of this? Pick up a pen/pencil and point it away from you. Close one eye and then slowly tilt the pen/pencil down slighty. The pencil will appear to get longer as you tilt it into your "plane" of vision. (You have to think two-dimensionally here, hence closing one eye helps.) The comet tail appears to get longer as it comes into the spacecraft's "plane of sky".

July 14, 2010 -- Peaceful summer days...

If you glance at the list of SOHO comet discoveries, you might notice that certain times of year have a lot more comet discoveries than others. Why is that, you ask? Here's how it works...

Most of SOHO's discoveries -- about 85% -- come from the Kreutz group of comets. If we look at the number of Kreutz comet discoveries each month during the history of the SOHO mission, it is obvious that we get a lot more comet discoveries at certain times of the the year than others*:
plot showing monthly comet discoveries
It's pretty clear that (approximately) May and June, and October through December are very busy times of year, while January-March and July/August are noticeably quieter for discovering Kreutz-group comets. This was illustrated just recently when we had twenty comet discoveries in seventeen days in early June, and then since June 23 until now, we have only had three! (As an interesting aside, there is a slight peak in the number of non-Kreutz comet SOHO discoveries at these times too. A possible reason is that the comet hunters are paying more attention to the LASCO C2 images at these periods, so happen to notice more of the less-predictable non-Kreutz comets.)
So why is this?
The reason we see this 'seasonality' of Kreutz comet discoveries is due primarily to the geometry of the comet orbits. Let me explain that in more detail...

The Kreutz group is a stream of (mostly very small) comets all following the same path (or orbit) through space. A good analogy would be cars on a race track, where the cars are the comets and the race track is their orbit (and it takes each car about 900 years do do one lap!). The Kreutz orbit is fixed in location in the solar system and does not move (appreciably, at least... Jupiter, for example, does actually tweak it somewhat, but on fairly slow timescales). But our location in the solar system -- and hence SOHO's location, since it stays between the Sun and the Earth at all times -- does move: we constantly travel around the Sun, completing one orbit every ~365 days. This means that at different time of the year, the Kreutz orbit is going to look like it is in a different location as we view it from different places in the solar system. Going back to the race car analogy, we can walk around the race track and the cars will look like they are going in a different direction depending on which side of the track we are -- but the track has not changed: just our view of it has. Therefore, at different times of year SOHO has a "better" view of the Kreutz orbit than at other times of year, leading to more SOHO Kreutz discoveries during these "better" times. But that's still not the full explanation...
Instrument sensitivity
You might also notice that during times of high comet discovery rates (e.g. May/June, October-December), the majority of the comets are only seen in the LASCO C2 camera. This is not a coincidence! LASCO C2 is more sensitive to Kreutz comets than LASCO C3 is. It has better resolution (~12 arc-seconds per pixel, compared to C3's ~56arc-seconds per pixel); it is more sensitive to fainter objects (limiting magnitude ~m9, whereas C3 is maybe ~m7.5 - m8); and its filter (Orange) corresponds to a wavelength of light that the Kreutz comets show up particularly well in (C3 has a 'clear' filter). Therefore, LASCO C2 allows us to see much smaller and fainter comets than C3 does, even though those comets must have passed through C3 to reach the C2 camera. Finally, as you can see from the tracks of the comets in C2, at certain times of the year, the comets do not appear in the C2 camera until they are much closer to the Sun, and thus only the larger brighter ones make it this close to the Sun while still remaining visible.
Viewing angle
There is actually one more factor in play, which is the 'phase angle' of the comet. The phase angle can be thought of as the angular distance that we observe between the comet and the Sun. The apparent brightness of the comet depends on the phase angle at which we see it, and varies according to how much of the comet appears to be illuminated by the Sun from our view-point. So at certain angles the comet can appear brighter, or fainter, than it actually is. This is not usually a strong effect for the SOHO Kreutz comets and so I will not go into it any further here, but interested readers can find easily more information about phase angle effects on many excellent web sites.
Those wishing for further reading on this subject might be interested in the following papers about light scattering in comets by Dr. J.Marcus: Also highly recommended reading for all SOHO comet enthusiasts is Dr. Matthew Knight's paper [PDF] "Photometric study of the Kreutz comets observed by SOHO from 1996 to 2005". Section 6.1 is most relevant to this news article (but the whole thing is very good!)

