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Latest News... Archive!

Fact-bites, stories, and points of interest relating to SOHO comets and SOHO images.

May 2008 through 2009

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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.
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!)

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.
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.

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...
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.

December 17,2008 -- SECCHI Makes a Fantastic Recovery!

You would think that, after 13-years of historic comet discoveries with SOHO and two years of amazing STEREO/SECCHI observations and discoveries, we had put a check-mark in most of the boxes for comet-related achievements. But last week, Australian comet-hunter Alan Watson helped us with yet another historic achievement -- the recovery of a comet! Here's how it unfolded...

While diligently scouring the latest STEREO/SECCHI Heliospheric Imager ("HI") HI-1B data for comets and asteroids, Alan noticed the appearance of an object rising up from the lower-edge of the images. Immediately recognizing it as a "non-group comet" (not belonging to any known population), he reported it to the popular 'stereohunter' chat group page. It was soon established that this was not one of the known bright comets currenty gracing the skies and so we assumed it to be a new discovery for SECCHI. Meanwhile, veteran comet hunter Rainer Kracht (Germany) recorded a few positions of the comet in the data and produced a set of very approximate orbital elements for it. Maik Meyer, Rainer's fellow countryman and another of our most esteemed comet hunters, saw these orbital elements and noticed their similarity to those of a comet that was discovered back in 2003, but lost in the depths of space sometime thereafter. This theory was confirmed when Dr. Brian Marsden released the electronic circular designating it P/2008 X4 and linking the SECCHI observations to the comet P/2003 K2 (Christensen). And so it was official -- SECCHI had made a historic recovery of a comet!

P/2003 K2 (Christensen) was discovered by Eric Christensen and then observed from the ground for a few weeks in May/June 2003. The 2003 observations indicated that the comet was a short-period object with period of ~5.7yrs. The angular elements of its orbit were determined to a reasonable accuracy, but there was inherent ambiguity as to exactly where along its orbital path the comet was. Regardless, a prediction was set that it would return close to the Sun in January 2009. In late 2008, knowing that it was getting closer to the Sun, astronomers began searching for the comet again, but observations were turning up negative. In the course of email discussion about this object, Maik Meyer remarked:
"About three weeks ago ... I searched remotely from Tahiti for the comet. I used at first the nominal orbit with T=2009-Jan-8 and covered the line of variation of ±10 days. Nothing was found down to 18 mag. I then recalculated the orbit without the last set of observations, which lead to a T at the end of January. I again covered about ±10 days. Alas, nothing found down to 18 mag. This also suggests that the comet was on a different part of its orbit (although the magnitude could still have been too faint)."
The appearance of P/2003 K2 in HI-1B data on December 8, 2008 confirmed Maik's suspicions -- namely that the comet was indeed on a different part of its orbit.

This latest STEREO/SECCHI discovery ("recovery") is noteworthy for a couple of reasons. First, it means that we now know much more accurately where the comet is in space, so now ground-based observers can find it when it moves away from the Sun in January as a fading 10th magnitude object. Secondly, it means the comet can be declared "officially" periodic, and redesignated as such, instead of bearing just the provisional "P/2003 K2" designation. And finally, as a major coup for the STEREO/SECCHI project, this is the very first recovery of a single-apparition comet by a spacecraft! ("Single-apparition" means it has only been observed to pass the Sun once.) Not even SOHO has managed to do this in over 13-years of observing the near-Sun environment!

So this is a wonderful achievement for the STEREO/SECCHI mission and the fantastic group of international comet hunters that volunteer so much of their time to this work. Of course, particular congratulations must go to Alan Watson, Maik Meyer and Rainer Kracht for their work on identifying this object. And now the best bit -- movies and pictures!
    The image on the left (click for a larger version) shows the location of the comet in a HI1-B image taken on Dec 13,2008 at 00:49UT. Also of note in the image is a beautiful coronal mass ejection.

The image on the right (again, click for a larger version) shows the track of the comet through the data. This image is more useful to be viewed along-side one of the following movies.

(We strongly recommend right-clicking the movie links and saving to your computer, rather than playing them in the browser.) Finally, there is also a bonus comet in the upper-two extended movies for the sharp-eyed among you. At the very start of the sequence, a Kreutz-group comet (SOHO-1583) can be seen flying into the lower-left edge of the images, about halfway between the solar outflow and the bottom of the image. (Can't see it? Here's a hint.) The images used here have been highly processed in such a way that solar features (CMEs, outflow, etc) are enhanced, and the stars are subdued. From time to time, the images may 'jump' or 'flash'. That is due to both instrumental and image processing affects. You can download more 'traditional' HI-1 images from the SECCHI Images web page.