* Many thanks to Dr. Matthew Knight for getting the data to me for the plot (so that I didn't have to count the discoveries myself!), and also for allowing the direct link to his paper.

June 16, 2010 -- The Sungrazer Project is back!!

It was a pleasure to recently announce that, for at least twelve more months, the Sungrazer project is back in action! So to kick things off, here is a summary of some of the excitement that has passed since last September... Click here to read on...



Probably the most noteworthy objects over the past few months have been a series of very bright Kreutz-group comets that flew through our cameras during the early part of 2010. The first was discovered in STEREO/SECCHI HI1-A data in early January by Alan Watson, and became STEREO-23. It exhibited some beautiful tail dynamics in HI1-A, so I actually made an exception and wrote a short news item about it ("January 11, 2010 -- Very Bright Comet!"). Over the next few weeks a series of bright Kreutz-group comet graced our fields of view, most notably SOHO-1786 (Jiangao Ruan), SOHO-1787 (Masanori Uchina), and STEREO-34 (Alan Watson). One month later -- February 15, 2010 -- Masanori Uchina and Michal Kusiak reported two components of a small but interesting pair of non-group comets which became SOHO comets #1805 and #1806.

Comets SOHO-1805, SOHO-1806, and STEREO-34 can be seen in the images below. Click the thumbnails for larger versions of the images.
               
    Non-group comets SOHO1805 and 1806 in LASCO C2. The inset shows a contrast-enhanced close-up of the two objects.      Kreutz-group comet STEREO-34 in SOHO/LASCO C2.       Kreutz-group comet STEREO-34 in STEREO/SECCHi Cor2-A.
For a time we went back to seeing just "ordinary" Kreutz comets, but then in early March we were treated to a rather spectacular example of the group. It began as a tiny, faint dot moving slowly upwards in the STEREO/SECCHI HI1-A camera on March 8th, 2010 that was picked up at the edge of the LASCO C3 camera three days later on March 11. Thereafter, it brightened rapidly and dramatically, forming a classic dust tail. By the end of the day it had also reached the COR-2 instruments on both STEREO spacecraft, and with the two of them so well distanced in space, we got to see this comet from three different angles. And to the delight of observers, one of those angles showed us something quite wonderful... a second tail! But, in spite of its size and brightness, STEREO-37 aka C/2010 E6 (STEREO) was not spared the fate of all SOHO and STEREO-observed Kreutz-group comets, and was evaporated in the solar atmosphere late in the day on March 12.

    Opposite is a COR2-B image of STEREO-37, complete with its forked tail! (Click for a larger version.) You can also see STEREO-37 in a the following movies. The first is LASCO C3 movie where not only will you see STEREO-37, but also (if you look carefully enough) SOHO-1814 a few hours ahead of it and then a very close pair of Kreutz comets, SOHO-1815 and SOHO-1816, a few hours behind it. The second movie shows STEREO-37 and its two tails in the STEREO/SECCHI COR2-B field of view.

We haven't seen any more spectacular objects since STEREO-37, with all subsequent being somewhat more typical of the comets we see -- namely, very small and quite faint. But the next big one could be just around the corner, so keep hunting!

Thanks to one of our veteran comet hunters, Polish amateur astronomer Michal Kusiak, who generously helped keep track of comet discoveries during Sungrazer's inactive phase, we can announce that the SOHO comet count currently stands at an astounding 1,864! The STEREO comet count is at a healthy 37 comets, which is impressive given the competition it faces with SOHO, and (by my count) already makes it the eighth most prolific comet discoverer in history. So we now have both the milestones of SOHO-2000 and STEREO-50 now looming on the horizon...

As a final note, I'll just make a couple of remarks about the status of the Sungrazer project. As stated at the start of this article, we now have funding for a year, and in the meantime hope to secure funding for the year following that. Due to the inactivity of the project from September 2009 to May 2010, there is (was) a substantial backlog of work to be performed such as verifying/measuring ~160 comets and website updates, including the discovery tables, image gallery, confirmation lists and astrometry. I'm pleased to say that with the help of a fast-learning and fast-working summer student, the backlog of unmeasured comets is clearing up quickly, but other parts of the website may take longer to get updated. But you can now keep up-to-date with the latest breaking news form the Sungrazer project by following SungrazerComets on Twitter! If any particular information is needed, or something seems to be wrong on the site, you are always welcome, and encouraged, to contact me (just try and use relevant subject lines... I get spam...).



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January 11, 2010 -- Very bright comet!

This news feature on the Sungrazer site has been very quiet of late due to unfortunate circumstances. But we just have to put in a mention of a wonderful bright Kreutz comet that was found by Alan Watson in the STEREO HI-1A images. Read on to see the awesome movie...


On January 1st, 2010, expert STEREO comet hunter Alan Watson spotted a new Kreutz object in the STEREO/SECCHI HI1-A data. The comet quickly brightened and was soon seen in delayed LASCO C3 and C2 images. Movies of the comet in LASCO can be found in several places, some with surprising popularity!

Those movies are very nice, but the pick of the bunch is the fantastic sequence you can (hopefully) see below.
(If you don't see a movie above, you can view/download it here as an .avi (10MB),.wmv (1.3MB), or .mov (<1MB).)

This movie was created by the finder himself, Alan Watson, who processed the HI-1 images before cropping and rotating them to make the sequence. It shows the comet in the SECCHI HI-1A images dramatically increasing in brightness as it nears the Sun. At the same time, it grows a beautiful long tail that starts to suffer the consequences of being buffeted by solar outflow. Hmmm... does this remind you of anything?! (Bit of trivia: it was also Alan that first alerted us to the comet Encke "disconnection event".)

So very many congratulations to Alan for finding this fantastic comet.



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May 15,2009 -- Mysterious comets and igniting planets? No, they're mostly just ghosts...

Regular viewers of SOHO and STEREO data are well familiar with the variety of strange artifacts we see in the satellites images sometimes. We see various strange blobs, reflections and streaks, and I frequently get emails about them (which is something I strongly encourage: you learn by asking questions, so ask away!). Of course, all of these things we see in the data are completely explainable when armed with the appropriate knowledge of CCD detectors (like in digital cameras) and instrument optics (telescopes, lenses, etc). So after over 13 years of SOHO/LASCO images, we have seen and explained every weird artefact that has appeared in the data, and occasionally responded to a few popular myths. More recently (October 2006), we launched the STEREO/SECCHI mission and began send back data from that too. As expected, the STEREO/SECCHI 'COR2' telescopes see exactly the same blobs and streaks (dust, cosmic rays, etc) that we see in LASCO. So no explanation needed there. But the Heliospheric Imagers (HI) are a new kind of telescope and with that comes a new set of strange image effects. So what I'm going to do here is address the two most commonly questioned artifacts that we see in the HI images and explain what they are and why we see them. Read on for the full explanation...

Did that planet just explode?

A few hours before this image was taken, Venus was typical looking planet in the HI-1 field of view: it was bright, as expected, and had the vertical saturation spikes like those you see in Mercury in the image (the other bright spot). But then just as it reached the edge of the images, it got huge! And prior to that, it seemed to throw a mass ejection (a "VME" perhaps??) of its own out at the Sun. What happened?

Check out the movie here (~8Mb .mov) or here (~5Mb mpeg4). Again, the other bright dot is Mercury.