November 24, 2008 -- Website news

Today I am pleased to announce that the sungrazer site has found a new, permanent home at the easy-to-remember web address of!! Please update your bookmarks, and tell all your friends! All old links will eventually be updated to point to this new site, and we do not plan on moving it again. Many thanks to you all for your patience while this site was offline a couple of months ago and while we have been temporarily hosted by the wonderful Upper Atmospheric Group here at NRL!

Also, in related news, the LASCO and SECCHI websites are now back online too! At the time of writing this, some parts of those sites are still not fully functioning, but they will be within the next week or so.

October 2nd, 2008 -- Three exciting pieces of news!

The past few weeks have been the source of a few exciting SOHO-comet-related events. We'll go through them in chronological order:

Eclipse Kreutz

August 1st saw a total solar eclipse over the far northern reaches of North America and across Northern Europe and Asia. These are always exciting events as they give us the opportunity to see the Sun's faint outer atmosphere ("corona") from Earth. Normally, the direct light from the Sun is far too blinding for us to see anything. But on these rare occasions, the Moon takes on the function of a coronagraph occulting disk and, for a brief time, lets us see what LASCO and SECCHI see all day, every day!
Now, as fortune would have it, at noon on July 31st, the highly successful Chinese comet hunter, Hua Su, spotted a small Kreutz in the LASCO C3 field of view -- an object that would be very close to the Sun at the peak of the eclipse! I began performing astrometric measurements as soon as we have a reasonable number of images. From these, and a subsequent set, Dr. Brian Marsden released a couple of MPECs giving the (approximate) orbit and ephemeris of the comet (named SOHO-1517 or, officially, C/2008 O1). Then we sat back and waited for the reports of ground sightings to come rolling in... And we waited... And waited...
So after a week or two without reports of sightings, we pretty much concluded that the comet was too small and faint to be seen from the ground, despite its near-perfect placement in the sky. However, all that changed on September 20th when Polish comet hunter Michal Kusiak spotted a extra star in the fantastic eclipse image taken by Miloslav Druckmuller (Brno University of Technology), Peter Aniol and Vojtech Rusin. Subsequent follow-ups by other SOHO comet hunters and enthusiasts led to the conclusion that this was C/2008 O1!
So what's the significance here? Well, it's two-fold. First, this is only the second LASCO-discovered comet to have been observed from the ground -- the first being C/1998 J1. (A few comets discovered by the SOHO/SWAN instrument have been observed from the ground.) Second, this is the first Kreutz-group comet to be seen from the ground since Comet White-Ortiz-Bolelli in 1970! So while not a spectacular object, it is an exciting discovery nonetheless! Congratulations to Hua, Michal and Dr Druckmuller et al.!

A new short period comet!

On September 18, Rainer Kracht (the famous SOHO comet hunter), found a small non-group object (SOHO-1532) in the LASCO C2 images. His initial reaction was that this object looked like it could be related to C/2004 X7 (SOHO), and his initial computations indicated it to be so. However, not stopping there, he pursued the idea that rather than being related to C/2004 X7, SOHO-1532 was in fact identical to it, implying an orbital period of approximately 3.8yrs. If true, this would mean that is should also have passed through the LASCO C2 camera in 2001. And sure enough, there was C/2001 D1 (SOHO), which was in the right place at the right time... but with completely the wrong orbital elements. However, such is the nature of the SOHO comet measurements, the observations recorded for many of these objects lead to highly ambiguous and uncertain orbital solutions. In the case of C/2001 D1, Rainer was able to identify a whole new set of images of the comet that were missed back in 2001. So armed with new observations, Dr Brian Marsden was able to officially confirm the link, and SOHO had it second short-period comet! (The first being about one year ago.)

Another new short period comet!