On a certain popular Internet video sharing website there are a couple of videos of this particular time sequence and a number of... interesting... interpretations of what's happening, the primary conclusion being that Jupiter had just been ignited by a passing CME! It's an intriguing prospect but a little wide of the mark for a couple of reasons:
1. It's Venus, not Jupiter. This image shows the inner solar-system planets on January 31, 2009 (the particular date in question), and Venus is clearly in the field-of-view of the STEREO-B spacecraft, just to the 'right' of Mercury as seen from the instruments.
2. Coronal Mass Ejections (CMEs) can not and do not "ignite" things. They are not made of fire or any other flammable material! They are a diffuse cloud of plasma with an embedded magnetic field, and the only reason we can see them is because of sunlight scattering off of electrons in the CME.
3. Planets do not ignite under ordinary circumstances (and these were very ordinary circumstances!). Eventually the Sun will expand and engulf the inner planets of the solar system. But that's a few billion years away yet, and an entirely different situation.

So what is this? Well, this is an artefact of optics and reflections. The 'Venus Mass Ejection' is what we call a ghost, and is the result of bright light reflecting off part of the instrument and entering the optical system of the telescope. (Another example is marked in this neat 1.8Mb .avi movie sent to me by SOHO/STEREO comet hunter Alan Watson.) The 'exploding/igniting' is slightly different, and is caused by light diffracting at the very edge of the instrument optical system. I'll give a little more detail about these in a moment, but first will address the other strange feature that gets me a few extra messages in my inbox.

Where did that bright comet come from?
At first glance, you would say that the image opposite shows a comet. It certainly looks like one. Check out the movies here (~5Mb .mov) or here (~3Mb mpeg4). Once again, the other bright dot is Mercury.

There's a crucial clue that proves it isn't a comet: in this image, the Sun is on the left. A comet's tail always points away from the Sun, because it is formed by the solar wind flowing out from the Sun, so clearly this can not be a comet. So what's happening here? Well, it's essentially the same thing as described above, namely that a very bright object is at a certain point outside of the field of view of the camera, and is shining brightly on just the correct spot on the instrument such that its light is reflected in the detector and lens barrel cavities, creating the artefact you see.

In this particular example, this artefact was seen immediately after Venus left the images, so it's pretty obvious that the two are related. But the same thing can happen if a bright object is about to enter the images, giving the appearance of an abruptly appearing and disappearing bright "comet".
Slightly more technical explanation
So why did Venus appear to explode? What of the apparent ejection from Venus? As I mentioned, they are a result what we call "stray light" and "ghosting", and are basically due to a very bright object just creeping out of direct view of our cameras and the light from that object reflecting off of various parts of the instrument. To explain it better, it's helpful to look at the design of the HI instrument. The Heliospheric Imager instrument has a series of lenses designed to give us the optimum performance and sensitivity while minimizing the effect of extremely bright objects in the field of view. Remember: we're trying to detect the incredibly faint signal of solar outflow and corona (atmosphere), so the telescope has to be really, really sensitive to faint signals. In addition, this camera uses a very long exposure times -- 40-minutes per image, in fact! Planets like Venus are incredibly bright, especially when you're up in space with no atmosphere to diffuse their light, and so when they shine into our camera, there is only so much we can do to stop 'bad' reflections.

As you can probably image, before launching a mission like STEREO it is vital that we design our instruments so that the stray light does not blind us. To help do this, we do 'ray tracing' simulations on computer models of our telescopes to simulate how beams of light will react as they enter the instrument. If you click on the image to the right, it will show an expanded diagram that I put together (which is why it's not very good...) that illustrates how a bright object is causing the 'ghosts' I have described above. At the top of the image is a 'ray tracing simulation' that shows bright light from some object being focused onto a point away from the CCD detector. This bright light then reflects back into the lens system and ultimately reaches the CCD as a very faint signal.

The example given in the image is more specifically for the ring that emanated from Venus (the "VME"!). In the case of the 'mysterious comet', the explanation is partly as above but is more related to diffraction, which in this instance is the 'bending' of light waves around the edge of the telescope and instrument. Refraction is very common in optical systems but the effect is amplified in the HI instrument because of the very sensitive optics and the fact that our exposure times are so long. Any faint refraction (or reflection) that reaches the camera is going to accumulate in brightness for the entire duration of the exposure (~40-mins). So it's is actually an incredible feat of design and engineering that our telescopes have so very few artifacts in them!

So as cool as it may seem for comets to suddenly appear, and planets to suddenly ignite, the true explanation is a little less exciting but hopefully, at least, a little interesting...

(Many thanks to Jean-Philippe Halain (Centre Spacial de Liege, Belgium) for the ray-tracing diagram and technical assistance with this article!)

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April 9,2009 -- STEREO-20: Coming soon to a night sky near you!*
*Assuming you live in the Southern Hemisphere...

STEREO's 20th comet has been discovered... and it's a pretty exciting one! Comet C/2009 G1 (STEREO), also known as STEREO-20, was announced earlier today on MPEC 2009-G30. Discovered yesterday by Chinese amateur astronomer Jiangao Ruan, it is a small but relatively bright (~mag 10-11) comet that, unlike most of SOHO and STEREO's comet discoveries, does not belong to any known population or group of objects. This in itself makes it an interesting target, but the most exciting part of this discovery is that it is very likely to be visible from Earth to observers with relatively small telescopes! This may not seem like a particularly big deal, but of the more than 1,600 comets discovered by SOHO, only a very small number have ever been seen from the ground (perhaps most notably C/1998 J1 (SOHO)), and none of STEREO's other nineteen discoveries have been ground-observable at all. Read more about this exciting discovery...


Jiangao first reported C/2009 G1 as a possible moving object in SECCHI HI1-B images from April 5th. It was quickly established that this was indeed a real object, but nobody was sure if it was already "known". Fortunately, its brightness narrowed down the list of possible known objects to just a few, and it took little time after that to establish that this was indeed a brand new discovery. At the time of writing, the orbit of the comet is based entirely on SECCHI HI1-B measurements, so has a degree of uncertainty to it. However, the comet is currently placed some 40-degrees from the pre-dawn Sun for observers in the Southern Hemisphere. Hopefully they will be able to find comet STEREO in their early-morning skies and allow the determination of a more certain orbit.

Comet C/2009 G1 is not predicted to get much brighter than it is now, as it is predicted to reach perihelion (closest approach to the Sun) on April 12, 2009. But maybe we will be lucky and it will put on an unexpected show for us...

I will continue to update this article as and when ground observations/images are announced. In the meantime, sit back and enjoy a view of comet C/2009 G1 (STEREO) from your desk or armchair, in much the same fashion as it was discovered!

    The image opposite shows C/2009 G1 (STEREO) in SECCHI HI1-B on April 6th, 2009 at ~23UT. The image has been processed in such a way as to enhance moving features (such as comets and solar outflow), and minimize the effect of stars. It has also been cropped and then enlarged. Click for a larger version.
Also available are the following movies: (We recommend you right-click the movies and save to your computer.)
  • A 48Mb QuickTime movie here
  • A 29Mb AVI movie here
  • A 2.9Mb MPEG-4 movie here
I will update these movies as more data becomes available.

April 10 UPDATE: STEREO-20 seen from the ground!
Just a couple of hours after the release of the initial orbit of C/2009 G1 (STEREO), it was located from the ground by Japanese astronomers, just a few arc-minutes from its predicted location! (Considering how poor orbit determinations can be from solar instrument data, this is an impressive result!). The orbit was updated to reflect these observations and released on MPEC 2009-G32. C/2009 G1 (STEREO) will reach peak brightness in mid-May at ~mag10.2, and should be a reasonably easy target for Southern Hemisphere observers. Hopefully we'll soon get some ground-based images...

April 14 UPDATE: Ground-based image of STEREO-20!
    This image (click for full version) shows C/2009 G1 (STEREO) as seen from Earth! It was taken remotely on April 10th by Belgian amateur astronomer Erik Bryssinck using a telescope at the Moorook Observatory in Australia (part of the Remote Astronomical Society's Global Rent-a-Scope program). The image shows the comet looking much the same as it does in the SECCHI data -- namely, small and diffuse with no apparent tail.