On September 24, while working on the previous comet (above), it occurred to Rainer that maybe there were other short-period comets in the SOHO archive that had just been overlooked due to the uncertain orbital solutions for the comets. Sure enough, it did not take him too long to find one! In his own words:

"The second trio needed a bit of luck and much patience. I had looked many times at the orbital elements of the non-group comets, but could not identify any possible pair (or trio).
After I had found the first trio, I looked again at my list of non-group comets and searched for low inclination, low q comets with short observational arcs. I found C/1999 X3, C/2004 E2 and C/2008 K10. The comparison of the apparent pathes looked terrible, but they were rather evently spaced indicating a orbital period of just over 4 years. The short observed arcs (0.11, 0.12, 0.16 days) meant that the orbits were not determined very well. So I continued the work.
I tried C/1999 X3 = C/2004 E2 with no success. I tried C/1999 X3 = C/2008 K10 with no success. I stopped the work on this trio.
I knew that I should also try C/2004 E2 = C/2008 K10. When I did this some hours later, all fell into one place.

Thus, Rainer was able to link the SOHO comets C/1999 X3, C/2004 E2 and C/2008 K10, finding SOHO's third confirmed short period comet, with a 4.22yr orbit around the Sun!

As a final point of interest about these short-period objects, there is a potential relation between them. Both of the above are in " 1:3 resonance orbits" with Jupiter, which makes them somewhat similar to a collective group of objects known as Alinda asteroids. However, the orbital eccentricity of the SOHO-discovered objects is really too high to make them Alindas, but it is theorized that with enough orbital evolution, their eccentricities could increase sufficiently to pass them close to (or into) the Sun.

So does that mean that these are asteroids and not comets? The problem there is in the definition of what exactly is an asteroid and a comet, and we have already seen that SOHO has already started to blur that line...

June 27, 2008 -- 1,500 Comets!!

This week, SOHO has achieved yet another astounding milestone -- the discovery of it's 1,500th comet! This most prestigious of objects was discovered by the US-based veteran comet-hunter and amateur astronomer Rob Matson. Rob is no stranger to SOHO-comet publicity, having co-discovered two SOHO/SWAN comets and 74 SOHO/LASCO comets! The signficance of his latest discovery came somewhat unexpectedly to him, however. "I knew we were close to 1500, but hadn't bothered to backtrack through the reports and see HOW close. So a very nice surprise!", said Rob.

While this may rank as one of his more prominent SOHO discoveries, sadly it does not rank among the brightest! As you can see from the following images, SOHO-1500 was a very small and faint Kreutz-group comet, barely visible in the lower-right corner of the LASCO C2 data.

    The image on the left (click for larger version or here for a medium-sized one) shows increasingly zoomed inserts, illustrating just how small and faint the comet was.
The image on the right will animate once clicked, and shows the comet moving through the images. The box in the lower-left corner of the animation shows a contrast-enhanced zoom of the comet.

With all these comet discoveries by SOHO, you may be wondering what percentage of all comets have been discovered by SOHO. Well, the last time I got "official" figures (courtesy of Dr. B.Marsden, Minor Planet Center) was mid-January 2008, when there were 2785 single and multiple-apparition comets on record. By remarkable coincidence, I had submitted astrometry for SOHO-1393 the previous day -- and it was that comet that meant SOHO had discovered more than half of all officially recorded comets! SOHO has, of course, discovered many more comets since then, putting it well beyond the 50% mark (more like 54%...) of all officially recognized comet discoveries.

Now, if you're sad that you did not find the 1,500th SOHO comet, don't worry -- we're only 500 away from 2000! When will we find it? Well let's look at the statistics...
SOHO-500 -- August 12, 2002
SOHO-1000 -- August 6, 2005
SOHO-1500 -- June 25, 2008

So, based on the last three major milestones, my prediction is:
SOHO-2000 -- April 15, 2011!

If you want to join in the fun, feel free to email me your prediction for SOHO-2000! Of course, this is rather dependent on SOHO/LASCO still being operational in 2011. After all, it was only designed for a nominal two-year mission and we are already into it's thirteenth year! But hopefully it will still be around to see in yet another fantastic milestone.

Finally, I always like to take these opportunities to thank the heroes behind SOHO's comet-finding successes; namely the fantastic team of "comet hunters" who have dedicated their free time to searching for these objects. As is often stated, SOHO is a project of international cooperation between NASA and ESA. So I find it very fitting that this comet data set is the work of an international team comprised of over 60 people representing (by my count) 15 nations around the world. So thank you to everyone who has contributed to this project!

This story has been picked up in a European Space Agency press release and a SOHO HotShot.

May 29, 2008 -- Six eyes are better than one!

Anyone taking even a cursory glance at the images from the SOHO or STEREO satellites in the past week could not have failed to spot the fantastically bright Kreutz-group comet, now dubbed "SOHO-1476". This beautiful comet was first spotted as a tiny, faint dot in the LASCO C3 field of view by veteran comet-hunter Rob Matson. Within 24 hours the comet formed a long, bright tail as it plunged unnervingly towards its fiery demise.