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March 20,2009 -- New STEREO Comets

SOHO has to start looking over its shoulder if it wants to remain History's Most Successful Comet Discoverer -- STEREO is catching up! Well, sort of... Read more about these latest STEREO comets...


This week I finally got around to confirming the past three months of STEREO/SECCHI comet reports and I'm pleased to say that there have now been nineteen new objects discovered in SECCHI images! OK, so it's not quite up there with the one-thousand six-hundred and twenty-one that SOHO currently has, but it's a start!

Eighteen of STEREO's nineteen comets are members of the Kreutz-group of comets, most of them discovered in the SECCHI HI-1 imager. The nineteenth was a non-group object, also found in HI-1 images. Currently, Australian comet hunter Alan Watson is leading the way with eleven SECCHI discoveries, with Rainer Kracht (Germany) and Rob Matson (USA) trailing with three discoveries each.

Here is the recent set of SECCHI objects that I confirmed:
STEREO-#    Reported       Discoverer   Cam(s)    Group    Images of...
------------------------------------------------------------------------------
STEREO-008  Jan07,09 04:23  A.Watson    HI1-A     Kreutz   Jan04,09 
STEREO-009  Jan07,09 05:12  A.Watson    HI1-A     Kreutz   Jan02,09
STEREO-010  Jan07,09 04:23  A.Watson    HI1-A     Kreutz   Dec31,08-Jan01,09
STEREO-011  Jan13,09 04:42  A.Watson    HI1-A,B   NonGrp   Jan10-12 
STEREO-012  Jan22,09 19:31  R.Matson    HI1-A     Kreutz   Jan19-20,09
STEREO-013  Jan31,09 20:42  R.Matson    HI1-A     Kreutz   Jan29,09
STEREO-014  Feb12,09 18:40  R.Kracht    HI1-A     Kreutz   Feb10,09
STEREO-015  Feb13,09 21:40  K.Battams   HI1-A     Kreutz   Feb11-12,09
STEREO-016  Mar12,09 13:24  K.Battams   HI1-A     Kreutz   Dec31,08-Jan01,09
STEREO-017  Mar11,08 05:33  A.Watson    HI1-A     Kreutz   Mar07-08,08
STEREO-018  Jan12,09 22:13  R.Matson    HI1-A     Kreutz   Jan10,09
STEREO-019  Jan13,09 05:17  A.Watson    HI1-A     Kreutz   Jan10,09
Test time...
    So do you think of yourself as an expert comet hunter? Or still learning how it all works? Either way, here is a nice little test for your comet hunting skills! Between December 31,2008 and January 02, 2009 there were SEVEN Kreutz-group comets visible in the SECCHI HI-1A images. Download and play the movie below (not the "spoiler"!) and see if you can spot all seven of them. Some are brighter than others, and the seventh is the trickiest to spot. Maybe there's one in there that we haven't noticed yet! (We strongly recommend right-clicking the movie links and saving to your computer, rather than playing them in the browser.)

And in case you are wondering, the seven comets in the movie are named (and were discovered by) the following:
  • Comet #1: SOHO-1598, H.Su (China)
  • Comet #2: STEREO-10, A.Watson (Australia)
  • Comet #3: STEREO-16, K.Battams (US)
  • Comet #4: SOHO-1599, H.Su (China)
  • Comet #5: SOHO-1600, H.Su (China) -- his 200th SOHO comet discovery!
  • Comet #6: SOHO-1601, H.Su (China)
  • Comet #7: STEREO-09, A.Watson (Australia)
Happy Hunting!

Mar 24 Update: I should clarify that the images in the above movies are cropped from full-size SECCHI HI-1A images, and processed to minimize the effect of stars and enhance moving features such as solar outflow and comets.

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Older news stories (permanent archives): Oct-Dec 2005, 2006/2007, 2008/2009

Have an interesting SOHO comet story? Send it to sungrazer@nrl.navy.mil