Regulars here will know that comets like this are not entirely uncommon for SOHO to see. In fact SOHO has discovered well over one thousand comets belonging to the Kreutz group of "sungrazing" comets. But what made this object more interesting was not the fantastic appearance, great as it was, but the fact that as it raced through the SOHO/LASCO cameras, it was also being watched by another two spacecraft!

The NASA STEREO "A" and "B" satellites, launched in late 2006, are now well distanced from Earth and each other meaning that near-sun objects are now seen from very different viewpoints in space. There is no better illustration of this concept than by showing the data from the STEREO/SECCHI and SOHO/LASCO instruments.

    This image (click for larger version) shows SOHO-1476 at different times in images from LASCO C3, LASCO C2, SECCHI COR2-A, SECCHI COR2-B, SECCHI COR1-A, and SECCHI COR1-B.
A larger version of the image on the left can be obtained here.

On the right I have a short MPEG movie (~1.1MB) showing the comet in simultaneous SECCHI COR2 A and B images. Click to play or right-click and download to save to your computer.

As you can see, the comet looks quite different when seen from the different viewing angles offered by the three sun-gazing satellites. But in addition to giving us information on the morphology (shape) of the comet and its tail, the multiple views have a secondary advantage when it comes to calculating the orbits for these objects. Ordinarily, we only see comets in SOHO data for a relatively short time and only from one location in space. This means that any measurements we make of the comet's location in space are highly ambiguous because we are only seeing it in two dimensions. However, now that we see brighter comets in SOHO and both STEREO spacecraft, we are able to remove a great deal of the uncertainty in the comet's true location in space and calculate a significantly more accurate and confident orbit for it.

One final thing that may also not escape your attention is the faint eruption ("Coronal Mass Ejection" or CME) that appears to be caused by the comet. Actually, the two events are completely unrelated and it is again thanks to the multiple viewpoints offered by STEREO that we can say and prove this. When you watch movies of the eruption, particularly in COR1-A but also when compared to COR1-B, you see that the eruption occurred well away from the comet. Additionally, the sun is HUGE and the comet is TINY (maybe a couple of hundred meters)... it's like a bug hitting a freight train, but with less mess.

May 27, 2008 -- Introducing SECCHI comets!

No one needs reminding of SOHO's ability to discover comets. At time of writing, the discovery count stands at an astounding 1,465 comets -- a figure that means that SOHO has actually discovered more than 50% of all officially designated comets throughout history! Now, while SOHO's position on the podium is not at risk, we do have a new instrument on the block that has just started to notch up its first comet discoveries. I am referring, of course to STEREO/SECCHI.

It took a while -- over a year in fact -- but in February 2008 the first comets were discovered in data returned by the SECCHI instruments on board NASA's STEREO mission. In just a few days, Alan Watson and Rainer Kracht found four (3 for Alan, 1 for Rainer) previously unknown Kreutz-group comets in SECCHI data. Three of these were visible only in the HI-1 images on spacecraft 'A', but one (SECCHI-003) was also seen clearly in the COR2 instruments on both 'A' and 'B' satellites. More recently, two more SECCHI discoveries by Alan and Rainer were made official, bringing the SECCHI/STEREO comet discovery total to six.

    These images (click for larger version) show SECCHI-003 (C/2008 D3 STEREO) animated in HI1-A (left) and stacked in both COR2 instruments (right).
Image credit: A.Watson

STEREO/SECCHI is clearly never going to rival SOHO/LASCO for comet discoveries, and while LASCO remains operational and transmitting data in near-realtime, it is going to hog the bulk of the Kreutz-group discoveries. But we can at this point anticipate a small but steady stream of SECCHI comet discoveries over the next few years and so with that in mind, the time has come to merge the "SOHO Comets" program with the SECCHI program and create a joint "Sungrazing Comets" program!

You will notice some subtle changes to the sungrazer website, namely the addition of pages giving a SECCHI overview, information on reporting SECCHI comets, and a tabular list of SECCHI comet discoveries. Also, the website heading now has changed to reflect the addition of the SECCHI comets into the sungrazing comet program. Otherwise, no other major changes are happening any time soon, and comets will continue to be posted in the same way as always.

Older news stories: Oct-Dec 2005, Jan-Dec 2006

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